A Level Chemistry Paper 2 Planning Stoichiometry 1 To determine the percentage composition by mass of Na2CO3 using gas collection. 2 To determine the percentage of calcium carbonate in a 1.0 g sample of mussel shell fragments by a back titration method. 3 To determine which of the equations that correctly represents the neutralization between citric acid and NaOH using titrimetric analysis. 4 To determine the purity of a sample of commercial aspirin which has been exposed to air for some time using titrimetric analysis. 5 To determine the percentage by mass of MgCO3 in the contaminated sample using gravimetric analysis. 6 To confirm that the formula of the basic carbonate is consistent with the equation written using gravimetric analysis. Redox Titration 1 To use a standard solution of potassium manganate(VII) to measure the percentage purity of a given sample of solid iron(II) ethanedioate, FeC2O4. 2 5 7 11 14 17 19 To carry out a titrimetric analysis to determine the percentage by mass of Fe3+ ions present in the sample of oxidised iron tablets. Energetics 1 To determine the standard enthalpy change of combustion of cyclohexa-­‐1,3-­‐diene or cyclohexa-­‐1,4-­‐diene 2 To determine the enthalpy change of combustion, ΔHc, under laboratory conditions, for each of the five alcohols 3 To plan a simple experiment to identify FA1 and FA2. FA1 and FA2 could be 1.00 mol dm–3 H2SO4 or 1.00 mol dm–3 CH3CO2H. 4 To determine the enthalpy change of solution of the two salts, with ammonium chloride. Kinetics 1 To study the effect of concentration changes on the rate of a reaction, using Mg and HCl. 2 To determine the order of reaction with respect to sodium thiosulphate(VI) and hydrochloric acid by using the initial rate method. Chemical Equilibrium 1 To determine the Partition Coefficient, K for ethanoic acid, CH3COOH, between water and cyclohexane, (C6H12). Organic Chemistry 1 To identify 5 unlabelled bottles each containing the following organic liquids or solutions 2 To identify organic acids. 3 To identify each of the organic substances, butan-­‐2-­‐ol, butanal and butanone. 4 Preparation of methylamine hydrochloride, CH3NH3Cl Electrochemistry 1 To determine a value for the Avogadro constant by electrolysis Qualitative Analysis 1 To identify each of the solutions aluminium nitrate, lead nitrate and zinc nitrate 20 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 1 2 24 26 30 32 36 38 40 43 46 49 51 53 55 Paper 2 Planning Questions Stoichiometry (Gas collection) 1 FA2 is a mixture of solid sodium carbonate and sodium chloride. The estimated percentage range by mass of sodium carbonate in the mixture is 90 – 100 %. You are required to plan an experiment that will allow you to determine the percentage composition by mass of sodium carbonate in FA2. Sodium carbonate reacts with hydrochloric acid as shown in the equation below. Since carbon dioxide gas is produced, the method of gas collection is proposed. Na2CO3(s) + 2HCl(aq) ⎯→ 2NaCl(aq) + CO2(g) + H2O(l) Chemicals available 1.00 g of FA2 0.200 mol dm 3 HCl solution Apparatus available 100.0 cm3 graduated gas syringe Other common laboratory apparatus (a) Calculate an appropriate mass of FA2 to use, given that the volume of gas collected, at r.t.p., should not exceed 50.0 cm3. State any other assumptions that you make in your calculations. (b) Calculate an appropriate volume of HCl(aq) that should be used in your experiment. (c)
With the aid of a labeled diagram, briefly outline how you would carry out the experiment on FA2. In your outline, you should pay particular attention to the method of ensuring that no gas is lost on mixing the solid and solution. Answer (a) Amount of CO2 evolved = 50.0 ÷ 24000 = 2.08 × 10 3 mol Na2CO3(s) + 2HCl(aq) ⎯→ 2NaCl(aq) + CO2(g) + H2O(l) Amount of Na2CO3 present in weighed sample of FA2 = amount of CO2 evolved = 2.08 × 10 3 mol [1] −
−
−
Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 2 Molar mass of Na2CO3 = (23.0 × 2) + 12.0 + (16.0 × 3) = 106.0 g mol 1 −
Mass of Na2CO3 in weighed sample of FA2 = 2.08 × 10 3 × 106.0 = 0.221 g [1] −
Assume FA2 contains 100 % by mass of Na2CO3(s). Then mass of FA2 to be used = 0.221 g. A suitable mass of FA2 to be used is 0.220 g (If student assumes 90%, mass to be used = 100/90 x 0.221 = 0.246g. Accept any answer between 0.220-­‐0.246g) (b)
[1] Assume FA2 contains 100 % by mass of Na2CO3(s). Amount of Na2CO3 present = 2.08 × 10 3 mol Minimum amount of HCl(aq) used = 2 x 2.08 × 10 3 mol = 4.16 x 10-­‐3 mol [1] Minimum volume of HCl(aq) used = 4.16 x 10-­‐3 ÷ 0.200 = 0.0208 dm3 = 20.8 cm3 [1] HCl(aq) should be used in excess, thus we could use 30 cm3 of 0.200 mol dm-­‐3 HCl(aq). (Accept any answer above 20.8 cm3) −
−
(c) Graduated gas
syringe
Stopper
Diagram showing small
Small tube containing
test tube with FA 1 in
FA1 tied to a string
Retort stand
conical flask + showing
gas syringe[1m]
HCl(aq)
Weigh accurately about 0.220 g of FA2 and place it in a small test tube tied to a string. Introduce 30 cm3 of HCl(aq) using a measuring cylinder into a clean and dry 250 cm3 conical flask. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 3 Set up the apparatus as shown above in the diagram. Lower the filled tube into the conical flask taking care that the reagents do not mix. Stopper the conical flask. Check that the initial reading of the 100.0 cm3 graduated gas syringe is set at the zero mark. At a suitable time, loosen the stopper slightly to release the string. Stopper the conical flask immediately. (Or other forms of starting the reaction) Swirl the conical flask to ensure that the reagents are well mixed. Allow the reaction to progress until it has ceased as indicated by a constant volume reading of the syringe. Record the final volume, V1 cm3, on the graduated gas syringe. Repeat the experiment to get consistent results. Actual Marking Scheme Weigh accurately about 0.220 g of FA2 and place it in a small test tube tied to a string. [1] 3
Introduce 30 cm of HCl(aq) using a measuring cylinder into a clean and dry 250 cm3 conical flask. [1] Lower the filled tube into the conical flask taking care that the reagents do not mix. Stopper the conical flask. [1] At a suitable time, loosen the stopper slightly to release the string. Stopper the conical flask immediately. (Or other forms of starting the reaction) [1] Record the volume of the gas collected. [1] Max: 4 marks (for outline) Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 4 Stoichiometry (Acid-­‐carbonate reaction) 2 New Zealand green-­‐lipped mussels are rich in omega-­‐3 fatty acids. Freeze-­‐dried tissues of such mussels are sold in many countries as nutritional supplement to relieve arthritic symptoms. Mussel shells are a good source of calcium carbonate and may be used as ‘fertilisers’ to regulate soil acidity. When hydrochloric acid is added to mussel shells, the following reaction occurs. CaCO3(s) + 2Hcl(aq) → CaCl2(aq) + CO2(g) + H2O(l) You are to determine the percentage of calcium carbonate in a 1.0 g sample of mussel shell fragments by a back titration method. The shell fragments are first treated with an excess of hydrochloric acid and the unreacted acid is then determined by titration with standard sodium hydroxide solution. (a) Calculate the minimum volume of 1.0 mol dm–3 hydrochloric acid that must be added to the 1.0 g sample of mussel shell fragments for complete reaction to occur. [2] Mr of CaCO3 = 40.0 + 12.0 + 3(16.0) = 100 1.0 = 0.010 mol mol of CaCO3 = 100
mol of HCl = 2 × mol of CaCO3 = 2 × 0.010 = 0.020 mol ∴ vol of HCl = 0.020
× 1000 = 20 cm3 1
(b)(i) Upon complete reaction of the calcium carbonate in a 1.0 g sample of mussel shell fragments, the excess unreacted hydrochloric acid was diluted to prepare a 250 cm3 standard solution for titration with 0.10 mol dm-­‐3 NaOH solution. [4] If 25.0 cm3 of sodium hydroxide solution was required to react with 25.0 cm3 of the standard solution of hydrochloric acid prepared, calculate the number of moles of HCl present in the 250 cm3 standard solution prepared. Mol of NaOH = 25.0
x 0.1 = 2.5 x 10-­‐3 mol 1000
mol of HCl in 25.0 cm3 std solution = 2.5 x 10-­‐3 mol mol of HCl in 250 cm3 std solution = 0.025 mol Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 5 (ii) If 40.0 cm3 of the original 1.0 mol dm-­‐3 hydrochloric acid was added to the 1.0 g sample of mussel shell fragments, calculate the number of moles of CaCO3 present in the sample of mussel shell fragments and hence determine the percentage by mass of CaCO3 in the mussel shells. Total mol of HCl added to react with mussel shells = 40.0
x 1 = 0.04 mol 1000
mol of HCl reacted with CaCO3 = 0.04 – 0.025 = 0.015 mol 1
2
mol of CaCO3 present in 1 g mussel shell = x 0.15 = 7.5 x 10-­‐3 mol (c)
(d)
mass of CaCO3 present in 1 g mussel shell = 7.5 x 10-­‐3 x 100 = 0.75 g % by mass of CaCO3 present in mussel shell = 0.75 / 1 x 100% = 75% Outline the procedure you would carry out in the back titration method. In your description, you should include details of the apparatus and chemicals used and measure(s) taken to ensure reliability of results. [4] 1. Using a burette, run 40.00 cm3 of 1 mol dm–3 HCl into a 100 cm3 beaker containing the 1.0 g sample of mussel shell fragments. Reaction is complete when there is no more effervescence of CO2(g). 2. Filter with a filter funnel and filter paper to remove any impurities. 3. Transfer the filtrate and washings into a 250 cm3 volumetric flask and make up to the mark with distilled water. Shake well to obtain a homogeneous solution. 4. Pipette 25.0 cm3 of diluted HCl (from the volumetric flask) into a 250 cm3 conical flask. 5. Add 2 drops of phenolphthalein indicator. 6. Titrate with 0.10 mol dm–3 sodium hydroxide (placed in a 50 cm3 burette) until the solution in the conical flask changed from colourless to pink. 7. Repeat the titration until consistent results (two titres within ±0.10 cm3) are obtained. An alternate method to determine the percentage of calcium carbonate in mussel shells involves measuring the volume of carbon dioxide gas evolved when an excess of hydrochloric acid is added. Draw a 6nlabel diagram of the apparatus (set-­‐up) that could be used to collect and measure, as accurately as possible, the volume of carbon dioxide gas produced. [2] divided flask / thistle funnel graduated gas syringe / collected over water saturated with CO2(g) Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 6 Stoichiometry (Acid-­‐base reaction) 3
Weak organic acids such as tartaric acid, malic acid, lactic acid, acetic acid and citric acid, are significant constituents in determining the sensory properties of wines and beverages. Soft drinks often contain varying quantities of several acids, which give sodas their tart flavour. Acids can be classified as monobasic, dibasic, or tribasic, depending on how many protons donated to the base during neutralization. In sodas such as Sprite and 7-­‐Up, the acids are carbonic acid (from carbonated water) and citric acid. Citric acid, C6H8O7, reacts with NaOH(aq) in one of the following ways: C6H8O7(aq) + NaOH(aq) → NaC6H7O7(aq) + H2O(l) C6H8O7(aq) + 2NaOH(aq) → Na2C6H6O7(aq) + 2H2O(l) C6H8O7(aq) + 3NaOH(aq) → Na3C6H5O7(aq) + 3H2O(l) You are to plan a titration to determine which of the above equations correctly represents the neutralisation between citric acid and NaOH. You are provided with a can (330 cm3) of 7-­‐Up which has been opened so that it is decarbonated and you can assume that the only acid present is citric acid and its concentration is 0.0420 mol dm-­‐3, Solid NaOH; Deionised water You are also provided with the standard apparatus present in the laboratory. (a) A standard aqueous NaOH solution may be prepared using solid NaOH so that it can be titrated with citric acid. Suggest an appropriate concentration of the standard aqueous NaOH solution to be prepared. Show your working. [You are to assume that the pipette provided has a capacity of 25.0 cm3 and that 10.00 cm3 ≤ titre values ≤ 35.00 cm3] [2] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 7 For minimum conc, c = (25 x 0.042) x 3/35 = 0.09 mol dm-­‐3 For maximum conc, c= (25 x 0.042 )/10= 0.105 mol dm-­‐3 Any answers between 0.09 mol dm-­‐3 and 0.105 mol dm-­‐3 is acceptable Any appropriate workings (b) Outline, step by step, how you would prepare, in a graduated flask, 250 cm3 of an aqueous solution of NaOH with exactly the concentration you have given in (a). [Ar: Na,23.0; H,1.0; O,16.0] [4] Assume student’s concentration value is 0.09 mol dm-­‐3 no of moles of NaOH in 250 cm3 = 250
× 0.09 = 0.0225 1000
Mass of NaOH in 250 cm3 = 0.0225 x [23.0+ 16.0 + 1.0] = 0.900 g 1. Weigh out 0.900 g of the NaOH in a weighing bottle using a weighing balance. 2. Dissolve the NaOH in about 100 cm3 deionised water in a 250 cm3 beaker. 3. Using a filter funnel, transfer the solution into a 250 cm3 graduated flask . 4. Rinse the beaker,glass rod and filter funnel with deionised water and add washings into the graduated flask OR (NaOH dissolved in less than 250 cm3 of distilled water in beaker and transferred to flask along with rinsings.) 5. Make up to 250 cm3 mark using deionised water. 6. Stopper and shake well to get a homogenous solution. (c) Draw a labelled diagram of the apparatus you would use to titrate the citric acid in 7-­‐Up with the standard aqueous NaOH solution which you have prepared. The following details should be included in your labels: - the volume of citric acid used - name and capacity of apparatus used. [1] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 8 50 cm3 burette
containing NaOH
250 cm 3 conical flask containing 25.0 cm3 citric acid
(d) Suggest a suitable indicator to be used during this titration and state the colour change observed at end-­‐point of the titration. [1] Phenolphthalein (colourless to permanent pink if citric acid is in conical flask) OR (pink to colourless if NaOH is in conical flask) (e) Draw up tables with appropriate headings to show the data you would record in (b) and (c). [1] Mass of bottle + NaOH/g Mass of empty weighing bottle/g Mass of NaOH/g Final burette reading/cm3 3
Initial burette reading/cm 3
Volume of NaOH used/cm (f) Assuming that the average titre of NaOH used is y.00 cm3, show how you would process the results to find the correct equation for the neutralisation reaction between citric acid and NaOH. [1] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 9 y
x 0.09 = 9.00 x 10-­‐5 y 1000
25
Number of moles of citric acid in 25.0 cm3 = x 0.042 = 1.05 x 10–3 1000
n( NaOH ) 9.00 x 10 -5 y
Basicity of the citric acid = =
n(acid )
1.05 x 10 3
Number of moles of NaOH used in y.00 cm3 = Knowing the basicity of the citric acid , the correct equation can be determined. (g) If the result shows that citric acid is tribasic and given the following informations: - citric acid is a tertiary alcohol, - citric acid is optically inactive, draw a possible structural formula of citric acid. [1] HOC(CH2CO2H)2COOH also accept HOC(CO2H)2CH2CH2COOH (h) Identify a safety risk in the experiment and explain how you would minimise it when carrying out the experiment. [1] Solid NaOH is corrosive, hence use gloves/lab coat/goggles ACJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 10 Stoichiometry (Acid-­‐base reaction) 4 Commercially prepared aspirin is not 100% pure as it hydrolyses slowly when exposed to moisture in the air. Aspirin which has not been properly sealed may give off a vinegar smell. COOH
OCOCH3 Aspirin, Mr 180 You are to design a titrimetic analysis to determine the purity of a sample of commercial aspirin which has been exposed to air for some time. You are provided with the following about 4.5 g crushed aspirin 0.100 mol dm-­‐3 NaOH 0.100 mol dm-­‐3 HCl any other chemicals and apparatus found commonly in the laboratory The experiment will take place in four stages: Stage 1 Preparation of aspirin solution Stage 2 Titration of a sample of aspirin solution with NaOH Stage 3 Hydrolysis of titrated solution from Stage 2 by warming with excess base Stage 4 Titration of resultant solution from Stage 3 with HCl (a) Explain why it is not advisable to simply use the titration results from Stage 2 to determine the purity of the aspirin tablet. [1] Since some hydrolysis has already taken place, ethanoic acid produced will also react with NaOH during titration. Hence volume of NaOH titrated will be higher than expected. 1 mark for both points Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 11 (b) Name a suitable indicator for the experiment and state the colour changes seen at both end-­‐points in Stage 2 and Stage 4. [2] Phenolphthalein; [1] Stage 2: colourless to pink Stage 4: pink to colourless [1 for both points] (c)(i) Briefly outline, in a step by step procedure, how you would prepare the aspirin solution in Stage 1 and use it to perform the titration in Stage 2. [8] In your answer, include the following: a. a suitable solvent of your choice, b. any measurements you would take, c. any suitable lab apparatus used and d. appropriate volumes or masses of reagents used. 1. With an electronic balance, weigh a clean and dry beaker. 2. Weigh 4.5 g of aspirin and place in the beaker. 3. Dissolve the aspirin in ethanol and transfer to a 250 cm3 graduated flask with a funnel. 4. Rinse the beaker and pour the washings into the flask. Repeat this at least twice. 5. Mix thoroughly by inverting and shaking. 6. Pipette 25.0 cm3 of the solution into a conical flask. 7. Place 2-­‐3 drops of phenolphthalein into the conical flask. 8. From a burette, slowly add NaOH until the mixture turns a permanent pale pink. 9. Repeat steps 6 – 8, as necessary, to achieve consistent results. - Solvent is a polar organic solvent e.g. alcohol [1]. Other alternatives like benzene can be accepted. - Correct procedure (Step 1, 2, 3, 4, 5) and apparatus for Stage 1 [1] - Correct procedure (Step 6, 7, 8, 9) and apparatus for Stage 2 [1] - Appropriate masses and volumes used [1] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 12 (d) (ii) Deduce the volume of NaOH that should be added in Stage 3, based on your proposed procedure from I(i). Minimum volume of NaOH required to hydrolyse aspirin sample = (25.0/250) × (4.5/180) / 0.100 = 25.00 cm3 Volume of NaOH used = 25.00 + 10.00 cm3 [1] (iii) By using volumes which you proposed in I(i) and (ii), show how the percentage purity of the commercial aspirin sample can be calculated from your titration results in Stage 4. Let vol of HCl titrated be x cm3 and mass of aspirin weighed be msample. Volume of NaOH in excess = x cm3 Volume of NaOH reacted with aspirin = (35.00 -­‐ x) cm 3 Amt of aspirin present in 25.0 cm3 = (35.00 – x) (0.100) mol [1] Amt of aspirin present in 250 cm3 = (35.00 – x) (0.100) (250 / 25.0) mol [1] Mass of aspirin, maspirin = (35.00 – x) (0.100) (250 / 25.0) (180) g Purity of aspirin = 100 maspirin / msample [1] The NaOH solution provided was not standardised before the experiment. Explain clearly the impact of this error on the calculated results. [1] NaOH solution can absorb CO2 if exposed to air. Actual concentration is lower than the stated value, hence calculated purity is higher than actual. NYJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 13 Stoichiometry (Gravimetric Analysis) CJC 2010 5 Magnesium sulfate, MgSO4, does not decompose on heating with a Bunsen burner while magnesium carbonate, MgCO3, decomposes on heating. MgCO3(s) → MgO (s) + CO2 (g) During a practical lesson, a student accidentally added some anhydrous magnesium sulfate into the only bottle of magnesium carbonate available in the laboratory. You are to design an experiment to determine the percentage by mass of MgCO3 in that contaminated sample of magnesium carbonate by heating and weighing alone. The only apparatus available consists of a boiling-­‐tube and holder, a heat-­‐proof mat, a chemical balance and a Bunsen burner. (a)
Outline, step by step, the practical sequence for the method you would use to • make appropriate weighings, • decompose MgCO3 in the sample by heating, • ensure that decomposition was complete. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 14 (b)
Show how you would tabulate your results. Include in your table any other masses you would calculate from the experimental results to enable you to determine the percentage by mass of MgCO3 in the contaminated sample. Insert in your table, the letters A, B, C etc. to represent each measurement of mass. (c) Use the letters you have entered in (b) to show how you would process the results to find (i) the mass of MgCO3 in the contaminated sample. [Ar: C, 12.0; O, 16.0; Mg, 24.3; S, 32.1] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 15 (ii) the percentage by mass of MgCO3 in the contaminated sample. [4] (d) Suggest an alternative experimental method, not involving weighing, which could be used to determine the percentage by mass of MgCO3 in the contaminated sample. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 16 Stoichiometry (Gravimetric Analysis) 6 The formula of basic magnesium carbonate is given as 3MgCO3.Mg(OH)2.3H2O On strong heating, basic magnesium carbonate decomposes to give magnesium oxide, carbon dioxide and water vapour. (a) Write a balanced equation for the decomposition of basic magnesium carbonate when heated strongly. (b) Plan an experiment that will enable you to confirm that the formula of the basic carbonate is consistent with the equation that you have written in (a), including how you would tabulate your results. All necessary measurements should be shown in the table (s). (c)
Use “specimen results” (numerical or algebraic) to show how the results obtained could be processed to confirm that the formula for the basic carbonate and the equation written in (a) is consistent. (d) Suggest another reaction of basic magnesium carbonate that you could use to confirm the formula is consistent with 3MgCO3.Mg(OH)2.3H2O. Ans: (a) 3MgCO3.Mg(OH)2.3H2O (s) ⎯ → 4MgO (s) + 3CO2 (g) + 4H2O (g) (b) Justification -­‐ Use 3 to 5 g of solid -­‐ Too much may result in reaction not proceeding to completion -­‐ Too little may result in large apparatus error (c)
€
uncertainty
% error =
x 100 measurement made
!!
Method -­‐ weigh an empty boiling tube accurately -­‐ Add sensible mass (3 to 5 g) of basic magnesium carbonate and weigh tube again -­‐ Heat gently first and then more strongly to drive off all the water of crystalisation from the salt. -­‐ Stop when the mass of tube and content becomes constant. -­‐ Cool the tube on a heat proof mat. -­‐ Repeat cycle Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 17 Mass of tube and basic salt Mass of empty tube Mass of basic salt Mass of tube and content after heating Mass of MgO (d)
€
/ g / g / g / g / g / g / g A B A – B C C1 C1 C1 – B Amount of basic salt (A − B) /Mr of basic salt (A − B) / 365.2 1
=
=
= Amount
of
MgO
(C
−
B)
/Mr
of
MgO
(C
−
B)
/
40.3
4
1
1
!!
React with dilute HCl acid. Measure the amount of CO2 produced and use molar ratio to compare to confirm formula is consistent. NJC 2010 CT1 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 18 Redox Titration HCI 2008 1 Potassium manganate(VII) is a useful oxidising agent, acidified solutions of which can be used to titrate reducing agents. (i) Describe in outline the practical details you would need to follow in order to use a standard solution of potassium manganate(VII) to measure the percentage purity of a given sample of solid iron(II) ethanedioate, FeC2O4. In your answer, you should include: • an equation for the titration reaction, • an outline of the sequence of steps to carry out the titration, • names of any other chemicals you would use. Do not include any details of the calculation. [3 mol of MnO4 ≡ 5 mol of FeC2O4] (ii) – When a 0.600 g sample of an impure iron(II) ethanedioate solid is dissolved in −3 3 acid and titrated with 0.100 mol dm KmnO4, 20.50 cm of oxidant solution is required to reach the end point. Calculate the percentage purity of the sample. Answer (i) Solid dissolved in dilute H2SO4 Withdraw a sample of the solution Titrate with the KmnO solution from a burette until the solution (in conical flask) turns from yellow to orange Add in a catalyst (eg, MnSO ) or heat the solution, then start the titration Repeat titration, noting the volume of KmnO4 each time, and stop when values are consistent (to within ±0.10 cm ) Equation: 24H + 3MnO4 + 5FeC2O4 → 3Mn + 5Fe + 10CO2 + 12H2O (ii) No. of moles of KmnO4 = (20.50/1000) x 0.1 = 2.05 x 10-­‐3 mol 4 4
3
+ – 2+ 3+ No. of moles of FeC2O4 = (2.05 x 10-­‐3) x (5/3) = 3.24 x 10-­‐3 mol Mass of FeC2O4 in sample = 0.491 g ∴ percentage purity = 81.8 % Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 19 Redox Titration 2 Some iron tablets, containing iron (II) sulfate, were left on the shelf for too long such that some of the iron (II) sulfate was oxidised into iron (III) sulfate. You are to carry out a titrimetric analysis to determine the percentage by mass of Fe3+ ions present in the sample of oxidised iron tablets. The following reagents are provided to enable the experiment to be carried out: - Sample of oxidised iron tablets -­‐3
- 1.00 mol dm sulfuric acid, H2SO4 -­‐3
- 0.0110 mol dm potassium dichromate (VI), K2Cr2O7 - N-­‐phenylanthranilic acid as indicator (1 cm3 of indicator produces a violet colour change at end-­‐point) - Zinc powder (a) The oxidised iron tablets must first be dissolved in water to liberate the Fe2+ and Fe3+ ions. A suitable quantity of sulfuric acid is then added to the solution. Suggest a reason why sulfuric acid is used in the preparation of the mixture of Fe2+ and Fe3+ solution. [1] Fe2+ is oxidised in an acidic medium OR To prevent oxidation of Fe2+ to Fe3+ (b) Write the ionic equation for the reaction between Fe2+ ions and potassium dichromate (VI) solution. [1] 6Fe2+ + Cr2O72-­‐ + 14H+ à 6Fe3+ + 2Cr3+ + 7H2O (c) An unoxidised sample of iron tablet was dissolved and made up to 250 cm3. 25.0 cm3 of the Fe2+ ions solution required 15.00 cm3 of potassium dichromate (VI) to reach an end-­‐point in a titration. Calculate the mass of FeSO4 in the unoxidised sample of iron tablet that was used. [2] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 20 No. of mol of Cr2O72-­‐ used = 0.0110 ×
15.00
1000
= 1.65 × 10-­‐4 6Fe2+ ≡ Cr2O72-­‐ No. of mol of Fe2+ present in 25.0 cm3 = 6 × 1.65 × 10-­‐4 = 9.91 × 10-­‐4 No. of mol of Fe2+ present in 250 cm3 = 9.91× 10−4 ×
= 9.91 × 10-­‐3 250
25
Mass of iron tablet (which contains FeSO4) = 9.91 × 10-­‐3 × (55.8+32.1+4(16.0)) = 1.51 g (d) A sample of oxidised iron tablet is equally divided and dissolved to give two portions of 250 cm3 acidified Fe2+ and Fe3+ ions solutions for analysis. Procedure 1 To one portion, the Fe3+ (aq) should be converted into Fe2+ (aq) first using excess zinc metal, before titration against potassium dichromate (VI) solution. Procedure 2 To the other portion, the Fe2+ (aq) can be directly titrated against potassium dichromate (VI) solution. List, in the order in which they will be made, the steps of how Procedure 1 can be carried out. Your plan should include any quantity or measurements required, as well as how the end of any reaction which occurs may be determined. You have the use of common apparatus that is available in a college laboratory. [5] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 21 1. Add Zn powder to the beaker containing the 250 cm3 solution of Fe2+/Fe3+ ions with stirring until no more Zn dissolves. 2. Filter the mixture using a filter funnel and collect the filtrate containing Fe2+ ions. 3. Pipette 25.0 cm3 of the Fe2+ solution (filtrate) into a conical flask and to it, add 1 cm3 of N-­‐phenylanthranilic acid using a 10 cm3 measuring cylinder / graduated dropper. 4. Fill a burette with the given standard K2Cr2O7 solution and note the initial reading on the burette 5. Titrate the Fe2+ solution in the conical flask until the solution turns (from pale green to yellow to) violet; note the final reading on the burette. 6. Repeat the titration (steps 3 – 5) for consistent results (ie. Titre values are within ± 0.10 cm3 of each other). Marking points Procedure should include appropriate apparatus commonly found in a college laboratory: 2+
3+
- Addition of excess Zn powder into Fe /Fe solution until no more Zn dissolves - Filtration and collection of filtrate - Proper preparation of aliquot - Use of correct volume of indicator added - Titration of aliquot against standard K2Cr2O7 with end-­‐point colour change to violet noted - Initial and final burette readings noted with repeat of titration for consistent results (e) Using the following information, calculate the percentage by mass of Fe3+ ions in the oxidised iron tablet sample. No. of mol of Fe2+ (in 250 cm3) from Procedure 1 = p No. of mol of Fe2+ (in 250 cm3) from Procedure 2 = q Ar: O, 16.0; S, 32.1; Fe, 55.8 [2] No. of mol of Fe3+ present in 250 cm3 = p – q No. of mol of Fe3+ present in sample = 2 × (p – q) Mass of Fe3+ in actual sample = 2(p – q) × 55.8 Mass of oxidised iron tablet (containing FeSO4 + Fe2(SO4)3) = (2q × 151.9) + [½ × 2(p – q) × 399.9] = 399.9p – 96.1q Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 22 Percentage by mass of iron(III) in sample 111.6(p - q)
×100% 399.9p - 96.1q
116.1(p - q)
= % 4.2p - q
= (f) By modifying Procedure 1, suggest another means by which the amount of Fe3+ in an oxidized iron tablet sample can be determined. [1] Add a known mass of excess Zn powder to the solution containing Fe2+/Fe3+ ions. Filter the resultant solution, recover and dry the residue containing the unreacted Zn powder. The difference in mass of Zn can be used to calculate the no. of mol of Fe3+ present. Alternative test: Iodometric titration (Fe3+ / I-­‐) MJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 23 Energetics AJC 2010 CT1 1 Cyclohexa-­‐1,3-­‐diene and cyclohexa-­‐1,4-­‐diene (both Mr = 80.0) are highly flammable cycloalkenes that occur as colourless clear liquids. Both are potential fuels that can be used by burning them in air. (a) Define the term standard enthalpy change of combustion. (b)(i) Using the following set-­‐up, give a brief outline of the procedure you would follow in order to determine the standard enthalpy change of combustion of cyclohexa-­‐
1,3-­‐diene or cyclohexa-­‐1,4-­‐diene. Your answer must make clear references to the amount of substances used and the measurements you would take during the experiment. (ii) (iii) Outline how you would use the experimental results to determine the enthalpy change of combustion of cyclohexa-­‐1,3-­‐diene or cyclohexa-­‐1,4-­‐diene. You may assume that the specific heat capacity of water is 4.18 J g-­‐1 K-­‐1. Other than heat loss to the surroundings, state another assumption you have made in your calculations and discuss how it will affect the standard enthalpy change of combustion calculated. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 24 Answer (a) Standard enthalpy change of combustion is the hest evolved when 1 mole of substance in its standard state is completely burned in oxygen under standard conditions. (b)(i) 1. Measure 100 cm3 of water into a calorimeter (with a 100cm3 measuring cylinder). 2. Record the initial temperature of the water, T1, (using a thermometer). 3. Fill the spirit lamp with cyclohexa-­‐1,3-­‐diene (or cyclohexa-­‐1,4-­‐diene) and weigh it. Record the mass as m1. 4. Heat the water until a temperature rise of about 10oC has been obtained. 5. Extinguish the flame but continue stirring. Record the maximum temperature, T2, reached. 6. Allow the apparatus to cool and reweigh the lamp and its content. Record the mass as m2. (b)(ii) n(cyclohexa-­‐1,3-­‐diene) burned = (m1 – m2)/80.0 mol Heat absorbed by the water = 100 x 4.18 x (T2 – T1) J heat absorbed by water
ΔHc = −
mol−1 n(cyclohexa− 1,3 − diene)
!!
(b)(iii) €
1. Assume that the calorimeter has negligible heat capacity. ΔHc calculated will be numerically lower / less exothermic than expected (as some heat is lost to the calorimeter) 2. Assume complete combustion of the hydrocarbon has taken place. ΔHc calculated will be numerically lower / less exothermic than expected (as some heat will be given off) Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 25 Energetics 2 The equations for the complete combustion of the first five members of the alcohol homologous series are shown below. You are to plan an experiment to determine the enthalpy change of combustion, ΔHc, under laboratory conditions, for each of the five alcohols, using the apparatus and information below. (a) Inspection of the products of the five equations shows that from methanol to pentan-­‐1-­‐ol there is one more mole of CO2 and one more mole of H2O produced per mole of alcohol than the alcohol above it. What difference between the structures of adjacent alcohols is responsible for these additional combustion products? [1] The addition of 2 C–H bonds and 1 C–C bond. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 26 (b) Based on your answer in (a), suggest the trend in ∆Hc, of these five alcohols with a sketch on the grid below. [1] Decreasing straight line graph (not necessary to pass through the origin) (c) Using the apparatus in the diagram an experiment to determine the enthalpy change of combustion of any particular alcohol can be carried out. The independent variable in the experiment is the mass of the alcohol used during combustion. [2] Identify the dependent variable in this experiment. Dependent variable: temperature rise Identify one variable to be controlled in this experiment. Controlled variable: - mass of water used - distance of flame from the calorimeter Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 27 (d) You are to plan a step-­‐by-­‐step method to determine the enthalpy change of combustion, ∆Hc, for each of the alcohols in turn. You should use the apparatus shown in the diagram on page 2 and any other apparatus required that is normally found in a school laboratory. Your method should cover the following points: - the precision of the apparatus to be used - the way in which the independent variable is varied between experiments - measurements of the dependent variable - appropriate quantities to be used - the raw data to be collected - how this data is to be processed for ethanol Given: 3
o
- 4.2 J of heat is needed to raise the temperature of 1 cm (1 g) of water by 1 C −1 −1
- Specific heat capacity of copper = 0.385 J g K Method [4] - the precision of the apparatus to be used (mention use of measuring cylinder or burette; thermometer precision and precision of balance). - the way in which the independent variable is varied between experiments (change sample of alcohol and weighing of alcohol). - measurements of the dependent variable (initial and final temperature of water). - appropriate quantities to be used (volume of water and volume of alcohol). - Correct general approach (weighing of alcohols and taking temperature rise of water in calorimeter). Processing of raw data for ethanol [Mr: CH3CH2OH, 46.0] [2] Mass of calorimeter = m1 g Mass of spirit lamp before combustion = m2 g Mass of spirit lamp after combustion = m3 g Mass of ethanol used for combustion = (m2−m3) g Initial temperature of water = t1 oC Final temperature of water = t2 oC Qty of heat produced in combustion = [50 × 4.2 × (t2 – t1) + m1 × 0.385 × (t2 – t1)] J Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 28 ∆Hc = (e) Prepare a single table to show, for each alcohol, all the measurements obtained directly from the experiment and the information processed from these data. Your table should show the correct units for each of the headings. [1] Mass of calorimeter = m1 g Mass of spirit Mass of spirit Initial Final lamp before lamp after temp. of temp. of ∆Hc Alcohol combustion combustion water water (J mol−1) m2(g) m3(g) t1(oC) t2(oC) methanol ethanol propan-­‐1-­‐ol butan-­‐1-­‐ol pentan-­‐1-­‐ol (f) (i) Identify and assess a risk associated with this experiment. [1] - Alcohol is volatile and toxic. - Alcohol is flammable and may cause a fire if ignited accidentally. (ii) Describe how the risk in (f)(i) can be kept to a minimum. [1] - Containers of alcohols should be covered when not in use; proper disposal of alcohols. - Start the fire to burn the wick away from other alcohols. HCI Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 29 Energetics 3 FA1 and FA2 could be 1.00 mol dm–3 H2SO4 or 1.00 mol dm–3 CH3CO2H. You are required to plan a simple experiment to identify FA1 and FA2. You are provided with 100 cm3 FA1 100 cm3 FA2 1.00 mol dm–3 NaOH(aq) a thermometer polystyrene cup common apparatus in the laboratory No pH indicators are provided. (a) Write down the full equation, including state symbols, for the reaction between
[1] (i) sulfuric acid and aqueous sodium hydroxide, 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l) (ii) ethanoic acid and aqueous sodium hydroxide. NaOH(aq) + CH3CO2H(l) → CH3CO2–Na+(aq) + H2O(l) (b) Outline the steps you will take to determine the identities of FA1 and FA2. [3] Step 1: Using a measuring cylinder, add 30 cm3 of FA1 into the polystyrene cup provided and record its initial temperature. Using another measuring cylinder, measure 60 cm3 of 1.00 mol dm–3 NaOH and record its initial temperature. Step 2: Add the 60 cm3 of 1.00 mol dm–3 NaOH into the polystyrene cup containing FA1, stir with the thermometer and record the highest temperature reached. Rinse the cup. Step 3: Repeat steps 1 and 2, this time replacing FA1 with FA2. [Accept any specific volume suggested by student.] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 30 (c)
(d)
(e)
Let ΔT1 be the temperature change for the reaction between FA1 and NaOH(aq) and ΔT2 be the temperature change for the reaction between FA2 and NaOH(aq). Illustrate how the identities of FA1 and FA2 can be deduced from the temperature changes obtained in the procedure above. [4] No of moles of NaOH(aq) used = 1.0 x 60/1000 = 0.060 mol For 1.00 mol dm–3 CH3CO2H, no of moles of H2O formed = 1.00 x 30/1000 = 0.030 mol For 1.00 mol dm–3 H2SO4, no of moles of H2O formed = 2 x 1.00 x 30/1000 = 0.060 mol Since number of moles of water formed in the two reactions is in the ratio of 1:2, temperature change for the two reactions is expected to be around 1:2. If Δ T2 = 2ΔT1, FA2 is H2SO4 and FA1 is CH3CO2H. [Other methods are acceptable.] [4 m; award according to the method used] Do you expect the enthalpy change of reaction per mole of acid to be more or less exothermic if CHCl2CO2H is used instead of CH3CO2H? Explain your answer. [2] Due to electron withdrawing inductive effect of chlorine, the negative charge on CHCl2CO2– is more dispersed than that of CH3CO2–, resulting in greater stability for CHCl2CO2–. CHCl2CO2H is a stronger acid than CH3CO2H and the enthalpy change of reaction per mole of CHCl2CO2H is more exothermic than for CH3CO2H. FA1 and FA2 can also be determined by titration with aqueous sodium hydroxide separately. State an indicator that could be used for each titration. [1] (f)
Titration between H2SO4 and NaOH: phenolphthalein or methyl orange Titration between CH3CO2H and NaOH: phenolphthalein Explain why the titration method is not able to distinguish between CHCl2CO2H and CH3CO2H? [1] Both acids are monobasic. Thus, the volume of NaOH(aq) required is the same for both acids. VJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 31 Energetics 4 Ammonium salts are commonly used in instant cold packs. The cold pack contains water, and in the water is another pouch containing the ammonium salt. When the pack is squeezed, this inner pouch is broken, releasing the salt, which quickly dissolves and lowers the pack’s temperature. To determine whether ammonium chloride or ammonium nitrate is more effective as the ingredient in the cold pack, a student decided to conduct an experiment to find out the enthalpy change of solution of the two salts, starting with ammonium chloride. He added ammonium chloride to water and found the temperature change by plotting a suitable graph to correct for surrounding heat transfer. Solubility of ammonium chloride at 25 oC = 6.95 mol dm–3 4.3 J are required to raise the temperature of 1.0 cm3 of any solution by 1°C. In a preliminary investigation, the enthalpy change of solution of ammonium chloride was found to be approximately +15 kJ mol–1. Draw a labelled diagram to show the apparatus you could use in a school laboratory to carry out the experiment. By considering the apparatus chosen in your diagram, state a volume of water you could use in the experiment 100 cm3 (accept 10 cm3 to 200 cm3) Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 32 Calculate the maximum mass of ammonium chloride that can be added to the volume of water stated in (b)(i). The maximum mass that can dissolve in 100 cm3 of water = (6.95 / 1000) x 100 x 53.5 = 37.2 g Suggest a minimum mass of ammonium chloride that could be used. Justify your choice with relevant calculations, stating any assumptions you made. Assuming a temperature change of 5 °C to be measured and no heat loss to surroundings, nsalt x 15 000 = 100 x 4.3 x 5 ⇒ nsalt = 0.143 mol minimum mass to use = 0.143 x 53.5 = 7.65 g Describe the procedure to find the enthalpy change of solution of ammonium chloride. In your plan, you should: • allow for the plotting of a suitable graph • include appropriate apparatus, masses and volumes of reagents, using your answers in parts (a), (b) and (c), and • draw table(s) with headings to show the measurements you would make. 1. Weigh accurately 8.00 g of ammonium chloride in a weighing bottle, using a weighing balance. 2. Using a 100 cm3 measuring cylinder, add 100 cm3 of water into a dry styrofoam cup calorimeter. Support the styrofoam cup on a 250 cm3 beaker. 3. Stir the water gently using the thermometer. 4. Start the stopwatch. 5. Record the temperature of the water in the styrofoam cup using a 0.1 °C thermometer, at 30 s intervals (accept 1 min) until 2.5 min. 6. At exactly 3 min (accept 1–5 min), tip the ammonium chloride into the water. Do not measure the temperature at this time. 7. Stir the solution gently and record the temperature of the solution at 3.5 min. Continue to stir and record the temperature at 30 s intervals until solution returns to room temperature. 8. Reweigh the weighing bottle. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 33 Table with headings: Mass of weighing bottle and salt / g m1 Mass of weighing bottle after experiment / g m2 Time / min Temperature of solution / °C 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 s Draw a sketch of the graph that you expect to obtain in the experiment. Indicate clearly on the graph the initial and final temperatures that you would read. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 34 In the student’s experiment, he dissolved m g of ammonium chloride in v cm3 of water, and obtained a temperature change of T oC. Outline how you would use his results to calculate a value for the enthalpy change of solution of ammonium chloride. ∆Hsolution (in J mol-­‐1) = v x 4.3 x T
m
53.5
The student repeats the same procedure for ammonium nitrate. How might he conclude which of the two salts is more effective as the ingredient in the cold pack? The salt with the more endothermic enthalpy change of solution will be the more effective ingredient in the cold pack. OR The salt that gives the fastest drop in temperature will be the more effective ingredient. HCI P3 Prelim 2011 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 35 Reaction Kinetics PJC 2010 1 In order to study the effect of concentration changes on the rate of a reaction, an experiment was carried out using magnesium ribbon of mass 0.121 g placed in a conical flask with 15.0 cm3 of 0.500 moldm-­‐3 hydrochloric acid. (a) (i) Show that the acid is the limiting reagent. (1m) (ii) Calculate the maximum volume of hydrogen gas produced. (1m) (b) The process of this reaction is followed by using the gas collection method. Draw the experimental set-­‐up and describe how you would collect the data required to find the order of reaction with respect to hydrochloric acid. Prepare a table, showing all the data you would record, including columns for processing the data recorded. (6m) (c) A textbook lists the order of the reaction with respect to H+ to be first order. How would you use the data collected to confirm the order of the reaction? Show clearly your reasoning. (3m) (d) State and explain one safety precaution you would take when carrying out this experiment. (1m) Answer (a)(i) Amount of Mg = 0.121/24.3 = 0.00498 mol Amount of HCl = 15.0/1000 x 0.500 = 0.00750 mol Only 0.00750/2 = 0.00375 mol of Mg will be reacted, Mg is excess. (OR 0.00498 mol of Mg will require 0.00498 x 2 = 0.00996 mol of HCl.) HCl is the limiting reagent. (ii) Maximum volume of H2 = 0.00750 / 2 x 24000 = 90 cm3 (b) Procedures 1. Use a sand paper to clean the Mg ribbon. On the weighing balance, measure 0.121 g of Mg ribbon. 2. Using a 25 cm3 (or 50 cm3) measuring cylinder, measure 15 cm3 of 0.500 moldm-­‐3 hydrochloric acid. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 36 3. Pour it into a 50 cm3 (or 100 cm3) conical flask. Setup the apparatus as shown below. Read the initial volume of the syringe. (Accept inverted 100 cm3 measuring cylinder to collect gas, but reject burette.) 4. Drop the magnesium ribbon into the conical flask, start the stopwatch and stopper it immediately. (Swirl the conical flask gently throughout the reaction.) 5. Read the volume of gas collected in the syringe every 30 seconds in the following table until at least 67.5 cm3 of gas has been collected. Mark allocations [1m] record initial volume [1m] setup diagram (state size of flask with stopper) [1m] gas syringe (state size) [1m] appropriate time interval (between 10 s to 60 s) and stopwatch [1m] specify minimum volume time, t / s reading / cm3 volume of gas evolved / cm3 0 30 60 90 120 150 [1] tabulation (excluding zero, show at least 5 data sets) (c)
For 1st order reaction, the graph of volume of hydrogen against time is a curve, starting from the origin. Find two half lives (the time for volume of hydrogen to increase from 0 to 45 cm3 , then from 45 to 67.5 cm3 ). If they are constant, order of reaction w.r.t. H+ can be confirmed to be first order. (d) Hydrogen produced is flammable. Ensure that there is no naked flame. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 37 Reaction Kinetics 2 Hydrochloric acid reacts with sodium thiosulphate(VI) to produce sulphur. You are required to determine the order of reaction with respect to sodium thiosulphate(VI) and hydrochloric acid by using the initial rate method using the table below. Fill in the table for the 3rd experiment. 2H+(aq) + S2O32 (aq) → S(s) + SO2(g) + H2O(l) Apparatus: Stopwatch printed material 100 cm3 measuring cylinder (3) 250 cm3 beaker Chemicals: FA 1 0.4 mol dm–3 aqueous sodium thiosulphate(VI) FA 2 2.0 mol dm–3 aqueous hydrochloric acid −
Volume of FA 1 / cm3 Volume of FA 2 / cm3 Volume of water / cm3 Time / s 40 30 0 20 30 20 Procedure 1.
Measure 40 cm3 of FA 1 into one of the measuring cylinders provided. Pour the 40 cm3 of FA 1 into the beaker and stand the beaker over the printed material. 2.
Measure 30 cm3 of FA 2 into the other measuring cylinder provided. 3.
Pour the 30 cm3 of FA 2 into the beaker containing FA 1, simultaneously starting the stopwatch. Note the time taken to the nearest second for the printed material within the circle to become obscured by the sulphur produced in the reaction. (Discard the mixture and wash the beaker immediately.) 4.
To be continued with your plan. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 38 (a)
Volume of FA 1 / cm3 Volume of FA 2 / cm3 Volume of water / cm3 Time / s 40 30 0 20 30 20 40 15 15 Why is the volume of water varied in each experiment? So that total volume of the reaction mixture remains constant for the different sets of experiment, hence volume of reagent used is proportional to its concentration in the reaction mixture. (b) What is the relationship between the volume of sodium thiosulphate(VI) and its concentration? The volume of sodium thiosulphate(VI) used is directly proportional to its concentration since the total volume of reaction mixture is kept constant. (c) What is the relationship between the rate of reaction and the time taken for the printed material to be obscured? Rate of reaction is inversely proportional to the time taken for the printed material to be obscured since the amount of sulphur produced in every set of experiment is similar. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 39 Chemical Equilibrium 1 When a solute is added to two solvents, A and B, which do not mix, some of the solute dissolves in each of the solvents and an equilibrium is set up between the two solvents. It has been shown that for dilute solutions, at equilibrium the ratio of the two concentrations is a constant known as the Partition Coefficient, K, and it will remain a constant if the solute remains in the same molecular state in the two solvents. Concentration of solute in solvent A
= K Concentration of solute in solvent B
An experiment was conducted to verify the above observation. Ethanoic acid (solute) was shaken with two immiscible solvents (water and cyclohexane, (C6H12)), so that the solute distributed itself between the two solvents. The concentrations of the two solutions were then determined so that the ratio [CH3COOH]aq/[CH3COOH]S known as the Partition Coefficient, K can be calculated at a given temperature. Using apparatus which are found as standard items in a school laboratory, an experiment was carried out to determine the Partition Coefficient, K for ethanoic acid, CH3COOH, between water and cyclohexane, (C6H12). The following steps were carried out: 1. 5.0 g of the sample was first dissolved in 50 cm3 of water in a beaker and the aqueous solution was then transferred into a separating funnel. 2. 50 cm3 of cyclohexane was then poured into the separating funnel containing the aqueous solution, stoppered and shaken intermittently. 3. The concentration of ethanoic acid in the two layers was then determined by titration with 0.05 mol dm-­‐3 NaOH. (a) (i) Outline how a fixed volume of the sample of the aqueous layer can be removed from the mixture in the separating funnel for titrimetric analysis. (Note: The organic layer is less dense compared with the aqueous layer.) Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 40 Either • Place a finger on the open end of a 10.0 cm3 pipette and insert it into the bottom aqueous layer. • Gently blow lightly into the pipette to dispel any organic layer that may have entered the pipette before withdrawing the required amount of sample Or • Drain the aqueous layer into one dry beaker and the organic layer into another • Pipette 10.0 cm3 of the aqueous layer into a conical flask (ii) Outline stepwise how the removed sample could be analysed by titration to determine the concentration of the solute that had dissolved in that layer. • Place the pipetted sample into a 250 cm3 conical flask • Add a drop of phenolphthalein • Fill a burette with 0.05 mol dm-­‐3 aqueous NaOH • Titrate the sample until a faint pink colour is obtained • Repeat titration for consistent results (b) When analysing the organic layer, a fixed volume of water (about twice the volume) is added to the pipetted volume of cyclohexane solution in the conical flask and shaken vigorously, prior to the titration. Explain the purpose of this action. • NaOH(aq) will not be able to neutralise the ethanoic acid in the organic layer • The ethanoic acid must be first extracted into the water for complete reaction with the alkali during neutralization. (c) Based on your procedure, state and explain one significant error or limitation that could be encountered in determining an accurate value for the distribution ratio. Any one of them: • Short equilibration time for partitioning • Some organic layer could have been sucked up the pipette when collecting aqueous layer for analysis (if student withdrew sample Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 41 directly from separating funnel) • Titre reading for organic layer may be lesser than expected due to limited or partial partitioning of acid into water (d) Suggest one possible modification that would minimise the error or limitation that you have stated in I. Explain how this modification leads to an improvement in the accuracy and reliability of the results. Any one of them: • Allow a longer time for partitioning/distribution of acid between the two solvents • Add/use more water to extract acid quantitatively into aqueous layer before titration or use a fixed volume of standard alkali for extraction and determine solute by back titration with standard acid. (e) A student did the above experiment but carried out only one titration using a fixed aliquot of the aqueous layer and then proceeded to use the titre value to calculate a value for K. Assuming the student obtained a titre value of y cm3 of M moldm-­‐3 NaOH (aq) when 10.0 cm3 of the aqueous layer was titrated, outline how the student could have used the result to determine a value for K. No of moles of CH3COOH in 50 cm3 of aqueous layer = (5My / 1000) mol Thus [CH3COOH]aq = (My / 10) moldm-­‐3 Total No of moles of CH3COOH used = 5 / 60 No of moles of CH3COOH in 50 cm3 of organic layer = (5/60 – 5My/1000) Thus, [CH3COOH]s = (5/60 – 5My/1000) x (1000/50) moldm-­‐3 = [5/3 – My / 10] moldm-­‐3 K = (My / 10) / [5/3 – My / 10] or (0.1My) / (1.67 – 0.1My) (f) When a solute is distributed between the two immiscible solvents, it is important to ensure that the resulting solutions are dilute for the distribution to occur and the partition coefficient to remain constant. Suggest why this is critical in this experiment. Ethanoic acid may dimerise by way of hydrogen bonding in the organic layer when it is concentrated and thus affect the distribution ratio. ACJC P3 Prelim 2011 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 42 Organic Chemistry RJC 2010 1 You are tasked to identify 5 unlabelled bottles each containing the following organic liquids or solutions: • ammonium propanoate, CH3CH2CO2NH4 • pentan-­‐2-­‐one, CH3CH2CH2COCH3 • butan-­‐2-­‐ol, CH3CH2CH(OH)CH3 • benzaldehyde, C6H5CHO • pentanoic acid, CH3CH2CH2CH2CO2H Chemicals provided: • iodine water • aqueous sodium hydroxide • Tollens’ reagent • aqueous sodium carbonate • 2,4-­‐dinitrophenylhydrazine • aqueous hydrochloric acid • Fehling’s reagent • aqueous calcium hydroxide • acidified potassium manganate (VII) (a) Using suitable chemicals provided in the list above, outline a sequence of simple chemical tests to positively identify each of the organic substances. Your plan should include a detailed procedure, indicating clearly the apparatus and quantity of chemicals used for each test as illustrated in test (1) below. Your plan should also state the expected observations and compounds identified at each stage. Procedure for each test Expected Compound observation for identified positive test Test (1) 3
Using a dropper, add 1 cm of each of unknown samples into 5 separate test tubes. Then to each test-­‐tube, add 2 cm3 of Na2CO3(aq) solution dropwise and shake gently. Bubble any gas evolved into freshly prepared Ca(OH)2(aq) using a delivery tube. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 43 Test (2) Procedure for each test Expected observation for positive test Compound identified Test (3) Test (4) Test (5) [9] (b) If one of the labeled bottles contains butanamide instead of ammonium propanoate, suggest how you would modify one of the chemical tests in your plan to identify butanamide. Describe the expected observation clearly. [2]
(c) Many organic compounds are potentially flammable. In the light of this, suggest a safety precaution that should be enforced when a series of experiments is carried out to identify these unknown substances. [1] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 44 Answer Procedure for each test Expected observations Test (1) Effervescence of 3
Using a dropper, add 1 cm of each of CO2(g) which unknown samples into 5 separate test tubes. gives a white 3
Then to each test-­‐tube, add 2 cm of precipitate when Na2CO3(aq) solution dropwise and shake bubbled into gently. Bubble any gas evolved into freshly Ca(OH)2(aq) prepared Ca(OH)2(aq) using a delivery tube. observed. Test (2) 3
Using a dropper, place 1cm of the remaining Silver mirror unknowns in 4 different test tube, add 2 drops observed of freshly prepared Tollens’ reagent and warm the mixture in a water bath. Test (3) 3
Using a dropper, place 1cm of each remaining Orange unknown to 3 different test-­‐tubes, add 2 drops precipitate is of 2,4-­‐dinitrophenylhydrazine at room observed temperature Test (4) 3
Using a dropper, place 1cm of each remaining Yellow unknown to 2 different test-­‐tubes, add 2 drops precipitate is of I2 followed by 2cm3 of NaOH and warm observed Test (5) 3
Using a dropper, place 1cm of each remaining NH3 produced unknown to a test-­‐tube, add NaOH(aq) and turns moist red gently warm the mixture for about one minute litmus turns blue in a water bath. (b) Add NaOH(aq) but heat strongly (or for a longer period of time) NH3 gas produced turns moist red litmus blue Compound identified Pentanoic acid Benzaldehyde Penta-­‐2-­‐one Butan-­‐2-­‐ol Ammonium propanonate (c) Heating should be done in a water bath which is heated using a hot plate. No naked flame (e.g. Bunsen burner) should be used. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 45 Organic Chemistry 2 You are provided with 5 unlabelled bottles containing pure natural acids from the extracts of fruits. Each bottle contains one of the following natural acids. You are also provided with any other common laboratory reagents and apparatus. (a) All natural acids contain one or more carboxylic acid functional groups. Other than the carboxylic acid functional group, what other functional groups are present in these 5 natural acids? [1] quinic acid: maleic acid: pyruvic acid: oxalacetic acid: lactic acid: quinic acid: Secondary alcohol and tertiary alcohol maleic acid: alkene pyruvic acid: ketone oxalacetic acid: ketone lactic acid: secondary alcohol Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 46 (b) Two of these natural acids are colourless liquids and the rest are white crystalline solids. Using relevant chemical knowledge, identify the two liquids. Explain your choices. [2] The two liquids are: pyruvic acid and lactic acid. [1m] These two compounds have a relatively smaller Mr and less extensive hydrogen bonding/weaker intermolecular forces between their respective molecules than the other 3 compounds. [1m] (c) Suggest a reagent that could be used to carry out a test-­‐tube experiment to distinguish these two liquids. [2] Reagent: Describe what would be observed for each compound in the experiment. Observation: Brady’s reagent or 2,4-­‐DNPH Pyruvic acid will give an orange ppt. but lactic acid will not. OR acidified KMnO4(aq) or K2Cr2O7(aq) Lactic acid will turn purple KMnO4 colourless but pyruvic acid will not. (d) Outline a logical sequence of chemical tests that would enable you to identify the remaining 3 solids. Your plan should include - different positive test for each compound; - a detailed procedure (including apparatus, quantities of chemical and conditions used); - the expected observations for each compound in each test. Step 1: Prepare aqueous solutions of each of the 3 solid samples by dissolving 1 cm depth of solid in about 5 cm depth of deionised water. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 47 Step 2: To 1 cm depth of each of the 3 samples in separate test-­‐tubes, add equal volume of Brady’s reagent. Oxalacetic acid will give an orange ppt. while the other 2 samples will not. Step 3: To 1 cm depth of each of the 2 remaining samples in separate test-­‐tubes, add a few drops of Br2(aq). Maleic acid will decolourise brown Br2 while the other will not. Step 4: To 1 cm depth of the last sample in a test-­‐tube, add a few drops of acidified KMnO4(aq). Heat the mixture in a hot water-­‐bath. Quinic acid will turn purple KMnO4 colourless. (e) How would you ensure the reliability of the test result for quinic acid. [1] To ensure colour change of KMnO4, the acidified KMnO4(aq) must be added slowly and dropwise. It should not be added in excess. The mixture must also be heated. OR Hydrochloric acid cannot be used to hydrolyse the ester or to acidify KMnO4 as the chloride ions can be oxidised by KMnO4. As a result, there may be a colour change of KMnO4 due to oxidation of chloride to Cl2. (f) Suggest a safety precaution that you will consider in carrying out your plan. [1] Use a hot water bath for heating instead of using a naked flame directly from bunsen burner as some organic compounds are highly flammable. OR Use a test-­‐tube holder and dropper when adding chemicals to avoid direct contact with chemicals as chemicals may be toxic/poisonous/ harmful/corrosive JJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 48 Organic Chemistry 3 The labels of 4 bottles have fallen off. Each bottle contains one of the following: - butan-­‐2-­‐ol - butanal - butanone (a) [8] (i) Describe the procedures of simple chemical tests by which you could identify each of the organic substances. You are not allowed to identify the substances by elimination. You do not need to identify the aqueous solution of iron(III) ions anymore. Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 49 (ii) For the tests you proposed in (i), state and briefly explain any one issue of reliability or limitations that may be present. - (if oxidation is performed) Butan-­‐2-­‐ol needs to be identified before butanal since both can be oxidised, or vice-­‐versa. - (if tri-­‐iodomethane test is performed) Butan-­‐2-­‐ol needs to be identified before butanone since both give yellow ppt, or vice-­‐versa. - (if 2,4-­‐DNPH is used) Butanal needs to be identified before butanone since both give an orange ppt, or vice-­‐versa. - (if sodium is used) Ensure the unknowns are anhydrous, as water can react with sodium metal to give a false positive test. [any one] (b) State and explain two safety precautions employed in the course to distinguish the four unknowns given. [2] - Compounds like butan-­‐2-­‐ol, butanal and butanone are inflammable / flammable. Ensure no naked flame is used and a water bath is used for heating. - Compound like butan-­‐2-­‐ol, butanal and butanone are volatile and toxic. Perform experiment in a fume cupboard to minimise fumes inhaled. - Compound like butan-­‐2-­‐ol, butanal and butanone is toxic (and corrosive). Use test tube holder, add chemicals using droppers, wear gloves and goggles to avoid direct contact with chemicals. - (if sodium is used) Hydrogen gas is explosive. Ensures absence of naked flame / only used a small piece of sodium metal OR ensure anhydrous condition for sodium. (award only once in (b)(ii) or (c).) [any 2, reject precaution without reason] PJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 50 Organic Chemistry 4
The following describes the preparation of methylamine hydrochloride, CH3NH3Cl from ethanamide, CH3CONH2. The equations involving the chemicals in the preparation are as shown: CH3CONH2 + 4NaOH + Br2 → CH3NH2 + Na2CO3 + 2NaBr + H2O …………. [1] CH3NH2 + HCl → CH3NH3Cl ………….. [2] The procedure in the preparation of methylamine, CH3NH2 is as follows: 1. Dissolve 36 g of sodium hydroxide, NaOH in 160 cm3 water contained in a 500 cm3 conical flask and chilled the stirred solution to 0-­‐5° C in ice water. 2. Add 10.8 cm3 (32.4 g) of bromine to the stirred solution. 3. Add 12 g ethanamide, CH3CONH2 to the stirred bromite(I) solution. 4. Remove the conical flask containing the alkaline solution from the ice water and set it aside at room temperature for 30 minutes. (a) What is the purpose of using ice water? [1] To prevent volatile bromine from vapourising or prevent side reactions (b) What apparatus are used to measure out (i) 36 g NaOH (ii) 160 cm3 (iii) 10.8 cm3 [3] 36g NaOH -­‐ weighing balance 160 cm3 -­‐ measuring cylinder 10.8 cm3 -­‐ burette (c) The boiling point of CH3NH2 is 70° – 80° C. Draw the apparatus to collect CH3NH2 from the reaction mixture. [4] Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 51 (d) The last stage of reaction is to obtain colourless crystals of CH3NH3+Cl−. The procedure is as follows: 1. CH3NH2 is absorbed by hydrochloric acid in a beaker. 2. Transfer the resultant solution to a small evaporating dish and evaporate to dryness on a boiling water bath to obtain the crude dry solid. [4] (i) Why is water bath used instead of direct heating? CH3NH3+Cl-­‐ might decompose over the Bunsen flame (ii) The crude dry solid from d(2) consists of CH3NH3+Cl− with small amount of NH4Cl. Suggest how you could purify CH3NH3+Cl−. Transfer CH3NH3+Cl-­‐ with NH4Cl to a round bottom flask fitted with a condenser. Add about 60-­‐70 cm3 of absolute alcohol (NH4Cl is insoluble). Boil the mixture for 5 min. Pour hot supernatant liquid quickly through a small fluted filter paper into a conical flask. When cool, collect the crystals. YJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 52 Electrochemistry 1(a) Describe how you could determine a value for the Avogadro constant by electrolysis. You should describe the apparatus and chemicals that you would use, the measurements you would make, any other data you need to know, and the calculations you would carry out. (d) Calculate the ratio of the mass of silver to the mass of nickel deposited on the cathodes when the same current is passed through electrolytic cells containing aqueous silver(l) nitrate and aqueous nickel(ll) sulphate, connected in series. Answer (a) The apparatus is set up as shown using two pieces of Cu electrodes of known mass, immersed in an aqueous solution of CuSO4. A steady current I (maintained by adjusting the rheostat and read from the ammeter) is passed through for t seconds. The two Cu electrodes are then dried and reweighed. Let the gain in mass of cathode = m g Quantity of electricity passed = It C Amount of Cu deposited by It C = m/63.5 mol Quantity of electricity required to deposit 1 mol of Cu = (63.5/m) x It C Cu2+ (aq) + 2e → Cu (s) Since 2 mol of e are required to deposit 1 mol of Cu, Charge on 2 mol of e = (63.5/m) x It C 2 x L x 1.60 x 10-­‐19 = (63.5/m) x It C L = 1.98 x 1020 x (It/m) mol-­‐1 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 53 (b) Ag+ (aq) + e → Ag (s) Ni2+ (aq) + 2e → Ni (s) For the same current passed, Mole ratio of Ag : Ni deposited = 2 : 1 Mass of Ag : Ni deposited = (2 x 108) : (1 x 58.7) = 216 : 58.7 = 3.68 : 1 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 54 Qualitative Analysis 1
Labels have come off three bottles each containing an aqueous solution of the following chemicals: aluminium nitrate lead nitrate zinc nitrate You are provided with the reagents NaOH(aq), NH3(aq), Na2CO3(aq), dil H2SO4 and dilute HCl. Using only the above reagents, but not necessarily all of them, you are to plan a method to identify each of the solutions. Maximum credit will be given for the minimum number of tests to positively identify each of the solutions. Marks will be deducted for additional or unnecessary tests. Record your expected results in the table below. [4] Expected observation with each Experiment solution tested and deductions from these observations 1 2 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 55 Experiment 1 Add dil HCl or dil H2SO4 to separate samples of the three solutions. 2 Add NH3(aq) to the two solutions where no precipitate is formed with the first reagent. OR Add NH3(aq) to separate samples of the three solutions. 1 2 1 2 Add dil HCl or dil H2SO4 to the two solutions where the ppt formed was insoluble in excess NH3(aq). OR Add Na2CO3 to separate samples of the three solutions. Add NH3(aq) to the two solutions where no effervescence was seen with the first reagent. Expected observation with each solution tested and deductions from these observations Pb(NO3)2 : White precipitate Al(NO3)3: No precipitate Zn(NO3)2: No precipitate (No change/no reaction is acceptable) Zn(NO3)2: White precipitate soluble in excess ammonia Al(NO3)3: White precipitate insoluble in excess ammonia Zn(NO3)2: White precipitate. Soluble in excess ammonia. Pb(NO3)2 : White precipitate. Insoluble in excess ammonia. Al(NO3)3: White precipitate. Insoluble in excess ammonia. Pb(NO3)2: White precipitate Al(NO3)3: No precipitate (No change/no reaction is acceptable) Al(NO3)3: White precipitate. Effervescence or CO2 or gas which forms white ppt with lime water. Zn(NO3)2: White precipitate. Pb(NO3)2 : White precipitate. Pb(NO3)2 : White precipitate. Insoluble in excess ammonia. Zn(NO3)2: White precipitate. Soluble in excess ammonia. TJC Prelim 2010 Compiled by Twig Learning Center Pte Ltd (http://www.twig.com.sg) 56