CHEM163
School of Chemistry & Physics
Main Examination
Pietermaritzburg
June 2015
75 MARKS; 2 HOURS
Chemistry & Society I
Place Barcode Here
Student Number
Internal Examiners:
Moderating Board:
Dr V. Jeena (Course coordinator)
Dr P. Singh (UKZN – Westville)
Dr S. Sithebe
Dr I. Booysen
NOTE:
•
•
•
This paper consists of 19 pages including a Periodic Table and Data Sheet. Please
check that you have all pages.
The use of molecular models and calculators is permitted.
Answer ALL questions directly on the question paper legibly.
Section
A
B
C
Total
25
25
25
75
25
25
25
75
Internal
Marks
Moderation
Marks
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
Section A
1. State the number of significant figures in the following examples:
(i)
(ii)
(iii)
3.14159
0.000350
900.06
=
=
=
(3)
2.
A mechanic has 8 car bodies and 48 tires. Using the materials at his disposal he can
produce 8 cars and has 16 excess tires. Fully explain the concepts of excess reactant
and limiting reagent using the example below.
(4)
Page 2 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
3. Oxygen gas can be produced by decomposing potassium chlorate using the reaction
below. If 138.6 g of KClO 3 is heated and decomposes completely, what mass of
oxygen gas is produced?
KClO3 (s)
KCl (s) + O2 (g)
(7)
Page 3 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
4.
10.0 ml of a solution of potassium hydroxide was titrated with a 0.105 mol.dm-3
solution of hydrochloric acid. The titre values for the acid were 17.20 ml, 17.10 ml,
17.50 ml and 17.20 ml of the acid for neutralization. Calculate the concentration of the
potassium hydroxide solution. (HINT: Begin your answer by writing a balanced
chemical equation).
(6)
Page 4 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
5. Determine the rate law of the following reaction and find the value of the rate constant
(k) with the correct units.
2O3 (g)
3O2 (g)
Experiment No.
Initial [O 3 ]/M
Initial Rate (M/S)
1
2.00
0.500
2
4.00
1.00
3
6.00
1.50
(cont.)
Page 5 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
(5)
Total for Section A: [25]
Page 6 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
Section B
1. (i) 0.200 mol of H 2 (g) is allowed to react with 0.200 mol of I 2 (g) to form HI in a 2.0 L
container according to the following reaction. At 450 °C, K c = 49.5 for the reaction.
H 2 (g)
+
I 2 (g)
2HI (g)
What will the concentrations of both reactants and product be when the reaction
reaches equilibrium? Show all your workings.
(5)
Page 7 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
(ii) In which direction must the reaction proceed in order to reach equilibrium? Support
your answer through suitable calculations.
(2)
2. At 100 °C, K sp = 1.5 ×10-3 for K 3 PO 4 (s). What is the molar solubility of K 3 PO 4 (s) at this
temperature? Support your answer through suitable calculations.
(3)
Page 8 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
3.
What is a molar solubility of a solute?
(1)
4. Will a precipitate of AgCl (s) form on mixing 250.0 mL of 0.100 M NaCl (aq) with
250.0 mL
of
1.0 ×10–3 M AgNO 3 (aq)? Show all your calculations given
K sp (AgCl) (s) = 1.8 x 10-10.
(5)
Page 9 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
5.
What is the pH of the solution resulting from mixing 2.50 g NaOH and 3.50 g HCl in
2.0 L of water?
(3)
Page 10 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
6.
A buffer is prepared by adding 2.50 g of propanoic acid, H 6 C 3 O 2 , and 2.50 g of
sodium propionate, NaH 5 C 3 O 2 , to enough water to form 750.0 mL of solution.
What is the pH of the solution if 50.0 mL of 0.10 M NaOH solution is added? Show all
your calculations.(K a = 1.4 ×10-5)
(6)
Total for Section B: [25]
Page 11 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
Section C
1.
When gasoline burns in a car engine, the heat released causes the products CO 2 and
H 2 O to expand, which pushes the pistons outward. Excess heat is removed by the
car’s cooling system. If the expanding gases do 451 J of work on the pistons and the
system loses 325 J to the surroundings as heat, calculate the change in energy (∆E) in
KJ.
(3)
2. In each of the following cases, determine the sign of ∆H, state whether the reaction is
exothermic or endothermic:
(a)
H 2 (g) + ½O 2 (g) → H 2 O(l) + 285.8 kJ
(b)
40.7 kJ + H 2 O(l) → H 2 O(g)
Page 12 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
(2)
3. (i) A layer of copper welded to the bottom of a skillet weighs 125 g. How much heat is
needed to raise the temperature of the copper layer from 25˚C to 300˚C? The specific
heat capacity of Cu is 0.387 J/g.K
(3)
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CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
(ii) What is the molar heat capacity of Cu?
(1)
4.
Given the following reactions and ∆H values:
a) B 2 O 3 (s) + 3H 2 O(g) → B 2 H 6 (g) + 3O 2 (g)
∆H = + 2035 kJ
b) 2H 2 O(l) → 2H 2 O(g)
∆H = + 88 kJ
c) H 2 (g) + ½O 2 (g) → H 2 O(l)
∆H = - 286 kJ
d) 2B(s) + 3H 2 (g) → B 2 H 6 (g)
∆H = + 36 kJ
Calculate ∆H for the formation of 1.0 mol of B 2 O 3 (s) according to the following reaction:
3
2B(s) + 2O 2 (g) → B 2 O 3 (s)
(3)
Page 14 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
5.
What is the oxidation state of Cr in Cr 2 O 7 2-?
(1)
6. Balance the following redox reaction in acidic media?
Cr 2 O 7 2-(aq) + I-(aq) → Cr3+(aq) + I 2 (s)
(5)
Page 15 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
7.
A voltaic cell houses the reaction between aqueous bromine and zinc metal:
Br 2 (aq) + Zn(s) → Zn2+(aq) + 2Br-(aq)
Calculate E˚ bromine , given E˚ zinc = - 0.76 V.
Half reactions and their standard reduction potentials:
Ag+(aq) + e- → Ag(s)
E˚ = + 0.80 V
Br 2 (l) + 2e-→ Br2-(aq)
E˚ = + 1.07 V
(3)
Page 16 of 17
CHEM163 - School of Chemistry and Physics, Pietermaritzburg – June 2015 Main Examination
8. Lead can displace silver from solution:
Pb(s) + 2Ag+(aq) → Pb2+(aq) + 2Ag(s)
As a consequence silver is a valuable by-product in the industrial extraction of lead
from its ore. Calculate K at 25˚C for this reaction.
Half reactions and their standard reduction potentials:
Ag+(aq) + e- → Ag(s)
E˚ = + 0.80 V
Pb2+(aq) + 2 e- → Pb(s)
E˚ = - 0.13 V
(4)
Total for Section C: [25]
Page 17 of 17
Grand Total: [75]