NAME:___________________________ Spring 2015 INSTRUCTIONS: Section:_____ Student Number:__________________ Chemistry 2000 Midterm #2C ____/ 50 marks 1) Please read over the test carefully before beginning. You should have 6 pages of questions, a blank “overflow” page and two pages of data sheets with periodic table. 2) If your work is not legible, it will be given a mark of zero. 3) Marks will be deducted for incorrect information added to an otherwise correct answer. 4) Marks will be deducted for improper use of significant figures and for missing or incorrect units. 5) Show your work for all calculations. Numerical answers without supporting calculations will not be given full credit. 6) You may use a calculator but only for the purposes of calculation. No text capable calculators are allowed. 7) You have 90 minutes to complete this test. Confidentiality Agreement: I agree not to discuss (or in any other way divulge) the contents of this exam until after 8pm Mountain Time on Tuesday, March 24th, 2015. I understand that breaking this agreement would constitute academic misconduct, a serious offense with serious consequences. The minimum punishment would be a mark of 0/50 on this exam and removal of the “overwrite midterm mark with final exam mark” option for my grade in this course; the maximum punishment would include expulsion from this university. Signature: ___________________________ Course: CHEM 2000 (General Chemistry II) Semester: Spring 2015 The University of Lethbridge Date: _____________________________ Question Breakdown Q1 Q2 Q3 Q4 Q5 Q6 /6 /2 /6 /5 / 11 / 20 Total / 50 NAME:___________________________ 1. (a) Section:_____ Student Number:__________________ The table below shows how boiling point increases as each hydrogen atom in methane (CH4) is replaced by a chlorine atom. [6 marks] Molecular Formula Boiling Point (°C) CH4 -161.5 CH3Cl -23.8 CH2Cl2 39.6 CHCl3 61.2 CCl4 76.7 Explain why the increase in boiling point from CH4 to CH3Cl is larger than any of the other increases in boiling point. [2 marks] This is the only case in which replacing a hydrogen atom with a chlorine atom generates a polar molecule (CH3Cl) from a nonpolar molecule (CH4). There are three types of intermolecular forces active in polar liquids like CH3Cl (dipole-dipole, dipole-induced dipole and induced dipole-induced dipole) while there is only one type of intermolecular force active in nonpolar liquids like CH4 (induced dipole-induced dipole). Since boiling point increases with the strength of the active intermolecular forces, it is reasonable that the introduction of two new types of intermolecular force would result in a significant increase in boiling point. (b) Explain why the increase in boiling point from CHCl3 to CCl4 is smaller than any of the other increases in boiling point. [2 marks] This is the only case in which replacing a hydrogen atom with a chlorine atom generates a nonpolar molecule (CCl4) from a polar molecule (CHCl3). Thus, replacing the last hydrogen atom in CHCl3 with a chlorine atom gives a compound which experiences fewer types of intermolecular forces (see answer to part (a) for list of active IMF). Since boiling point increases with the strength of the active intermolecular forces, it is reasonable that the elimination of two types of intermolecular forces would result in a smaller increase in boiling point. (c) What can you conclude from the fact that the boiling point increases every time a hydrogen atom is replaced by a chlorine atom? [2 marks] Based on the answer to part (b), one might expect the boiling point of CCl4 to be lower than that of CHCl3. The fact that it is not tells us that the increase in strength of the induced dipoleinduced dipole forces is greater than the loss of the dipole-dipole and dipole-induced dipole forces in this set of molecules. The increase in strength of induced dipole-induced dipole forces will be due to the fact that chlorine atoms are much larger than hydrogen atoms. This tells us that the strength of the induced dipole-induced dipole forces is significant (and that they are likely the major intermolecular force active in most of the molecules in this series). IMF act between molecules; physical changes like boiling DO NOT BREAK CHEMICAL BONDS. A molecule is only polar if its overall shape is polar; POLAR BONDS DO NOT MAKE A MOLECULE POLAR so that CH4 and CBr4 are both non-polar because tetrahedral. Please provide short, to the point answers based on facts; waffling does not help your cause! NAME:___________________________ 2. Section:_____ Student Number:__________________ Why does water “ball up” into spheres on wax paper but spread out (and soak in) on regular paper? [2 marks] The surface of wax paper is nonpolar. The surface of regular paper is polar. Water molecules are polar. As such, they experience stronger intermolecular forces toward other water molecules than to a nonpolar surface such as that of wax paper. Since the water molecules are more strongly attracted to each other than to the surface, they “ball up” into spheres. On regular paper, the attractions of the water molecules to the surface of the paper are also strong, so this effect is not observed. 3. (a) Consider the following reaction: 2 Na (s) + 2 H 2 O (l) → 2 NaOH (aq) + H 2 (g) Is this reaction entropy-favoured? Why or why not? [6 marks] [2 marks] There are more moles of gas in the products (1 moles) than in the reactants (0 moles). Therefore, the reaction IS entropy-favoured. (b) If this reaction was exothermic, would it be thermodynamically favoured at all temperatures? Why or why not? [2 marks] If the reaction is exothermic then it is favoured by enthalpy (∆H < 0). Since it is also favoured by entropy (∆S < 0), it will be favoured at all temperatures. To be favoured, a reaction must have ∆G < 0. ∆G = ∆H - T∆S If ∆H < 0 and ∆S > 0, then ∆G < 0 at all temperatures since T is always a positive number (Kelvin scale). (c) If this reaction was endothermic, would there be any conditions under which it would be thermodynamically favoured? If yes, what would they be? If not, why not? [2 marks] If the reaction is endothermic then it is not favoured by enthalpy (∆H > 0). To be favoured, a reaction must have ∆G < 0. ∆G = ∆H - T∆S If ∆H > 0 and ∆S > 0, then ∆G < 0 at temperatures which are high enough that T∆S is greater than ∆H. NAME:___________________________ 4. Section:_____ Student Number:__________________ Consider a container divided into two equal compartments. One compartment contains gas particles while the other is completely empty: [5 marks] Picture A The divider is removed: Picture B (a) The pictures above each have 10 dots representing gas particles. Draw what the system looks like once it reaches equilibrium: [1 mark] Picture C (b) Draw what the system looks like 10 minutes after it reaches equilibrium. [1 mark] Picture D (c) It must be clear whether Picture D is intended to look the same as Picture C or different from it. They must look different. The gas particles keep moving. Equilibrium is a dynamic state. Use the kinetic molecular theory of gases to explain why the system changes from Picture B to Picture C. [3 marks] According to the kinetic molecular theory, gas particles move in straight lines in random directions until they collide with something (usually the container wall; sometimes another gas particle). Since the gas particles all start in the left half of the container, the ones moving generally toward the left will encounter the container walls sooner and be redirected. The ones moving generally toward the right will be able to continue moving in a straight line for longer before colliding with a container wall and being redirected. Thus, the tendency will be for more gas particles to be moving from the left half of the container toward the right half (than from the right half to the left half), and this will continue until both halves contain equal numbers of gas particles. Please review the postulates of the KMT, found on slides 17 & 18 of “Lecture 15-17” notes. Explanations based on entropy as a driving force were accepted but the postulates of the KMT had to be used to obtain full credit for this question. NAME:___________________________ 5. Section:_____ Student Number:__________________ Phosphorus has two common allotropes, white phosphorus and red phosphorus. [11 marks] (a) Write a balanced chemical equation for the reversible conversion of white phosphorus to red phosphorus. Note that the data is given per mole of P atoms, not P4 molecules. [1 mark] P (white) ⇔ P (red) or P4 (white) ⇔ 4 P (red) (b) Determine the standard free energy change for the conversion of white phosphorus to red phosphorus. [5 marks] The data sheet provides ∆fH° and S° values for these two species (but not ∆fG° values). ∆rG° can be calculated from ∆rH° and ∆rS° – which can, in turn be calculated from ∆fH° and S°. Answers built on the S8 version of the question will have numerical values x 8 from what is given here. Step 1: Calculate ∆rH° using ∆rH° = Σ ∆fH°(products) - Σ ∆fH°(reactants) 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 Δ𝑟𝑟 𝐻𝐻° = �−17.6 𝑚𝑚𝑚𝑚𝑚𝑚� − �0 𝑚𝑚𝑚𝑚𝑚𝑚� = −17.6 𝑚𝑚𝑚𝑚𝑚𝑚 1 decimal place = 3 sig. fig. Step 2: Calculate ∆rS° using ∆rS° = Σ S°(products) - Σ S°(reactants) 𝐽𝐽 𝐽𝐽 𝐽𝐽 Δ𝑟𝑟 𝑆𝑆° = �22.80 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� − �41.10 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� = −18.30 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 2 decimal places = 4 sig. fig. Step 3: Calculate ∆rG° using ∆rG° = ∆rH° - T∆rS° 𝑘𝑘𝑘𝑘 𝐽𝐽 1𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 Δ𝑟𝑟 𝐺𝐺° = �−17.6 𝑚𝑚𝑚𝑚𝑚𝑚� − (298.15𝐾𝐾) �−18.30 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� �1000𝐽𝐽� = −12.1 𝑚𝑚𝑚𝑚𝑚𝑚 (c) 1 decimal place = 3 sig. fig. Which is the more stable allotrope at room temperature? Explain. [2 marks] Assume that “room temperature” is relatively close to standard conditions. (~25 °C) ∆rG° < 0 therefore the forward reaction is thermodynamically favoured under standard conditions. Therefore, P (red) is the more stable allotrope under standard conditions. Merely stating that the allotrope with ∆fH° is the most stable was not acceptable, and is not always true! (d) Calculate the temperature (in K) at which these two allotropes reach equilibrium at 1 bar. [3 marks] ∆rG = ∆rH – T∆rS At equilibrium, ∆rG = 0. (reaction favoured in neither forward nor reverse direction) We must assume that ∆rH and ∆rS do not vary dramatically with temperature (so that we can use the values for ∆rH° and ∆rS° from part (b)). 1 bar is the standard pressure. Δ𝑟𝑟 𝐺𝐺° = Δ𝑟𝑟 𝐻𝐻° − 𝑇𝑇Δ𝑟𝑟 𝑆𝑆° therefore 𝑇𝑇 = 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 �−�0 � 𝑚𝑚𝑚𝑚𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚 𝐽𝐽 � �−18.30 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 �−17.6 × 1000𝐽𝐽 1𝑘𝑘𝑘𝑘 𝑇𝑇 = = 962 𝐾𝐾 Δ𝑟𝑟 𝐻𝐻°−Δ𝑟𝑟 𝐺𝐺° Δ𝑟𝑟 𝑆𝑆° 3 sig. fig. NAME:___________________________ 6. (a) (b) Section:_____ Student Number:__________________ Consider the following reaction in the gas phase: 2 NOBr (g) ⇔ 2 NO (g) + Br2 (g) Write the thermodynamic equilibrium constant expression for this reaction. 𝐾𝐾 = (𝑎𝑎𝑁𝑁𝑁𝑁 )2 �𝑎𝑎𝐵𝐵𝐵𝐵2 � (𝑎𝑎𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 )2 [20 marks] [2 marks] Q is rxn quotient! Activities are required for full marks here. Calculate the equilibrium constant for this reaction under standard conditions. [9 marks] The data sheet provides ∆fH° and S° values for the three species in this reaction. ∆rG° can be calculated from ∆rH° and ∆rS° (which can, in turn be calculated from ∆fH° and S°). K can be calculated from ∆rG°. Step 1: Calculate ∆rH° using ∆rH° = Σ ∆fH°(products) - Σ ∆fH°(reactants) 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 Δ𝑟𝑟 𝐻𝐻° = �2 �90.29 𝑚𝑚𝑚𝑚𝑚𝑚� + �30.91 𝑚𝑚𝑚𝑚𝑚𝑚�� − 2 �82.13 𝑚𝑚𝑚𝑚𝑚𝑚� = 47.23 𝑚𝑚𝑚𝑚𝑚𝑚 2 decimal places = 4 sig. fig. Step 2: Calculate ∆rS° using ∆rS° = Σ S°(products) - Σ S°(reactants) 𝐽𝐽 𝐽𝐽 𝐽𝐽 𝐽𝐽 Δ𝑟𝑟 𝑆𝑆° = �2 �210.65 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� + �245.38 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾�� − 2 �273.53 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� = 119.62 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 2 decimal places = 5 sig. fig. Step 3: Calculate ∆rG° using ∆rG° = ∆rH° - T∆rS° 𝑘𝑘𝑘𝑘 𝐽𝐽 1𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 Δ𝑟𝑟 𝐺𝐺° = �47.23 𝑚𝑚𝑚𝑚𝑚𝑚� − (298.15𝐾𝐾) �119.62 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� �1000𝐽𝐽� = 11.57 𝑚𝑚𝑚𝑚𝑚𝑚 2 decimal places = 4 sig. fig. Step 4: Calculate K using ∆rG° = –RTlnK Δ𝑟𝑟 𝐺𝐺° = −𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝐾𝐾 = 𝑒𝑒 − therefore 𝑘𝑘𝑘𝑘 �11.57 � 1000𝐽𝐽 𝑚𝑚𝑚𝑚𝑚𝑚 × 𝐽𝐽 �(298.15𝐾𝐾) 1𝑘𝑘𝑘𝑘 �8.314462 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 𝐾𝐾 = 𝑒𝑒 − Δ𝑟𝑟 𝐺𝐺° 𝑅𝑅𝑅𝑅 = 𝑒𝑒 −4.665 = 9.42 × 10−3 Many students chose to calculate all the ∆fG° values, and then did so again at a different temperature for part (c). This is a highly inefficient approach and often resulted in serious arithmetic answers. Note also that (1) the stoichiometry of the reaction must be considered in the ∆rH° and ∆rS° calculations, and (2) kJ and J must be rationalized when calculating ∆rG°. (c) Alternate answer: 𝑘𝑘𝑘𝑘 𝐽𝐽 1𝑘𝑘𝑘𝑘 𝑘𝑘𝑘𝑘 Δ𝑟𝑟 𝐺𝐺 280 = �47.23 𝑚𝑚𝑚𝑚𝑚𝑚� − (280𝐾𝐾) �119.62 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾� �1000𝐽𝐽� = +13.74 𝑚𝑚𝑚𝑚𝑚𝑚 − 𝑘𝑘𝑘𝑘 �−13.74 � 1000𝐽𝐽 𝑚𝑚𝑚𝑚𝑚𝑚 × 𝐽𝐽 �(280𝐾𝐾) 1𝑘𝑘𝑘𝑘 �8.314462 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 𝐾𝐾 = 𝑒𝑒 = 2.74 × 10−3 But it is not permissible to use ∆rG° with T = 280 K! Standard conditions means standard! NAME:___________________________ 6. (c) Section:_____ Student Number:__________________ continued… Calculate the equilibrium constant for this reaction at 280 K. [5 marks] From part (b), we know that K1 = 9.42 × 10-3 at T1 = 298.15 K We need to solve for K2 at T2 = 280 K. We also know that ∆H = 47.23 kJ/mol. 𝑘𝑘𝑘𝑘 � 𝑚𝑚𝑚𝑚𝑚𝑚 𝐽𝐽 � �8.314462 𝑚𝑚𝑚𝑚𝑚𝑚∙𝐾𝐾 �47.23 𝐾𝐾2 𝑙𝑙𝑙𝑙 �𝐾𝐾 � = 1 𝐾𝐾 � 1000𝐽𝐽 1𝑘𝑘𝑘𝑘 1 1 � �(298.15𝐾𝐾) − (280𝐾𝐾)� 1 𝑙𝑙𝑙𝑙 �𝐾𝐾2 � = (5680𝐾𝐾) �0.00022 𝐾𝐾� = −1.2 𝐾𝐾2 𝐾𝐾1 1 = 𝑒𝑒 −1.2 𝐾𝐾2 = (𝐾𝐾1 )(𝑒𝑒 −1.2 ) = (9.42 × 10−3 )(𝑒𝑒 −1.2 ) = (9.42 × 10−3 )(0.3) = 3 × 10−3 (d) A gas reactor is charged with 0.39 bar NO, 0.37 bar Br2 and 0.011 bar NOBr at 280 K. Determine whether the reaction will proceed in the forward or in the reverse reaction. [4 marks] Calculate Q for the system. (This requires calculation of activities for all reactants and products.) Compare Q to K. If Q < K, the reaction will proceed forward. If Q > K, the reaction will proceed in reverse. Step 1: Calculate activities of all reactants and products 𝑝𝑝 0.39 𝑏𝑏𝑏𝑏𝑏𝑏 𝑝𝑝𝐵𝐵𝐵𝐵 0.37 𝑏𝑏𝑏𝑏𝑏𝑏 𝑁𝑁𝑁𝑁 𝑎𝑎𝑁𝑁𝑁𝑁 = 1 𝑏𝑏𝑏𝑏𝑟𝑟 = 𝑎𝑎𝐵𝐵𝐵𝐵2 = 1 𝑏𝑏𝑏𝑏𝑏𝑏2 = 𝑎𝑎𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 = 𝑝𝑝𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 1 𝑏𝑏𝑏𝑏𝑏𝑏 1 𝑏𝑏𝑏𝑏𝑏𝑏 1 𝑏𝑏𝑏𝑏𝑏𝑏 = = 0.39 2 sig. fig. = 0.37 0.011 𝑏𝑏𝑏𝑏𝑏𝑏 1 𝑏𝑏𝑏𝑏𝑏𝑏 2 sig. fig. = 0.011 2 sig. fig. Step 2: Calculate Q 𝑄𝑄 = (𝑎𝑎𝑁𝑁𝑁𝑁 )2 �𝑎𝑎𝐵𝐵𝐵𝐵2 � (𝑎𝑎𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 )2 Step 3: Compare Q to K = (0.39)2 (0.37) (0.011)2 = 440 = 4.4 × 102 from part (c), K = 0.003 therefore Q > K Therefore, the reaction will proceed in reverse. 2 sig. fig. Some Useful Constants and Formulae Fundamental Constants and Conversion Factors Atomic mass unit (u) 1.660 539 × 10-27 kg Avogadro's number (NA) 6.022 141 × 1023 mol–1 Boltzmann constant (kB) 1.380 649 × 10-23 J·K-1 Ideal gas constant (R) 8.314 462 J·mol-1·K-1 8.314 462 m3·Pa·mol-1·K-1 Kelvin temperature scale Planck's constant (h) Speed of light in vacuum (c) Pressure conversions Volume conversion 0 K = -273.15 ˚C 6.626 070 × 10-34 J·Hz-1 2.997 925 x 108 m·s-1 1 bar = 100 kPa 1 atm = 1.01325 bar 1000 L = 1 m3 Formulae K = 3 1 mv 2 = RT 2 2 v rms = v 2 = ∆S = S = k B ln Ω q rev T o Δr G = Δr G + RT ln Q pK a = − log K a Activities Solid Pure liquid Ideal Solvent Ideal Solute Ideal Gas 3RT M o ∆ r G = −RT ln K pH = − log aH + PA = X A PAo a =1 a =1 a=X c a= c° P a= P° P (white) P (red) NOBr (g) NO (g) Br2 (g) kJ ∆ f H ° mol 0 -17.6 82.13 90.29 30.91 n2 P + a 2 (V − nb ) = nRT V ∆ r G = ∆ r H − T∆ r S Some Useful Thermodynamic Properties Substance PV = nRT J S ° mol ⋅ K 41.10 22.80 273.53 210.65 245.38 o K2 ∆ r H = ln R K1 [ A] = k H PA 1 1 − T1 T2 X = n ∑n 1 CHEM 2000 Standard Periodic Table 18 1.0079 4.0026 He H 2 13 14 15 16 17 6.941 9.0122 10.811 12.011 14.0067 15.9994 18.9984 Li Be B C N O F Ne 3 22.9898 4 24.3050 5 26.9815 6 28.0855 7 30.9738 8 32.066 9 35.4527 10 39.948 1 2 20.1797 Na Mg 11 39.0983 12 40.078 3 4 5 6 7 8 9 10 11 12 44.9559 47.88 50.9415 51.9961 54.9380 55.847 58.9332 58.693 63.546 65.39 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 19 85.4678 20 87.62 21 88.9059 22 91.224 23 92.9064 26 101.07 27 102.906 28 106.42 29 107.868 30 112.411 31 114.82 32 118.710 33 121.757 34 127.60 35 126.905 36 131.29 Rb Sr 37 132.905 38 137.327 Cs Ba 55 (223) 56 226.025 Fr 87 Ra Y 39 La-Lu Ac-Lr 88 24 95.94 25 (98) Zr Nb Mo Tc 40 178.49 41 180.948 42 183.85 43 186.207 Cl Ar 17 79.904 18 83.80 Rh Pd Ag Cd In Sn Sb Te I Xe 46 195.08 47 196.967 48 200.59 49 204.383 50 207.19 51 208.980 52 (210) 53 (210) 54 (222) Ta W Re Os Ir Pt 74 (271) 75 (270) 76 (277) 77 (276) 78 (281) Bh S 16 78.96 45 192.22 73 (268) Sg P 15 74.9216 Ru Hf Db Si 14 72.61 44 190.2 72 (265) Rf Al 13 69.723 Hs Mt Ds Au 79 (280) Rg Hg 80 (285) Cn Tl Pb Bi Po At 81 (284) 82 (289) 83 (288) 84 (293) 85 (294) Uut Fl Uup Lv Uus 104 105 106 107 108 109 110 111 112 113 114 115 116 117 138.906 140.115 140.908 144.24 (145) 150.36 151.965 157.25 158.925 162.50 164.930 167.26 168.934 173.04 174.967 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb 57 227.028 58 232.038 59 231.036 60 238.029 61 237.048 65 (247) 66 (251) Ac 89 Th 90 Pa 91 U 92 Np 93 62 (240) Pu 94 63 (243) Am 95 64 (247) Cm 96 Bk 97 Cf 98 67 (252) Es 99 68 (257) Fm 100 69 (258) Md 101 70 (259) No 102 Rn 86 (294) Uuo 118 Lu 71 (262) Lr 103 Developed by Prof. R. T. Boeré (updated 2014)
© Copyright 2026 Paperzz