CHM 152 - Thermodynamics (Entropy, Free Energy, and Equilibrium)

Name: ______

Spontaneity 1. True or false? All exothermic reactions are spontaneous? ______2. Which variable can tell you if a process is spontaneous without exception?

a. Hrxn b. Srxn c. Hsurr d. Ssurr e. Stot

Enthalpy and Entropy 3. Circle the correct words: Nature tends towards (higher or lower) energy and more (order or disorder)? 4. Which of the following reactions will have the most positive ΔHo ? (Hint: Draw Lewis structures.) a. N2(g) → 2 N(g) b. F2(g) → 2 F(g) c. O2(g) → 2 O(g) d. These reactions would all have the same ΔHo .

5. Which state of matter has the highest entropy? ______6. Predict whether the entropy change will be positive or negative for the following:

a. H2O (g)  H2O (l) S_____

b. C6H12O6(s)  2C2H5OH(l) + 2CO2(g) S_____

c. 2NH3(g) + CO2(g)  H2O(l) + NH2CONH2(aq) S_____ d. NaCl(s)  NaCl(aq) S_____ e. Cu(s) (100oC)  Cu(s) (25oC) S_____ f. 2NH3(g) ⇄ N2(g) + 3H2(g) S_____

7. If a process is endothermic and the process creates more order than existed before, the process is: a. always spontaneous b. never spontaneous c. spontaneous at high T d. spontaneous at low T

8. Which of the following substances has the greatest entropy per mole?

a. O2(g)

b. N2(g) c. CO(g)

d. CO2(g)

e. C4H10(g)

CHM 152 Thermodynamics page 1 of 7 Name: ______

9. Which of the following reactions will have an increase in entropy? Choose all that apply.

a. SO3(g) → 2SO2(g) + O2(g)

b. H2O(l) → H2O(s)

c. Br2(l) → Br2(g)

d. H2O2(l) → H2O(l) + ½ O2(g)

10. Without consulting entropy tables, predict the sign of ΔS for the following process and choose the correct reasoning for your prediction: The mass of nitrogen remains constant. N2(g, 10 atm) → N2(g, 1 atm) a. positive; there is an increase in the number of gas molecules b. positive; the gas expands into a larger volume c. negative; the gas is compressed into a smaller volume d. negative; the gas expands into a larger volume e. negative; there is a decrease in the number of gas molecules.

11. Without consulting entropy tables, predict the sign of ΔS for the following process: Pb(s) + Cl2(g) → PbCl2(s). a. ΔS > 0 b. ΔS < 0 c. ΔS = 0 d. More information is needed to make a reasonable prediction.

Standard Molar Entropies 12. Which of the following compounds has the lowest entropy at 25 oC?

a. CH3OH(l) b. CO(g)

c. MgCO3(s)

d. H2O(l)

e. H2O(g)

o 13. Using the data below, calculate ΔS rxn for the following reaction: 4 Cr(s) + 3 O2(g) → 2 Cr2O3(s) a. 548.1 J/K Substance Δ S, J/K·mol b. 147.7 J/K Cr(s) 23.77 c. -147.7 J/K O2(g) 205.138 Cr O (s) 81.2 d. -548.1 J/K 2 3 e. None of the above.

14. Calculate the standard entropy change, ΔS, for the following reaction at 25 °C:

4Al(s) + 3O2(g)  2Al2O3(s) J J J S[Al(s)] = 28.32 S[O2(g)] = 205 S[Al2O3(s)] = 51.0 K × mol K × mol K × mol

Free Energy 15. In 1774 Joseph Priestly prepared oxygen by heating mercury(II) oxide according to the reaction HgO(l) → o o Hg(l) + ½O2, for which ΔH = 90.84 kJ/mol and ΔS = 108 J/K∙mol. Which of the following statements is true for this reaction? a. The reaction is spontaneous only at low temperatures. b. The reaction is spontaneous at all temperatures. c. ΔGo becomes less favorable as temperature increases. d. The reaction is spontaneous only at high temperatures. e. The reaction is at equilibrium at 25 oC and 1 atm pressure.

16. For the decomposition reaction of hydrogen peroxide:

o o H2O2(g) → H2O(l) + 1/2 O2(g) , ΔH = -106 kJ/mol; ΔS = 58 J/K

Is H2O2(g) stable? a. Yes, under all conditions. b. Yes, if the temperature is low enough. c. Yes, if the rate of decomposition is low. d. Yes, if the O-O bond energy is greater than the O-H bond energy. e. No.

17. a. Calculate the standard free energy change, ΔG, for the following at 25 C: MgO(s) + C(graphite)  Mg(s) + CO(g) ΔH = 491.18 kJ ΔS = 197.67 J/K

b. Is this reaction spontaneous at 25 C? If not, at what temperature can we make this reaction spontaneous?

o o o 18. Calculate H rxn, S rxn and G rxn for Mg(s) + O2(g) ⇄ MgO(s)) (Hint – balance the rxn)

o o 19. From # 18 the results for H rxn means (exothermic or endothermic), the results for S rxn means (disorder or o order) increased, and G rxn means (products or reactants) are favored under standard conditions.

20. Can you measure enthalpy (H)? _____ entropy (S)? ____ Gibb’s free energy (G)? _____

Standard Free Energy of Formation 21. Which of the following does not have a standard enthalpy of formation of zero?

A. I2(l) B. Cl2(g) C. Au(s) D. Ne(g) E. Hg(l)

22. The standard enthalpy of formation equation for NH3(g) is:

a) N(g) + 3H(g)  NH3(g) b) N2(g) + 3H2(g)  2NH3(g) 1 3 1 3 c) N2(l) + H2(g)  NH3(g) d) N2(g) + H2(g)  NH3(g) 2 2 2 2

° 23. Calculate H rxn for the following reaction at 25 °C.

4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)

° kJ ° kJ ° kJ H f [NO(g)] = 90.3 H f [H2O(g)] = -241.8 H f [NH3(g)] = -45.9 mol mol mol

24. Use the following data to calculate ΔGo at 298 K for the combustion of propane: C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(l)

o a. 2109.6 kJ/mol Substance  G f, kJ/mol b. 608.8 kJ/mol C3H8(g) -23.0 O (g) 0 c. -608.8 kJ/mol 2 CO2(g) -394.6 d. -2109.6 kJ/mol H2O(l) -237.2 e. None of the above.

o o o 25. Calculate G rxn at 25 C for this reaction: 2 NaHCO3(s) g  Na2CO3(s) + CO2(g) + H2O(g). Given: H rxn = 128.9 o kJ/mol and S rxn = 321 J/molK

Free Energy and Equilibrium 26. a) Calculate G for the following reaction. b) What is the value of the equilibrium constant at 298 K? c) Is this reaction spontaneous at 298 K?

° kJ ° kJ 3C2H2(g)  C6H6(g) G f (C2 H 2 )= 209.2 ; G f (C6 H 6 )= 129.7 mol mol

-5 24. a) Calculate G for the following reaction at 298 K. Kb = 1.8x10

+ - NH3(aq) + H2O(l) ⇄ NH 4 (aq) + OH (aq)

+ - b) Calculate G at 298 K, when [NH 4 ] = 0.10 M, [OH ] = 0.050 M and [NH3] = 0.10 M.

25. If G is a negative number fill in the following: i. Is the reaction spontaneous or not? ______ii. Q relates how to K? ______iii. Is the reaction going forwards or backwards? ______iv. Eventually the value of G will reach ______

26. Circle the correct sign: When ice melts S is ( + or - ) and H is ( + or - ). Under what conditions will this process be spontaneous, if ever? Be specific and refer to the appropriate temperature.

27. For carbon disulfide, CS2, the enthalpy and entropy for vaporization is 27.7 kJ/mol and 86.4 J/mol·K, respectively. What is the boiling point (°C) for CS2? Will CS2 boil above or below this temperature?

28. At 25ºC the equilibrium constant for this reaction CO(g) + 2H2(g)  CH3OH(g) has the value Kp = 2.1 x 4 10 . Calculate ΔGºrxn for this reaction at this temperature.

29. For the unbalanced reaction SO2(g) + O2(g) → SO3(g) calculate ΔG at 25.0ºC when the reactants and product are at the following partial pressures: 10.0 atm SO2 , 10.0 atm O2 , and 1.00 atm SO3.

30. Calculate K for MgCO3(s)  MgO(s) + CO2(g)

31. Calculate the temperature at which this reaction changes from being spontaneous to non spontaneous: Mg(s) + O2(g)  MgO(s). Is the reaction spontaneous above or below this temperature?

o 32. Calculate G for Mg(s) + O2(g)  MgO(s) at 30.5 C if the pressure of oxygen gas is 1.33 atm.

CHM 152 Thermodynamics page 7 of 7