CHM 152 Group Work 7C Names



CHM 152 - Thermodynamics (Ch. 16)

Spontaneity

1. True or false? All exothermic reactions are spontaneous? ______________

2. Which variable can tell you if a process is spontaneous without exception?

a. ΔHrxn b. ΔSrxn c. ΔHsurr d. ΔSsurr e. ΔStot

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) γ H2O (l) ΔS_____

b. C6H12O6(s) γ 2C2H5OH(l) + 2CO2(g) ΔS_____

c. 2NH3(g) + CO2(g) γ H2O(l) + NH2CONH2(aq) ΔS_____

d. NaCl(s) γ NaCl(aq) ΔS_____

e. Cu(s) (100oC) γ Cu(s) (25oC) ΔS_____

f. 2NH3(g) Δ N2(g) + 3H2(g) ΔS_____

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)

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)

13. Using the data below, calculate ΔSorxn for the following reaction:

4 Cr(s) + 3 O2(g) → 2 Cr2O3(s)

|Substance |Δ S, J/K·mol |

|Cr(s) |23.77 |

|O2(g) |205.138 |

|Cr2O3(s) |81.2 |

a. 548.1 J/K

b. 147.7 J/K

c. -147.7 J/K

d. -548.1 J/K

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)

S([Al(s)] = 28.32 [pic] S([O2(g)] = 205 [pic] S([Al2O3(s)] = 51.0 [pic]

Free Energy

15. In 1774 Joseph Priestly prepared oxygen by heating mercury(II) oxide according to the reaction HgO(l) → Hg(l) + ½O2, for which ΔHo = 90.84 kJ/mol and ΔSo = 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:

H2O2(g) → H2O(l) + 1/2 O2(g) , ΔHo = -106 kJ/mol; ΔSo = 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?

18. Calculate ΔHorxn, ΔSorxn and ΔGorxn for Mg(s) + O2(g) Δ MgO(s)) (Hint – balance the rxn)

19. From # 18 the results for ΔHorxn means (exothermic or endothermic), the results for ΔSorxn means (disorder or order) increased, and ΔGorxn 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

17. 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)

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

a) N(g) + 3H(g) ( NH3(g) b) N2(g) + 3H2(g) ( 2NH3(g)

c) [pic]N2(l) + [pic]H2(g) ( NH3(g) d) [pic]N2(g) + [pic]H2(g) ( NH3(g)

19. Calculate ΔH[pic] for the following reaction at 25 °C.

4NH3(g) + 5O2(g) ( 4NO(g) + 6H2O(g)

ΔH[pic][NO(g)] = 90.3 [pic] ΔH[pic] [H2O(g)] = -241.8 [pic] ΔH[pic][NH3(g)] = -45.9 [pic]

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)

|Substance |Δ Gof, kJ/mol |

|C3H8(g) |-23.0 |

|O2(g) |0 |

|CO2(g) |-394.6 |

|H2O(l) |-237.2 |

a. 2109.6 kJ/mol

b. 608.8 kJ/mol

c. -608.8 kJ/mol

d. -2109.6 kJ/mol

e. None of the above.

25. Calculate ΔGorxn at 25oC for this reaction: 2 NaHCO3(s) g Na2CO3(s) + CO2(g) + H2O(g). Given: ΔHorxn = 128.9 kJ/mol and ΔSorxn = 321 J/molK

Free Energy and Equilibrium

17. a) Calculate ΔG( for the following reaction. b) What is the value of the equilibrium constant at 298 K? c) Is this reaction spontaneous at 298 K?

3C2H2(g) ( C6H6(g) ΔG[pic]= 209.2 [pic]; ΔG[pic]= 129.7 [pic]

20. a) Calculate ΔG( for the following reaction at 298 K. Kb = 1.8x10-5

NH3(aq) + H2O(l) ( NH[pic](aq) + OH-(aq)

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

21. If ΔG 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 ΔG will reach ______________

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

23. 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?

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

25. 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.

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

27. 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?

28. Calculate ΔG for Mg(s) + O2(g) ( MgO(s) at 30.5oC if the pressure of oxygen gas is 1.33 atm.

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