University of South Florida CHM 2046 16

University of South Florida

CHM 2046

16.1 Multiple Choice Questions

1)Which of the following statements is not true?

1. 1. The reverse of a spontaneous reaction is always nonspontaneous.
   2. A spontaneous process always moves toward equilibrium.
   3. A nonspontaneous process cannot be caused to occur.
   4. A highly spontaneous process need not occur rapidly.

2. Which forward reaction is a nonspontaneous process?
   1. the expansion of a gas into a vacuum

B) N2(g) + 3 H2(g) ? 2 NH3(g) if PH? = PN? = 1 atm, PNH? = 0, and Kp = 4 × 105

C) 2 NH3(g) ? N2(g) + 3 H2(g) if PNH? = 1 atm, PH? = PN? = 0, and Kp = 2 × 10-6

D) none of the above

3. The chemical system shown below is at equilibrium. Which change in conditions will not

result in a spontaneous forward reaction?

N2(g) + 3 H2(g) ? 2 NH3(g)                  Kp = 4 × 105

1. adding a catalyst
2. adding more H2
3. adding more N2
4. reducing the volume

4. Classify each of the following processes as spontaneous or nonspontaneous.

1. H2O(l) → H2O(g)              T = 25°C, vessel open to atmosphere with 50% relative

humidity

2. H2O(s) → H2O(l)              T = 25°C, P = 1 atm
   1. I and II are both spontaneous.
   2. I is spontaneous and II is nonspontaneous.
   3. I is nonspontaneous and II is spontaneous.
   4. I and II are both nonspontaneous.

5. The reaction A(g) → B(g) is spontaneous under standard conditions. Which of the following statements must be true?

1. The reaction B(g) → A(g) is nonspontaneous under standard conditions.
2. A(g) will be completely converted to B(g) if sufficient time is allowed.
3. A(g) will be completely converted to B(g) rapidly.
   1. none of these
   2. I
   3. I and II
   4. I, II, and III

6. Which of the following processes are spontaneous?

1. dissolving more solute in an unsaturated solution
2. dissolving more solute in a saturated solution
3. dissolving more solute in a supersaturated solution
   1. none of these
   2. I
   3. I and II
   4. I, II, and III

7. Which of the following processes is spontaneous?
   1. a mixture of two gases separating into pure compounds
   2. reaction of sodium with oxygen
   3. precipitation of solute from a saturated solution
   4. water flowing uphill

8. Entropy is a measure of
   1. free energy.
   2. the heat of a reaction.
   3. molecular randomness.
   4. the rate of a reaction.

9. For which of the following will the entropy of the system increase?
   1. condensation of steam
   2. reaction of magnesium with oxygen to form magnesium oxide
   3. reaction of nitrogen and hydrogen to form ammonia
   4. sublimation of dry ice

10. For which process is the sign of ?S negative in the system? A) 2 H2(g) + O2(g) → 2 H2O(g)

B) 2 H2O(l) + 2 K(s) → 2 K+(aq) +2 OH–(aq) + H2(g)

3. H2O(s) → H2O(g)
4. H2O(l) → H2O(g)

11. Predict the sign of ΔS of the system for both of the following.

I.     2 C(graphite) + O2(g) → 2 CO(g)

II. C4H10(g) → C4H10(l)

1. 1. ΔS should be negative for I and negative for II.
   2. ΔS should be negative for I and positive for II.
   3. ΔS should be positive for I and negative for II.
   4. ΔS should be positive for I and positive for II.

12. Sodium reacts violently with water according to the equation: 2 Na(s) + 2 H2O(l) → 2 NaOH(aq) + H2(g)

1. 1. ΔH° is negative and ΔS° is negative.
   2. ΔH° is negative and ΔS° is positive.
   3. ΔH° is positive and ΔS° is negative.
   4. ΔH° is positive and ΔS° is positive.

13. The brown color associated with photochemical smog is due to NO2(g), which is involved in an equilibrium with N2O4(g) in the atmosphere.

2 NO2(g) ? N2O4(g)

Predict the signs of the enthalpy and entropy change for the forward reaction.

1. 1. The enthalpy change is negative and the entropy change is negative.
   2. The enthalpy change is negative and the entropy change is positive.
   3. The enthalpy change is positive and the entropy change is negative.
   4. The enthalpy change is positive and the entropy change is positive.

14. What is W in Boltzmann's formula, S = k ln W?
    1. a fraction indicating the probability of obtaining a result
    2. a random number
    3. the number of ways of obtaining the state
    4. the work times Avogadro's number

15. Which electron on an atom of copper would have the highest value of W in the Boltzmann formula?
    1. 3s
    2. 3d
    3. 4s
    4. 4p

16. An electron in an oxygen p orbital on which of the following would have the highest entropy?
    1. CH3CH2OH
    2. CH3CH2O–
    3. CH3CO2OH
    4. CH3CO2–

17. What is k in Boltzmann's formula, S = k ln W?
    1. the degeneracy of the state
    2. the equilibrium constant for the process
    3. the universal gas constant divided by Avogadro's number
    4. the universal gas constant times Avogadro's number

18. The entropy change associated with the expansion of one mole of an ideal gas from an initial volume of Vi to a final volume of Vf at constant temperature is given by the equation, ΔS = R ln (Vf/Vi). What is the entropy change associated with the expansion of three moles of an ideal gas from an initial volume of Vi to a final volume of Vf at constant temperature?
    1. ΔS = R ln (Vf/Vi)
    2. ΔS = 3 mol × R ln (Vf/Vi)
    3. ΔS = R ln (Vf × 23/Vi)
    4. ΔS = R ln (Vf × 3!/Vi)

19. What is the entropy change associated with the expansion of one mole of an ideal gas from an initial volume of V to a final volume of V of 2.50V at constant temperature?
    1. ΔS = 2.50 R ln (Vf/Vi)
    2. ΔS = -2.50 R ln (Vf/Vi)
    3. ΔS = R ln 2.50
    4. ΔS = -R ln 2.50

20. Predict the sign of ΔS for each of the following processes, which occur at constant temperature.

1. The volume of 2.0 moles of O2(g) increases from 44 L to 52 L.
2. The pressure of 2.0 moles of O2(g) increases from 1.0 atm to 1.2 atm.
   1. I: ΔS= negative; II: ΔS= negative
   2. I: ΔS= negative; II: ΔS= positive
   3. I: ΔS= positive; II: ΔS= negative
   4. I: ΔS= positive; II: ΔS= positive

21. Assume a heteronuclear diatomic molecule, AB, forms a one-dimensional crystal by lining up along the x-axis. Also assume that each molecule can only have one of six possible orientations, corresponding to atom A facing in either the positive or negative direction along the x-, y-, or z-axis. If the molecules are arranged randomly in the six directions, the molar entropy at absolute zero should be
    1. R ln 6.
    2. R ln 66.
    3. R ln 6!
    4. 0.

22. The Boltzmann formula is S = k ln W. A perfect crystal has a molar entropy of 0 at absolute zero because
    1. W = 0.
    2. W = 1.
    3. W = NA.
    4. k = 1.

23. What is the sign of ?S for each of the following processes?

1. The separation of gaseous molecules of UF6, into 238UF6 and 235UF6 at constant

temperature and pressure.

2. The dissolving of I2(s) in CCl4(l).
   1. ΔS is negative for I and negative for II.
   2. ΔS is negative for I and positive for II.
   3. ΔS is positive for I and negative for II.
   4. ΔS is positive for I and positive for II.

24. Which has the lowest entropy?
    1. CH3OH(s, –25°C)
    2. CH3OH(s, –15°C)
    3. CH3OH(l, 15°C)
    4. CH3OH(l, 25°C)

25. Which has the highest entropy in each set?

I.     H2O(s), H2O(l), H2O(g) at 0.1°C, 4.58 atm

II. H2O(l) at 0°C, H2O(l) at 25°C, H2O(l) at 100°C (all at 1.0 atm pressure)

1. 1. H2O(l) in set I and H2O(l) at 0°C in set II
   2. H2O(s) in set I and H2O(l) at 100°C in set II
   3. H2O(g) in set I and H2O(l) at 0°C in set II
   4. H2O(g) in set I and H2O(l) at 100°C in set II

26. Which provides the greatest increase in entropy? A) H2O (s, 0°C) → H2O (l, 0°C)

B) H2O (l, 0°C) → H2O (l, 25°C)

C) H2O (g, 0.1°C) → H2O (s, 0.1°C)

D) H2O (l, 100°C) → H2O (g, 100°C)

27. According to the third law of thermodynamics,
    1. energy is conserved in any transformation of matter.
    2. the entropy increases for any spontaneous process.
    3. the entropy of a perfectly ordered, crystalline substance is zero at 0 Kelvin.
    4. the entropy of the universe increases for any spontaneous process.

28. Which of the following statements must be true for the entropy of a pure solid to be zero?

1. The temperature must be 0 K.
2. The solid must be crystalline, not amorphous.
3. The solid must be perfectly ordered.
4. The solid must be an element.
   1. I
   2. I and II
   3. I, II, and III
   4. I, II, III, and IV

29. Under which of the following conditions would one mole of Ne have the highest entropy, S?
    1. 27°C and 25 L
    2. 137°C and 25 L
    3. 27°C and 35 L
    4. 137°C and 35 L

30. Which has the highest standard molar entropy at 25°C?
    1. Al(s)
    2. Al(l)
    3. Al(g)
    4. All three should have a standard molar entropy of zero.

31. Which has the highest standard molar entropy at 25°C?
    1. F2(g)
    2. Cl2(g)
    3. Br2(g)
    4. I2(g)

32. Which of the following gas molecules has the greatest standard molar entropy at 25°C?
    1. C2H2
    2. CH2CH2
    3. CH3CH3
    4. All have the same entropy.

33. Which substance has the highest standard molar entropy at 25°C ?
    1. C(graphite)
    2. C2H4(g)
    3. CH3OH(l)
    4. MgCO3(s)

34. Which one of the following has the lowest standard molar entropy, S°, at 25°C?"
    1. C8H18(s)
    2. C8H18(l)

C) C12H26(s)

D) C12H26(l)

35. Calculate ΔS° for the following reaction. N2(g) + 2 O2(g) → 2 NO2(g)

A) -156.5 J/K

B) -121.5 J/K

C) 15.5 J/K

D) 636.5 J/K

36. ΔS° = – 198.7 J/K for the reaction shown below. Calculate S° for NH3(g). N2(g) + 3 H2(g) → 2 NH3(g)

1. 1. 61.7 J/K?mol
   2. 123.4 J/K?mol
   3. 192.3 J/K?mol
   4. 384.6 J/K?mol

37. Calculate ΔS° for the formation of one mole of solid sodium bromide from the elements at 25°C.

A) -116.7 J/K

B) -81.2 J/K

C) -40.5 J/K

D) 86.8 J/K

38. The standard molar entropy for Br2(g) is 245.46 J/(mol ? K) at 25°C. Given that ΔS° =

104.58 J/K for the dissociation of one mole of Br2(g) into Br(g) at 25°C, find the standard molar entropy for Br(g) at 25°C.

A) 70.44 J/(mol ? K)

B) 140.08 J/(mol ? K)

C) 175.02 J/(mol ? K)

D) 350.04 J/(mol ? K)

39. Which of the three laws of thermodynamics provides a criterion for spontaneity?
    1. the first law of thermodynamics
    2. the second law of thermodynamics
    3. the third law of thermodynamics
    4. both the second and third laws of thermodynamics

40. Which of the following is a criterion for spontaneity that holds for any process?
    1. ΔG < 0
    2. ΔG > 0
    3. ΔStotal < 0
    4. ΔStotal > 0

41. According to the second law of thermodynamics, all reactions proceed spontaneously in the direction that increases the entropy of the
    1. surroundings.
    2. system.
    3. system – surroundings
    4. system + surroundings

42. For a process to be at equilibrium, it is necessary that
    1. ΔSsys = ΔSsurr.
    2. ΔSsys = - ΔSsurr.
    3. ΔSsys = 0.
    4. ΔSsys = 0 and ΔSsurr = 0.

43. For a spontaneous process
    1. energy and entropy are conserved.
    2. energy is conserved and the entropy of the system and surroundings increases.
    3. the energy of the system and the surroundings decreases and the entropy of the system and surroundings increases.
    4. both the energy and the entropy of the system and surroundings decrease.

44. For the process

CaCO3(calcite) → CaCO3(aragonite)                        ΔH° = -0.21 kJ, ΔS° = -4.2 J/K Assuming that the surroundings can be considered a large heat reservoir at 25°C, calculate ΔSsurr and ΔStotal for the process at 25°C and 1 atm pressure. Is the process spontaneous at 25°C and 1 atm pressure?

1. 1. ΔSsurr = 4.2 J/K, Δtotal = 0, not spontaneous
   2. ΔSsurr = 0.7 J/K, ΔStotal = -3.5 J/K, not spontaneous
   3. ΔSsurr = -0.7 J/K, ΔStotal = -4.9 J/K, spontaneous
   4. ΔSsurr = -0.7 J/K, ΔStotal = -4.9 J/K, not spontaneous

45. During perspiration,
    1. the entropy of the water evaporated decreases and the entropy of the body decreases.
    2. the entropy of the water evaporated decreases and the entropy of the body increases.
    3. the entropy of the water evaporated increases and the entropy of the body decreases.
    4. the entropy of the water evaporated increases and the entropy of the body increases.

46. A hot penny is dropped into cold water inside a polystyrene foam cup. Assuming negligible heat loss to the atmosphere and the cup,
    1. the decrease in entropy of the penny is equal to the increase in entropy of the water.

| ΔSpenny | = | ΔSwater |

1. 2. the decrease in entropy of the penny is less than the increase in entropy of the water.

| ΔSpenny | < | ΔSwater |

1. 3. the decrease in entropy of the penny is more than the increase in entropy of the water.

| ΔSpenny | > | ΔSwater |

1. 4. the entropy of both the penny and the water increases.