Deck 17: Thermodynamics: Directionality of Chemical Reactions

Question 1

Five coins are tossed. Which combination of heads (H) and tails (T) is least likely?

A) THTHT
B) TTTHT
C) HTTHT
D) HHTHT
E) THHTT

Answer 1

TTTHT

Question 2

Which statement is correct?

A) A product-favored process requires a continuous input of energy to continue
B) A product-favored process may start on its own without much initial energy input
C) A product-favored process, when run in reverse, is also product-favored
D) Once started, a reactant-favored process will continue on its own
E) The term non-spontaneous is sometimes used to describe a product-favored process

Answer 2

A product-favored process may start on its own without much initial energy input

Question 3

Nature favors exothermic reactions because after such a reaction

A) the energy previously concentrated in a few particles is now dispersed over more particles in the system.
B) the energy previously concentrated in a few particles is now dispersed over more particles in the surroundings.
C) the energy previously concentrated in a few particles is now dispersed over more particles in both the system and the surroundings.
D) the energy previously held in many particles is now concentrated in a few, resulting in a temperature rise.
E) the energy previously held in many particles is now concentrated in a few, resulting in a temperature fall.

Answer 3

the energy previously concentrated in a few particles is now dispersed over more particles in both the system and the surroundings.

Question 4

Which statement is false?

A) entropy is a measurement of the nanoscale dispersal of energy in a sample of matter.
B) entropies of ionic solids with similar formulas are larger when the attractions among the ions are stronger
C) A perfect crystal of any substance at 0 K has minimum entropy.
D) The standard molar entropy of a substance at temperature T is a measure of the quantity of energy that must be dispersed in that substance for it to exist at T.
E) Temperatures of a few nanokelvins can be achieved in a Bose-Einstein condensate.

Question 5

Which has the highest entropy at a given temperature?

A) SO₂(s)
B) S(s)
C) O₂(g)
D) SO₂(l)
E) SO₂(g)

Question 6

Two substances, A and B, have identical enthalpies of vaporization. They boil at 164 ^$\circ$ C and 236 ^$\circ$ C, respectively. If the entropy of vaporization of A is 87.2 J/K, what is the entropy of vaporization of B, in J/K?

A) 102
B) 60.6
C) 126
D) 38.3
E) 74.9

Question 7

At a particular temperature, a reactant-favored process is one in which

A) reactants rapidly react with one another to form products
B) products rapidly react with one another to form reactants
C) products predominate over reactants
D) reactants predominate over products
E) large amounts of energy are produced

Question 8

Which statement is false?

A) entropies of gases are usually much larger that those of liquids, which in turn are usually larger than those of solids
B) entropy usually increases when a gas is dissolved in a liquid
C) entropies of more complex molecules are larger than those of simpler molecules
D) entropy usually increases when a pure liquid or solid dissolves in a solvent
E) entropies of ionic solids that have similar formulas are smaller when the attractions among the ions are stronger

Question 9

Which statement is false?

A) energy will disperse unless it is hindered from doing so
B) energy will not disperse unless it is forced to do so
C) energy has a higher probability of being spread out over many molecules than of being concentrated in a few
D) energy is dispersed when a system of atoms or molecules expands to occupy a larger volume
E) gases expanding into larger volumes illustrate energy dispersal

Question 10

How many of the following processes could be described as product-favored: dissolving the first spoonful of sugar in a cup of hot coffee; alcohol separating from a beer sample at 5 ^$\circ$ C; dissolving the ninth spoonful of sugar in a cup of hot coffee; sediment settling out of an muddy water sample?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 11

Consider the options given for the distribution of three units of energy to three particles. Which arrangement is least likely?

A) A*B*C*
B) A**B*
C) A**C*
D) A***
E) B**C*

Question 12

DS ^$\circ$ for a reaction can be calculated from the following equation.

A) DS ^$\circ$ = (Smoles product) ×S ^$\circ$ _product - (Smoles reactant) ×S ^$\circ$ _reactant
B) DS ^$\circ$ = S(moles product × S ^$\circ$ _product) - S(moles reactant × S ^$\circ$ _reactant)
C) DS ^$\circ$ = S(moles reactant × S ^$\circ$ _reactant) - S(moles product × S ^$\circ$ _product)
D) DS ^$\circ$ = S(moles product × S ^$\circ$ _reactant) - S(moles reactant × S ^$\circ$ _product)
E) DS ^$\circ$ = S(moles product × S ^$\circ$ _product) + S(moles reactant × S ^$\circ$ _reactant)

Question 13

How many of the following processes involve a decrease in disorder: humidifying dry air; distilling crude oil into gasoline, fuel oil, and jet fuel; filtering solid impurities out of a mixture; adding sand to water?

A) 0
B) 1
C) 2
D) 3
E) 4

Question 14

Which has the lowest entropy at a given temperature?

A) H₂(g)
B) O₂(g)
C) H₂O(g)
D) H₂O(l)
E) H₂O(s)

Question 15

Which process involves a decrease of entropy for the system?

A) mixing aqueous solutions of NaCl and KNO₃ together
B) spreading grass seed on a lawn
C) raking and bagging leaves in the fall
D) shuffling a deck of cards
E) raking and burning leaves in the fall

Question 16

Which has the highest entropy?

A) H₂O(g) at 150 ^$\circ$ C
B) H₂O(g) at 100 ^$\circ$ C
C) H₂O(l) at 100 ^$\circ$ C
D) H₂O(l) at 4 ^$\circ$ C (the temperature of maximum density)
E) H₂O(s) at -50 ^$\circ$ C

Question 17

The boiling point of tin is 232 ^$\circ$ C. The heat of vaporization of tin at its boiling point is 247 kJ. The entropy of vaporization is

A) 2045 J/K.
B) 1065 J/K.
C) 939 J/K.
D) 489 J/K.
E) 2.04 J/K.

Question 18

Which statement is correct?

A) The entropy of a pure substance is greater in the liquid state than in the gaseous state.
B) The entropy of a pure substance in a given state decreases as the temperature rises.
C) Because of the larger charges, the entropy of CaO(s) is greater than the entropy of KF(s).
D) Entropy decreases when ethanol, C₂H₅OH, dissolves in water.
E) A bottle of carbonated water has lower entropy than the same masses of carbon dioxide gas and distilled water.

Question 19

Compression of a gas is

A) reactant-favored, since energy in the compressed gas is more dispersed.
B) reactant-favored, since matter in the compressed gas is more dispersed.
C) reactant-favored, since energy in the compressed gas is less dispersed.
D) product-favored, since energy in the compressed gas is less dispersed.
E) product-favored, since energy in the compressed gas is more dispersed.

Question 20

Which process is reactant-favored?

A) decomposition of iron ore into pure iron
B) the melting of ice at 25 ^$\circ$ C
C) diffusion of the odor of cooking food
D) the freezing of water at -5 ^$\circ$ C
E) dissociation of a strong acid in water

Question 21

A reaction cannot change between being product-favored and being reactant-favored when

A) DH ^$\circ$ _rxn < 0 and DS ^$\circ$ _rxn > 0
B) DH ^$\circ$ _rxn < 0 and DS ^$\circ$ _rxn < 0
C) DH ^$\circ$ _rxn > 0 and DS ^$\circ$ _rxn < 0
D) DH ^$\circ$ _rxn > 0 and DS ^$\circ$ _rxn > 0
E) DH ^$\circ$ _rxn = 0 and DS ^$\circ$ _rxn < 0

Question 22

A certain reaction has DH ^$\circ$ _rxn = +177.8 kJ, and DS ^$\circ$ _rxn = +160.5 J/K. Above what temperature does it become product-favored ?

A) 384 ^$\circ$ C
B) 630 ^$\circ$ C
C) 835 ^$\circ$ C
D) 1108 ^$\circ$ C
E) 1381 ^$\circ$ C

Question 23

At constant T and P, in which situation will the reaction be product-favored at high temperature but not at low temperature?

A) DH > 0 and DS < 0
B) DH > 0 and DS > 0
C) DH < 0 and DS < 0
D) DH < 0 and DS > 0
E) DH = 0 and DS < 0

Question 24

Calculate the value of DS ^$\circ$ for the reaction shown: At 25 ^$\circ$ C the values of entropy in J K^-1 mol^-1 are ammonia, 192.77; nitrogen, 191.61; and hydrogen, 130.68.

A) +969.19 J/K
B) +393.20 J/K
C) +390.88 J/K
D) +259.03 J/K
E) +198.11 J/K

Question 25

At constant T and P, in which of the following situations will the reaction never be product-favored?

A) DH > 0 and DS < 0
B) DH > 0 and DS > 0
C) DH < 0 and DS < 0
D) DH < 0 and DS > 0
E) none of these

Question 26

Use the data at 25 ^$\circ$ C given below to calculate the value of DG ^$\circ$ _rxn for the reaction shown when it takes place at 120 ^$\circ$ C.

A) -2004 kJ
B) -2046 kJ
C) -2055 kJ
D) -2073 kJ
E) -2083 kJ

Question 27

According to the Second Law of Thermodynamics,

A) any process during which the entropy of the system increases will be product-favored
B) any process during which the entropy of the universe increases will be product-favored
C) any process which is endothermic will be product-favored
D) the total amount of matter in the universe remains constant
E) heat will always be transferred from a cooler object to a warmer object

Question 28

At constant T and P, in which of the following situations will the reaction always be product-favored?

A) DH > 0 and DS < 0
B) DH > 0 and DS > 0
C) DH < 0 and DS < 0
D) DH < 0 and DS > 0
E) there are no examples of reactions that are always product-favored

Question 29

Which statement is not correct?

A) the aqueous hydrogen ion (H⁺) is the only species for which DH ^$\circ$ _f, S ^$\circ$ , and DG ^$\circ$ _f each have a value of zero at 25 ^$\circ$ C
B) DH ^$\circ$ _f = 0 for each of the elements in its standard state at 25 ^$\circ$ C
C) DG ^$\circ$ _f = 0 for each of the elements in its standard state at 25 ^$\circ$ C
D) S ^$\circ$ ¹ 0 for each of the elements in its standard state at 25 ^$\circ$ C
E) DH ^$\circ$ _f, S ^$\circ$ , and DG ^$\circ$ _f each have a value of zero for each of the elements in its standard state at 25 ^$\circ$ C

Question 30

A reaction is exothermic and has a negative value of DS ^$\circ$ . The value of DG ^$\circ$ for this reaction is therefore:

A) negative at all temperatures.
B) positive at all temperatures.
C) positive above 0 ^$\circ$ C and negative below 0 ^$\circ$ C.
D) positive above a certain temperature and negative below it.
E) negative above a certain temperature and positive below it.

Question 31

Use the data given to calculate the value of DG ^$\circ$ _rxn for the reaction at 25 ^$\circ$ C

A) -75.2 kJ
B) -55.7 kJ
C) +32.5 kJ
D) +55.7 kJ
E) +75.2 kJ

Question 32

For a particular reaction, the value of DH ^$\circ$ = +98.8 kJ and DS ^$\circ$ = +141.5 J/K. This reaction is

A) product-favored, because DS ^$\circ$ _universe is positive
B) reactant-favored, because DS ^$\circ$ _universe is positive
C) reactant-favored, because DS ^$\circ$ _universe is negative
D) product-favored, because DS ^$\circ$ _universe is negative
E) cannot determine without further information

Question 33

At constant T and P, in which of the following situations will the reaction be product-favored at low temperature but not at high temperature?

A) DH > 0 and DS < 0
B) DH > 0 and DS > 0
C) DH < 0 and DS < 0
D) DH < 0 and DS > 0
E) all product-favored reactions proceed forward at all temperatures

Question 34

Use the data given to calculate the value of DG ^$\circ$ _rxn at 25 ^$\circ$ C for the reaction given below. DG ^$\circ$ _f for CO(g) is -137.16 kJ/mol.

A) -274.32 kJ
B) -137.16 kJ
C) -68.58 kJ
D) +137.16 kJ
E) +274.32 kJ

Question 35

Calculate the value of DS ^$\circ$ for the reaction shown: At 25 ^$\circ$ C the values of entropy in J K^-1 mol^-1 are nitrogen, 191.61: hydrogen, 130.68; and ammonia, 192.77.

A) -198.11 J/K
B) -259.03 J/K
C) -390.88 J/K
D) -393.20 J/K
E) -969.19 J/K

Question 36

A slight change in temperature can change the directionality of a reaction when

A) T = DH ^$\circ$ _rxn / DS ^$\circ$ _rxn
B) T = DS ^$\circ$ _rxn / DH ^$\circ$ _rxn
C) DG ^$\circ$ _rxn > 0
D) DG ^$\circ$ _rxn = 0
E) DG ^$\circ$ _rxn is a much larger positive value than DH ^$\circ$ _rxn

Question 37

If a reaction is product-favored at any temperature, then DH ^$\circ$ is ____ and DS ^$\circ$ is ____.

A) positive; positive
B) positive; negative
C) zero; positive
D) negative; positive
E) negative; negative

Question 38

For a specific chemical reaction, the value of DG ^$\circ$ calculated from tabulated values of the standard Gibbs free energy of formation, DG ^$\circ$ _f, of the reactants and products is valid at

A) all temperatures
B) standard temperature and pressure (STP)
C) 0 K
D) 273 K
E) the temperature specified for the tabulated values

Question 39

A specific chemical reaction must be product-favored if

A) DG ^$\circ$ _rxn > 0
B) DG ^$\circ$ _rxn = 0
C) DG ^$\circ$ _rxn < 0
D) DH ^$\circ$ _rxn > 0 and DS ^$\circ$ _rxn < 0
E) DH ^$\circ$ _rxn < 0 and DS ^$\circ$ _rxn < 0

Question 40

Use the data given to calculate the value of DG ^$\circ$ _rxn for the reaction at 25 ^$\circ$ C.

A) -291.4 kJ
B) -244.3 kJ
C) -226.8 kJ
D) -207.6 kJ
E) -64.6 kJ

Question 41

For the reaction shown, DG ^$\circ$ = -32.8 kJ at 25 ^$\circ$ C.
N2(g) + 3 H2(g) ? 2 NH3(g)
a. Calculate the equilibrium constant for this reaction at 25°C.
b. Is a mixture of the three gases where
P_N2 = 3.5 bar,
P_H2 = 1.2 bar, and
P_NH3 = 0.22 bar at equilibrium? Justify your answer.
c. What is the value of DG under the conditions of Part b?
d. Compare the answer from Part c with DG°. Explain the difference observed.

Question 42

It is possible for a substance, with respect to a given reaction, to

A) be kinetically and thermodynamically stable.
B) be kinetically stable but not thermodynamically stable.
C) be thermodynamically stable but not kinetically stable.
D) fall in either category a or category b.
E) fall in either category a or category c.

Question 43

Which statement correctly describes the meaning of the value of DG ^$\circ$ _rxn for a reactant-favored reaction?

A) It is the amount of energy that is required to overcome the disorder of the system.
B) It is the amount of additional energy that must be supplied in order to initiate reaction.
C) It is the minimum amount of heat released from the system when reaction occurs.
D) It is the maximum amount of useful work obtainable from the reaction when it occurs.
E) It is the minimum amount of work which must be done on the system to make reaction occur.

Question 44

If a chemical reaction is at equilibrium, it must be true that

A) DG ^$\circ$ = 1.
B) DG > 1.
C) DG ^$\circ$ < 1.
D) DG = 1.
E) DG = 0.

Question 45

Use the data given to calculate the value of K for the reaction at 25 ^$\circ$ C.

A) 6.61 × 10^-14
B) 3.41 × 10^-12
C) 4.75 × 10^-12
D) 1.76 × 10^-10
E) 5.69 × 10⁹

Question 46

Use the data given to calculate the value of K for the reaction at 5 ^$\circ$ C.

A) 1.9 × 10¹²
B) 4.5 × 10¹⁰
C) 5.7 × 10⁹
D) 1.3 × 10⁶
E) 1.0

Question 47

Which set of conditions describes a reaction that is least likely to proceed?

A) endothermic, decreasing entropy, high activation energy
B) exothermic, decreasing entropy, high activation energy
C) exothermic, increasing entropy, high activation energy
D) exothermic, increasing entropy, low activation energy
E) endothermic, decreasing entropy, low activation energy

Question 48

Recovering aluminum directly from its ore, which is primarily aluminum oxide, involves the following reaction, for which thermodynamic data is tabulated below:


a. Calculate DH°rxn and DS°rxn.
b. Explain how you could have predicted the signs of DH°rxn and DS°rxn without any calculations.
c. Without performing any further calculations, predict whether this reaction will be product-favored only above a certain temperature, or only below a certain temperature. Explain your answer.
d. Calculate the temperature alluded to in Part c.
e. Calculate DG° at this temperature.

Question 49

Since no chemical process is 100% efficient, some Gibbs free energy is always "wasted" and converted to ____ instead of doing useful work.

A) chemical energy
B) electricity
C) heat
D) light
E) mechanical energy

Question 50

Which set of conditions describes a reaction that is most likely to proceed?

A) endothermic, decreasing entropy, high activation energy
B) exothermic, decreasing entropy, high activation energy
C) exothermic, increasing entropy, high activation energy
D) exothermic, increasing entropy, low activation energy
E) endothermic, decreasing entropy, low activation energy

Question 51

What is the value of the equilibrium constant at 25 ^$\circ$ C for a reaction, if the value of DG ^$\circ$ _rxn is -47.8 kJ at 25 ^$\circ$ C?

A) 1.70
B) 6.88
C) 2.30 × 10²
D) 2.74 × 10⁵
E) 2.37 × 10⁸

Question 52

The value of DG ^$\circ$ _rxn at 45 ^$\circ$ C is -47.8 kJ. What is the value of the equilibrium constant for this reaction?

A) 1.14
B) 1.02
C) 1.05 × 10²⁴
D) 7.11 × 10⁷
E) 1.40 × 10⁹

Question 53

A reaction is product-favored when

A) K ^$\circ$ < 1 and DG ^$\circ$ > 0
B) K ^$\circ$ > 1 and DG ^$\circ$ < 0
C) K ^$\circ$ = 1 and DG ^$\circ$ = 0
D) K ^$\circ$ = 0 and DG ^$\circ$ = 0
E) K ^$\circ$ < 1 and DG ^$\circ$ = 0

Question 54

If two reactions are coupled, then

A) A product of one reaction is a reactant of the other, and the overall process is reactant-favored.
B) A product of one reaction is a reactant of the other, and the overall process is product-favored.
C) The heat released from an exothermic reaction is used in an endothermic reaction.
D) A product-favored reaction supplies the activation energy for another reaction.
E) The Gibbs free energy released from a product-favored reaction is released to the surroundings, causing an increase in entropy for a second reaction.

Question 55

The value of the equilibrium constant for a reaction is 2.65 × 10^-6 at 45 ^$\circ$ C. Calculate the value of DG ^$\circ$ _rxn at this temperature.

A) +47.4 kJ
B) +34.0 kJ
C) +14.8 kJ
D) +335 kJ
E) -14.8 kJ

Question 56

The enthalpy change for a reaction can be calculated using the equation
DH°_rxn It would be possible to use DS ^$\circ$ _f values to calculate DS ^$\circ$ _rxn in a similar manner; however, this is not done in practice.
a. Describe the procedure which is used instead.
b. Explain why this mathematical process is possible for entropy calculations but not for enthalpy calculations.