Deck 13: Chemical Thermodynamics

Question 1

Many of the following questions assume that a truncated table of standard-state thermodynamic data is attached to the exam. The table usually contains data for 6-10 compounds, which pertain to the particular questions on that exam.

-For which of the following processes would $\Delta$ S be negative?

A) the melting of ice
B) the freezing of water
C) the evaporation of a liquid
D) the breakup of a large molecule into smaller molecules
E) these all have positive $\Delta$ S values

Answer 1

the freezing of water

Question 2

Many of the following questions assume that a truncated table of standard-state thermodynamic data is attached to the exam. The table usually contains data for 6-10 compounds, which pertain to the particular questions on that exam.

-Which of the following processes would have a negative entropy change?

A) decomposition of hydrogen peroxide
B) conversion of dry ice to gaseous CO₂
C) melting of ice
D) formation of Al₂O₃ from its elements
E) dissolution of salt in water

Answer 2

formation of Al₂O₃ from its elements

Question 3

In any spontaneous process if the entropy change of the system is negative, the entropy change of the surroundings must be:

A) positive
B) negative
C) may be either positive or negative
D) must be zero

Answer 3

positive

Question 4

The second law of thermodynamics states that $\Delta$ S_universe is positive for a spontaneous process. However, our bodies are constantly carrying out spontaneous reactions in which simple molecules and compounds are used to make more complex ordered molecules such as proteins as well as more complex structures such as cells and tissues. According to the second law of thermodynamics how can these reactions be taking place spontaneously?

Question 5

Which of the following reactions would have a positive value for $\Delta$ S?

A) 3 NO(g) $\rightarrow$ NO₂(g) + N₂O(g)
B) 2 CO₂(g) $\rightarrow$ 2 CO(g) + O₂(g)
C) 2 I(g) $\rightarrow$ I₂(g)
D) NH₃(g) $\rightarrow$ NH₃(l)
E) None of these reactions would have a positive $\Delta$ S.

Question 6

Which of the following reaction would be more likely to have a positive value for $\Delta$ S°?

A) 2 H₂O(g) $\rightarrow$ 2 H₂(g) + O₂(g)
B) N₂(g) + 3 H₂(g) $\rightarrow$ 2 NH₃(g)
C) CO₂(g) + H₂(g) $\rightarrow$ CO(g) + H₂O(g)
D) H₂O(l) $\rightarrow$ H₂O(s)
E) None of these reactions would have a positive $\Delta$ S.

Question 7

Which of the following reactions would have a negative $\Delta$ S°?

A) H₂(g) $\rightarrow$ 2 H(g)
B) H₂(l) $\rightarrow$ H₂(g)
C) H₂(g) + Cl₂(g) $\rightarrow$ 2 HCl(g)
D) 2 H₂(g) + CO(g) $\rightarrow$ CH₃OH(g)
E) none of the above

Question 8

Without using thermodynamic tables, predict the signs of $\Delta$ H ° and $\Delta$ S ° for the following reaction. CCl₄(l) $\rightarrow$ CCl₄(g)

A) $\Delta$ H ° > 0 and $\Delta$ S ° > 0
B) $\Delta$ H ° < 0 and $\Delta$ S ° > 0
C) $\Delta$ H ° > 0 and $\Delta$ S ° < 0
D) $\Delta$ H ° < 0 and $\Delta$ S ° < 0
E) none of these

Question 9

Which of the following reactions will most likely have a positive $\Delta$ S°?

A) CO(g) + 2H₂(g) $\rightarrow$ CH₃OH(l)
B) NaCl(s) $\rightarrow$ Na⁺(aq) + Cl ^- (aq)
C) CH₄(g) + 2 O₂(g) $\rightarrow$ CO₂(g) + 2 H₂O(l)
D) 3H₂(g) + N₂(g) $\rightarrow$ 2 NH₃(g)
E) none of these

Question 10

Carbonated beverages can be made by bubbling carbon dioxide gas through the aqueous beverage and allowing the carbon dioxide to dissolve. Predict the signs of $\Delta$ H and $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 for the following reaction without using tables.
CO₂(g) $\rightarrow$ CO₂(aq)

A) $\Delta$ H 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = +, $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = +
B) $\Delta$ H 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = - , $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = +
C) $\Delta$ H 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = - , $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = -
D) $\Delta$ H 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = +, $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = 0
E) $\Delta$ H 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = 0, $\Delta$ S 11ee9e79_dbe5_b5a7_9fac_f7a63aa32526_TB9692_11 = -

Question 11

The value of $\Delta$ S for this reaction is:

2 Fe³⁺(aq) + Cu(s) $\rightarrow$ 2 Fe²⁺(aq) + Cu²⁺(aq)

A) between -400 and -200 J/mol_rxn-K
B) between -200 and 0 J/mol_rxn-K
C) between 0 and 200 J/mol_rxn-K
D) between 200 and 400 J/mol_rxn-K
E) more than 400 J/mol_rxn-K

Question 12

Use the $\Delta$ S_ac ° values for tin metal ( $\Delta$ S_ac ° = -124.35 J/mol_rxn-K), chlorine gas ( $\Delta$ S_ac ° = -107.33 J/mol_rxn-K), and tin tetrachloride ( $\Delta$ S_ac ° = -570.7 J/mol_rxn-K) to calculate $\Delta$ S ° for the following reaction.
Sn(s) + 2 Cl₂(g) $\rightarrow$ SnCl₄(l)

A) -339.0
B) -231.7
C) 231.7
D) 339.0
E) none of the above

Question 13

Use the following data to calculate $\Delta$ S for the reaction:

A) -198.8 J/mol_rxn-K
B) -91.26 J/mol_rxn-K
C) -106.22 J/mol_rxn-K
D) 91.26 J/mol_rxn-K
E) 198.8 J/mol_rxn-K

Question 14

If a reaction is spontaneous, what can be said about the reverse reaction under the same conditions?

A) The reverse reaction is nonspontaneous.
B) The reverse reaction is spontaneous also.
C) The reverse reaction is at equilibrium.
D) This cannot be determined.

Question 15

Given that $\Delta$ H ° for the reaction shown below is 92.2 kJ/mol_rxn, calculate $\Delta$ G ° at 25 ° C.
2 NH₃(g) $\rightarrow$ N₂(g) + 3 H₂(g)

A) -106.5 kJ/mol_rxn
B) -33.0 kJ/mol_rxn
C) 33.0 kJ/mol_rxn
D) 106.5 kJ/mol_rxn
E) none of these

Question 16

For which of the following highly exothermic processes would you expect $\Delta$ H ° and $\Delta$ G ° to be about the same?

A) 2 Al(s) + 3/2 O₂(g) $\rightarrow$ Al₂O₃(s)
B) 2 H₂(g) + O₂(g) $\rightarrow$ 2 H₂O(g)
C) 2 Na(s) + 2 H₂O(l) $\rightarrow$ 2 NaOH(aq) + H₂(g)
D) 2 NO(g) $\rightarrow$ N₂O₄(g)
E) 2 Al(s) + Fe₂O₃(s) $\rightarrow$ 2 Fe(s) + Al₂O₃(s)

Question 17

Which of the following combinations of $\Delta$ H ° and $\Delta$ S ° indicates a nonspontaneous reaction under all conditions of temperature?

A) $\Delta$ H ° > 0 and $\Delta$ S ° > 0
B) $\Delta$ H ° < 0 and $\Delta$ S ° > 0
C) F $\Delta$ H ° > 0 and $\Delta$ S ° < 0
D) $\Delta$ H ° < 0 and $\Delta$ S ° < 0
E) none of these

Question 18

Under which one of the following conditions will the following reaction be spontaneous as written?

A) spontaneous at all temperatures
B) spontaneous at high temperatures
C) spontaneous at low temperatures
D) not spontaneous at any temperature

Question 19

Which of the following combinations of $\Delta$ H^o and $\Delta$ S^o can result in a reaction changing from nonspontaneous to spontaneous when the temperature increases?

A) $\Delta$ H^o < 0 and $\Delta$ S^o > 0
B) $\Delta$ H^o < 0 and $\Delta$ S^o < 0
C) $\Delta$ H^o = 0 and $\Delta$ S^o < 0
D) $\Delta$ H^o = 0 and $\Delta$ S^o > 0
E) $\Delta$ H^o > 0 and $\Delta$ S^o > 0

Question 20

Which statement correctly describes the following reaction?

A) The reaction is spontaneous at all temperatures.
B) The reaction is spontaneous only below about 300 K.
C) The reaction becomes spontaneous at about 300 K.
D) The reaction becomes spontaneous at about 560 K.
E) The reaction is spontaneous only above 2000 K.

Question 21

The entropy change in the process: 2 H(g) $\rightarrow$ H₂(g) is -98.74 J/mol_rxnK. The enthalpy of atom combination of the H₂ molecule is 435 kJ/mol_rxn. Estimate the approximate temperature above which the formation of H₂ molecules from H atoms will no longer be a spontaneous process.

A) 500 K
B) 1250 K
C) 2300 K
D) 4400 K
E) 6500 K

Question 22

Which of the following relationships between K_p and K_c is correct for the reaction
N₂(g) + 3 H₂(g) $\rightarrow$ 2 NH₃(g)
When the reaction is carried out at 100 ℃?

A) K_p = K_c
B) K_p < K_c
C) K_p > K_c
D) K_p < 0, K_c < 0

Question 23

The K_p for the following equation is 3.2 x 10 ^{- 3}. Calculate the value of K_c at 25 o C.
COCl₂(g) $\rightarrow$ CO(g) + Cl₂(g)

A) 7.8 x 10 ^{- 2}
B) 6.6 x 10 ^{- 3}
C) 1.6 x 10 ^{- 3}
D) 1.3 x 10 ^{- 4}
E) none of these

Question 24

Assume that K_p = 1.75 for the reaction.
CO₂(g) + C(s) 2 CO(g)
Calculate K_p for the following reaction.
CO(g) 1/2 CO₂(g) + 1/2 C(s)

A) 0.756
B) 1.32
C) 1.75
D) 3.06
E) none of the above

Question 25

The composition of the following system at equilibrium at 1000 K is 0.202 atm CO₂, 0.594 atm CO, and 5 kg C
2 CO(g) CO₂(g) + C(s)
What is the equilibrium constant for this reaction?

A) 2.94
B) 14.56
C) 0.572
D) 0.337
E) none of the above

Question 26

Given the equilibrium constant for the following reaction,
H₂(g) + Cl₂(g) 2 HCl(g)   K_p = 2.5 x 10⁴
If equimolar amounts of H₂ and Cl₂ are allowed to reach equilibrium in a system in which no HCl was present initially, the ratio of the partial pressures of H₂ to Cl₂ at equilibrium will be

A) larger than 1.
B) smaller than 1.
C) equal to 1.
D) a value that can't be generalized by any of the above statements.

Question 27

Which statement concerning the following reaction isn't true?
2 HI(g) H₂(g) + I₂(g)

A) At equilibrium, the concentrations of the three gases do not change with time.
B) K_p = K_c
C) The partial pressures of I₂ and H₂ are equal at equilibrium, regardless of the initial pressures of HI, H₂, and I₂.
D) HI decomposes as fast as it is formed once equilibrium is established.
E) The percent dissociation of HI does not depend on the total pressure.

Question 28

The standard free energy change for the evaporation of water is found to be +8.56 kJ/mol_rxn.
H₂O(l) $\rightarrow$ H₂O(g)
This says that the evaporation of water is nonspontaneous under standard conditions. If a beaker of water is set out on a table in the room why will it spontaneously evaporate?

Question 29

Which of the following statements is true?

A) $\Delta$ G = $\Delta$ G o when the reaction is at equilibrium.
B) $\Delta$ G is a measure of how far the reaction is from equilibrium.
C) $\Delta$ G o is positive for reactions that have too much reactant in the standard state.
D) The value of $\Delta$ G does not depend on the temperature of the reaction.
E) None of these statements are true.

Question 30

Which of the following statements is false?

A) $\Delta$ G is equal to $\Delta$ G o when the system is at the standard state.
B) $\Delta$ G is zero when the system is at equilibrium.
C) $\Delta$ G measures how far the reaction is from equilibrium.
D) When $\Delta$ G is positive, the reaction should proceed forward to form more product.
E) All of the above statements are true.

Question 31

If the following reaction is spontaneous as written, under standard conditions,
Zn(s) + Cu²⁺(aq) $\rightarrow$ Zn²⁺(aq) + Cu(s)
Which of the following statements is true?

A) K_c > 1 and $\Delta$ G° > 0
B) K_c > 1 and $\Delta$ G° < 0
C) K_c < 1 and $\Delta$ G° < 0
D) K_c > 1 and $\Delta$ G° = 0
E) K_c < 1 and $\Delta$ G° = 0

Question 32

For the reaction, 2 SO₃(g) $\rightarrow$ 2 SO₂(g) + O₂(g), the following thermodynamic data are available.

What is the equilibrium constant for this reaction at 25 ° C?

A) between 10^-50 and 10^-30
B) between 10^-30 and 10^-20
C) roughly 1
D) between 10²⁰ and 10³⁰
E) between 10³⁰ and 10⁵⁰

Question 33

The equilibrium constant for this reaction at 25°C is:

A) between 10-30 and 10-16
B) between 10-16 and 10-10
C) between 10-10 and 106
D) between 106 and 1016
E) larger than 1016

Question 34

$\Delta$ G° = -53.6 kJ/mol_rxn for the following reaction at 25°C.
Zn²⁺(aq) + 4 NH₃(aq) $\rightarrow$ Zn(NH₃)₄²⁺(aq)
What is the value of the complex formation equilibrium constant for this reaction?

A) 5 x 10^-21
B) 4 x 10^-10
C) 2.5 x 10⁹
D) 2.0 x 10²⁰
E) 1.2 x 10²³

Question 35

Use the following information to answer this question.
H₂(g) + F₂(g) $\rightarrow$ 2 HF(g)   $\Delta$ G ° = -190 kJ/mol_rxn
H₂(g) + Cl₂(g) $\rightarrow$ 2 HCl(g)   $\Delta$ G ° = -546 kJ/mol_rxn
H₂(g) + Br₂(g) $\rightarrow$ 2 HBr(g)   $\Delta$ G ° = -106 kJ/mol_rxn
H₂(g) + I₂(g) $\rightarrow$ 2 HI(g)   $\Delta$ G ° = 4 kJ/mol_rxn
An excess of H₂ is allowed to react with equal amounts of F₂, Cl₂, Br₂, and I₂ in a sealed flask. Which of the following products is present in the largest concentration at equilibrium?

A) HF
B) HCl
C) HBr
D) HI
E) All four products are present in equal concentrations.

Question 36

What is the equilibrium constant for the following reaction if $\Delta$ G° at 298 K is -16.5 kJ/mol_rxn?
N₂(g) + 3 H₂(g) $\rightarrow$ 2 NH₃(g)

A) 2.0 x 10^-5
B) 1.3 x 10^-3
C) 780
D) 4.6 x 10⁶
E) 220

Question 37

K_a for benzoic acid (C₆H₅CO₂H) is 3.0 x 10^-5. What is the free energy change for the dissociation of this acid at 25°C?

A) -25.8 kJ/mol_rxn
B) -2.2 kJ/mol_rxn
C) 2.2 kJ/mol_rxn
D) 25.8 kJ/mol_rxn
E) none of the above

Question 38

Which statement is true if the following reaction is spontaneous as written?
Zn(s) + Cu²⁺(aq) $\rightarrow$ Zn²⁺(aq) + Cu(s)

A) K_c is larger than 1, $\Delta$ G° is negative, and E° is positive.
B) K_c is larger than 1, $\Delta$ G° is positive, and E° is negative.
C) K_c is larger than 1, $\Delta$ G° is negative, and E° is negative.
D) K_c is smaller than 1, $\Delta$ G° is negative, and E° is positive.
E) K_c is smaller than 1, $\Delta$ G° is positive, and E° is negative.

Question 39

What happens to the magnitude of the equilibrium constant for the following reaction as the temperature of the system increases?

A) K_c first increases and then decreases.
B) K_c first decreases and then increases.
C) K_c increases.
D) K_c decreases.
E) It is impossible to determine what happens to K_c.

Question 40

Which of the following statements is true about $\Delta$ G ° , and K for the following reaction when the temperature is increased?

A) As temperature increases, $\Delta$ G ° will increase (become more positive), and K will increase .
B) As temperature increases, $\Delta$ G ° will decrease (become more negative), and K will decrease.
C) As temperature increases, $\Delta$ G ° ncreases (become more positive) and K decreases.
D) As temperature increases, $\Delta$ G ° decreases (becomes more negative) and K increases.
E) None of these are true.

Question 41

Although the diatomic molecule N₂ has an extremely strong nitrogen/nitrogen triple bond, it will break apart into nitrogen atoms
(N₂(g) $\rightarrow$ 2 N(g)) if it is heated to a high enough temperature because

A) the value of $\Delta$ G becomes more positive as the temperature increases.
B) the unfavorable enthalpy change for the reaction disappears at higher temperatures.
C) the entropy change for the reaction is favorable and this factor becomes more important as the temperature of the system increases.
D) the equilibrium constant for the reaction decreases with temperature.
E) none of the above statements is a correct explanation.

Question 42

Calculate the standard free energy change, $\Delta$ G ° , and standard entropy change, $\Delta$ S ° , for the following reaction at 298 K.
Cl₂(g) + 3 F₂(g) $\rightarrow$ 2ClF₃(g)

A) -123 kJ/mol_rxn, -134 J/mol_rxn-K
B) -246 kJ/mol_rxn, -268 J/mol_rxn-K
C) -326 kJ/mol_rxn, -268 J/mol_rxn-K
D) -51 kJ/mol_rxn, -586 J/mol_rxn-K
E) -148 kJ/mol_rxn, -134 J/mol_rxn-K

Question 43

Calculate the standard free energy, $\Delta$ G ° , in kJ/mol_rxn for the reaction below.

A) -1232.0 kJ/mol_rxn
B) 1232.0 kJ/mol_rxn
C)-478.9 kJ/mol_rxn
D) 478.9 kJ/mol_rxn
E) none of these

Question 44

Calculate the standard enthalpy change, $\Delta$ H ° , and standard entropy change, $\Delta$ S ° , for the following reaction at 298 K.

A) 93.45 kJ/mol_rxn, 32.5 J/mol_rxn-K
B) 32.5 kJ/mol_rxn, 93.45 J/mol_rxn-K
C) -32.5 kJ/mol_rxn, 93.45 J/mol_rxn-K
D) -32.5 kJ/mol_rxn, -93.45 J/mol_rxn-K
E) -93.45 kJ/mol_rxn, 32.5 J/mol_rxn-K

Question 45

For each of the following variables what is the appropriate sign that is favorable for a spontaneous reaction?

A) $\Delta$ H ° = +, $\Delta$ S ° = +, $\Delta$ G ° = +, E ° = +
B) $\Delta$ H ° = -, $\Delta$ S ° = +, $\Delta$ G ° = +, E° = +
C) $\Delta$ H ° = +, $\Delta$ S ° = +, $\Delta$ G ° = - , E ° = +
D) $\Delta$ H ° = +, $\Delta$ S ° = +, $\Delta$ G ° = +, E ° = -
E) $\Delta$ H ° = -, F $\Delta$ S ° = +, $\Delta$ G ° = -, E °F +

Question 46

In chapter 12 you used E^o to predict when a redox reaction would be spontaneous. In chapter 13 $\Delta$ G^o was used to predict if a reaction was spontaneous. Which of the following describes the relationship between $\Delta$ G^o and E^o?

A) when $\Delta$ G^o is positive, E^o is negative
B) when $\Delta$ G^o is positive, E^o is positive
C) when $\Delta$ G^o is negative, E^o is negative
D) when $\Delta$ G^o is negative, E^o is zero
E) There is no relationship between $\Delta$ G^o and E^o.

Question 47

The equilibrium constant for the following reaction is K_p = 4.1 x 10^-3
COCl₂(g) CO(g) + Cl₂(g)
Calculate the equilibrium pressure of CO in a closed vessel that initially contained 0.30 atm of COCl₂ and no CO or Cl₂.

Question 48

The equilibrium constant for the following reaction is K_p = 1.0 x 10⁴.
2 NO(g) + O₂(g) 2 NO₂(g)
Calculate the equilibrium pressure of oxygen in a system that initially contains 0.50 atm of NO, 0.10 atm of O₂, and 0.20 atm of NO₂.

Question 49

What is the value of $\Delta$ S o per mole of CH₃OH produced in this reaction?

A) -219.2 J/mol_rxnK
B) -164.1 J/mol_rxnK
C) -88.4 J/mol_rxnK
D) 88.4 J/mol_rxnK
E) 219.2 J/mol_rxnK

Question 50

What is F $\Delta$ G o per mole of CH₃OH produced in this reaction at 25 o C?

A) -155.5 kJ/mol_rxn
B) -24.8 kJ/mol_rxn
C) +24.8 kJ/mol_rxn
D) 155.5 kJ/mol_rxn
E) none of the above

Question 51

Which of the following statements about this reaction is true?

A) The reaction can never become favorable.
B) The reaction can never become unfavorable.
C) The reaction will become more favorable as the temperature is increased.
D) The reaction will become less favorable as the temperature is increased.
E) Temperature has no effect on whether this reaction is favorable.

Question 52

K_p = 2.1 x 10⁶ for the following reaction at a given temperature.
2 NH₃(g) N₂(g) + 3 H₂(g)
Starting with ammonia at 4.00 atm in a fixed-volume container, calculate the equilibrium pressures of all three gases. Justify any approximations used in solving this problem.

Question 53

The following equilibrium is established in the gas phase at 600 K by heating phosgene, COCl₂.
COCl₂(g) CO(g) + Cl₂(g) K_p = 4.10 x 10^-3
If the initial pressure of phosgene is 0.120 atm, what is the equilibrium pressure of Cl₂?

A) 4.9 x 10^-4 atm
B) 2.0 x 10^-2 atm
C) 2.2 x 10^-2 atm
D) 0.10 atm
E) none of these

Question 54

The equilibrium constant for the following gas-phase reaction is 68.0.
2 IBr(g) I₂(g) + Br₂(g)
At equilibrium, twice as much Br₂ is present as IBr and the partial pressure of I₂ is 1 atm. What is the equilibrium partial pressure of IBr?

A) 0.0074 atm
B) 0.0037 atm
C) 0.029 atm
D) 0.059 atm
E) 136 atm

Question 55

The equilibrium constant for the reaction of NO(g) with Cl₂(g) to produce NOCl(g) is 52. If 0.50 atm of NO and 0.25 atm of Cl₂ react to equilibrium in a closed system that initially contained no NOCl, calculate the partial pressures of all three components at equilibrium.

Question 56

At a fixed temperature and 1.00 atm total pressure, NO₂ gas and N₂O₄ gas are in equilibrium.
2 NO₂(g) N₂O₄(g)
What is the effect of increasing the total pressure to 10.0 atm at a constant temperature?

A) K_p increases, but the partial pressure of N₂O₄ decreases.
B) K_p decreases, but the partial pressure of N₂O₄ increases.
C) Both K_p and the partial pressure of N₂O₄ decrease.
D) Both K_p and the partial pressure of N₂O₄ increase.
E) K_p remains the same and the partial pressure of N₂O₄ increases.