Question 1

What is the sign of ΔH (system) and ΔS (system) if a chemical reaction is spontaneous only at lower temperatures under standard conditions?
A) ΔH (system) is negative, and ΔS (system) is negative.
B) ΔH (system) is positive, and ΔS (system) is positive.
C) ΔH (system) is positive, and ΔS (system) is negative.
D) ΔH (system) is negative, and ΔS (system) is positive.
E) None of these

Accepted Answer

The answer of What is the sign of ΔH (system)...

Question 2

When a real gas is compressed from low pressure to high pressure, its temperature increases. Which of the following is true for ΔH and ΔS? A) ΔH < 0 and ΔS < 0 B) ΔH < 0 and ΔS > 0 C) ΔH > 0 and ΔS < 0 D) ΔH > 0 and ΔS > 0 E) ΔH < 0 and ΔS = 0

Accepted Answer

The answer of When a real gas is compressed from...

Question 3

Calculate ΔG° at 298 K for the reaction below. I₂(g) + Br₂(g) → 2IBr(g) Given: Δ_rH° = -11.6 kJ/mol-rxn; Δ_rS° = 12 J/K⋅mol-rxn at 298 K. A) -8.02 kJ/mol-rxn B) 15.2 kJ/mol-rxn C) -15.2 kJ/mol-rxn D) 3.59 × 10³ kJ/mol-rxn E) -3.59 × 10³ kJ/mol-rxn

Accepted Answer

ΔG° = ΔH° - TΔS°

ΔS° is given as 12 J/K⋅mol-rxn, which needs to be converted to kJ/K⋅mol-rxn.
ΔS° = 12 J/K⋅mol-rxn × (1 kJ/1000 J) = 0.012 kJ/K⋅mol-rxn

Substituting the values given:
ΔG° = (-11.6 kJ/mol-rxn) - (298 K)(0.012 kJ/K⋅mol-rxn) = -15.2 kJ/mol-rxn

Therefore, the answer is C, -15.2 kJ/mol-rxn.

Question 4

A change of state occurring in a system is accompanied by 60.7 kJ of heat that is transferred to the surroundings at a constant pressure and at a constant temperature of 300 K. Calculate the ΔS(surroundings) for the process.
A) -202 J/K
B) -60.7 kJ/K
C) 202 J/K
D) 60.7 kJ/K
E) 239 kJ/K

Accepted Answer

The answer of A change of state occurring in a...

Question 5

If a cube of ice at 0 °C is placed outside on a warm summer day, the ice will melt spontaneously. What are the signs of Δ_rH, Δ_rS, and Δ_rG for the process? A) Δ_rH < 0, Δ_rS > 0, and Δ_rG < 0 B) Δ_rH < 0, Δ_rS < 0, and Δ_rG < 0 C) Δ_rH < 0, Δ_rS > 0, and Δ_rG > 0 D) Δ_rH > 0, Δ_rS > 0, and Δ_rG < 0 E) Δ_rH > 0, Δ_rS < 0, and Δ_rG > 0

Accepted Answer

The melting of ice into water is an endothermic process (absorbs heat) so ΔH > 0. The disorder increases as solid ice turns into liquid water, so ΔS > 0. For a process to be spontaneous at constant temperature and pressure, ΔG must be < 0.

Question 6

For a chemical reaction, if Δ_rG° = 0, then _____. A) K > 0 B) K = 0 C) K < 0 D) K > 1 E) K = 1

Accepted Answer

If Δ_rG° = 0, then ΔG° = -RTlnK = 0, which means lnK = 0 and K = e^0 = 1. Therefore, K = 1.

Question 7

Which of the following is correct for the condensation of gaseous oxygen at -188 °C? (The normal boiling point of oxygen is -183 °C.) A) ΔH < 0, ΔS > 0, and ΔG > 0. B) ΔH < 0, ΔS < 0, and ΔG >0. C) ΔH > 0, ΔS < 0, and ΔG < 0. D) ΔH = 0, ΔS = 0, and ΔG < 0. E) ΔH > 0, ΔS > 0, and ΔG > 0.

Accepted Answer

Condensation is an exothermic process (releases heat), so ΔH < 0. It involves a gas becoming a liquid, which decreases the system's entropy (ΔS < 0). Since the process is spontaneous at -188 °C (below the boiling point), ΔG must be < 0, not > 0. Therefore, the correct statement should indicate ΔH < 0, ΔS < 0, and ΔG < 0, but none of the options correctly states ΔG < 0 for a spontaneous process. Given the options, B is the closest to being correct, acknowledging an error in the explanation regarding ΔG's sign based on the provided choices.

Question 8

For the reaction given below, ?H⁰ = -1516 kJ at 25°C and ?S⁰ = -432.8 J/K at 25°C. This reaction is spontaneous _____. SiH₄(g) + 2 O₂(g) ?SiO₂(s) + 2 H₂O $( \ell )$ A) only below a certain temperature B) only above a certain temperature C) at all temperatures D) only when entropy is zero E) None of these

Accepted Answer

The answer of For the reaction given below, ?H⁰ =...

Question 9

At what temperatures will a reaction be spontaneous if Δ_rH° = +117 kJ and Δ_rS° = -35 J/K? A) All temperatures below 94.1 K B) Temperatures between 12.7 K and 135 K C) All temperatures above 94.1 K D) The reaction will be spontaneous at any temperature. E) The reaction will never be spontaneous.

Accepted Answer

The answer of At what temperatures will a reaction be...

Question 10

While diluting concentrated sulfuric acid with water, the temperature of the solution increases rapidly. What are the signs of ΔrH, ΔrS, and ΔrG for the process? A) Δ_rH < 0, Δ_rS > 0, and Δ_rG < 0 B) Δ_rH < 0, Δ_rS < 0, and Δ_rG < 0 C) Δ_rH < 0, Δ_rS > 0, and Δ_rG > 0 D) Δ_rH > 0, Δ_rS > 0, and Δ_rG < 0 E) Δ_rH > 0, Δ_rS < 0, and Δ_rG > 0

Accepted Answer

The answer of While diluting concentrated sulfuric acid with water,...

Question 11

At what temperatures will a reaction be spontaneous if Δ_rH° = +62.4 kJ and Δ_rS° = +301 J/K? A) All temperatures below 207 K B) All temperatures above 207 K C) Temperatures between 179 K and 235 K D) The reaction will be spontaneous at any temperature E) The reaction will never be spontaneous

Accepted Answer

To determine the temperature range for spontaneity, we use the Gibbs free energy equation: ΔG = ΔH - TΔS. A reaction is spontaneous when ΔG < 0. Given ΔH° = +62.4 kJ = 62400 J (since 1 kJ = 1000 J) and ΔS° = +301 J/K, we find the temperature at which ΔG = 0 to determine the boundary of spontaneity. Setting ΔG = 0 gives 0 = ΔH - TΔS, or T = ΔH/ΔS. Plugging in the values, T = 62400 J / 301 J/K = 207.31 K. Since ΔH is positive and ΔS is positive, the reaction becomes more spontaneous at higher temperatures. Therefore, the reaction is spontaneous at temperatures above 207 K.

Question 12

Which of the following is true of a reaction that is product-favored? A) Q < K and $\Delta$ _rG < 0 B) Q < K and $\Delta$ _rG > 0 C) Q = K and $\Delta$ _rG = 0 D) Q > K and $\Delta$ _rG < 0 E) Q > K and $\Delta$ _rG > 0

Accepted Answer

A product-favored reaction has Q < K and ΔrG < 0. This means that the concentration of products is greater than the concentration of reactants at equilibrium, and the reaction is spontaneous in the forward direction.

Question 13

ΔG° = 0 for a reaction indicates that _____.
A) the reaction favors formation of products
B) the reaction is at equilibrium
C) the reaction is nonspontaneous
D) the reaction is spontaneous
E) the reaction cannot reach equilibrium

Accepted Answer

ΔG° = 0 for a reaction indicates that the reaction is at equilibrium, meaning the rate of the forward reaction is equal to the rate of the reverse reaction.

Question 14

The dissolution of ammonium nitrate occurs spontaneously in water at 25 °C. As ammonium nitrate dissolves, the temperature of the water decreases. What are the signs of Δ_rH, Δ_rS, and Δ_rG for this process? A) Δ_rH > 0, Δ_rS < 0, and Δ_rG > 0 B) Δ_rH > 0, Δ_rS > 0, and Δ_rG > 0 C) Δ_rH > 0, Δ_rS > 0, and Δ_rG < 0 D) Δ_rH < 0, Δ_rS < 0, and Δ_rG < 0 E) Δ_rH < 0, Δ_rS > 0, and Δ_rG > 0

Accepted Answer

The dissolution of ammonium nitrate in water is an endothermic process, meaning it absorbs heat from the surroundings. Therefore, Δ_rH > 0. Additionally, as the dissolution of ammonium nitrate results in a decrease in temperature, the disorder of the system decreases, indicating a decrease in entropy. This means that Δ_rS < 0. Finally, since the dissolution process is spontaneous and endothermic, the overall change in free energy is negative. This indicates that Δ_rG < 0. Therefore, the correct answer is C.

Question 15

A 100 mL sample of water is placed in a coffee cup calorimeter. When 1.0 g of an ionic solid is added, the temperature decreases from 21.5 °C to 20.8 °C as the solid dissolves. For the dissolving of the solid, _____. A) ΔH < 0 B) ΔS(universe) > 0 C) ΔS(system) < 0 D) ΔS(surroundings) > 0 E) None of these

Accepted Answer

The answer of A 100 mL sample of water is...

Question 16

If Δ_rG° > 0 for a reaction at all temperatures, then Δ_rH° is _____ and Δ_rS° is _____. A) Negative; positive B) Positive; negative C) Negative; negative D) Positive; positive E) Positive; either positive or negative

Accepted Answer

If Δ_rG° > 0, then ΔG° is positive, which means the reaction is not spontaneous at any temperature. For a reaction to be spontaneous at any temperature, ΔH° must be negative and/or ΔS° must be positive. Therefore, Δ_rH° must be negative to make the reaction spontaneous in terms of enthalpy, and Δ_rS° must be positive to make the reaction spontaneous in terms of entropy. This corresponds to option B.

Question 17

A flask containing helium gas is released into a closed room. Which of the following ideas concerning entropy is/are true?
A) ΔS(system) > 0
B) Matter is dispersed.
C) ΔS(universe) > 0
D) This process is spontaneous.
E) All of these statements are true.

Accepted Answer

A) ΔS(system) > 0 - true, as the gas molecules will occupy a larger volume compared to the flask.
B) Matter is dispersed - true, as the gas molecules are dispersed in the room.
C) ΔS(universe) > 0 - true, as the entropy of the system (helium gas) increases, and the entropy of the surroundings (room) also increases due to the dispersal of matter.
D) This process is spontaneous - true, as the gas molecules will naturally disperse in the room due to their random motion.

Therefore, all of the given statements are true.

Question 18

Hydrogen gas is prepared by electrolysis of water according to the reaction below. 2 H₂O( $\ell$ ) ? 2 H₂(g) + O₂(g) Predict the signs of ?_rH and ?_rS. A) ?_rH > 0 and ?_rS > 0 B) ?_rH < 0 and ?_rS > 0 C) ?_rH > 0 and ?_rS < 0 D) ?_rH < 0 and ?_rS < 0 E) ?_rH = 0 and ?_rS < 0

Accepted Answer

The reaction is endothermic, requiring energy input (electrolysis), so ?_rH > 0. The reaction produces gases from a liquid, increasing disorder, so ?_rS > 0.

Question 19

For a reaction, Δ_rH° = -208.8 kJ and Δ_rS° = -308.2 J/K. At what temperature will Δ_rG° = 0.00 kJ? A) 0.68 K B) 677.5 K C) 1476 K D) 6435 K E) 0.85 K

Accepted Answer

We know that ΔG° = ΔH° - TΔS°. At equilibrium, ΔG° = 0.
Thus, 0 = -208.8 kJ - T(-308.2 J/K)
0 = -208800 - T(-308.2)
T = 677.5 K. Therefore, the answer is B.

Question 20

If a chemical reaction is exothermic but not spontaneous, which of the following must be true? A) Δ_rG > 0, Δ_rS > 0, and Δ_rH > 0 B) Δ_rG < 0, Δ_rS > 0, and Δ_rH > 0 C) Δ_rG > 0, Δ_rS < 0, and Δ_rH > 0 D) Δ_rG < 0, Δ_rS < 0, and Δ_rH < 0 E) Δ_rG > 0, Δ_rS < 0, and Δ_rH < 0

Accepted Answer

The answer of If a chemical reaction is exothermic but...

Quiz 19: Principles of Chemical Reactivity: Entropy and Free Energy

What is the sign of ΔH (system) and ΔS (system) if a chemical reaction is spontaneous only at lower temperatures under standard conditions?

When a real gas is compressed from low pressure to high pressure, its temperature increases. Which of the following is true for ΔH and ΔS?

Calculate ΔG° at 298 K for the reaction below. I2(g) + Br2(g) → 2IBr(g) Given: ΔrH° = -11.6 kJ/mol-rxn; ΔrS° = 12 J/K⋅mol-rxn at 298 K.

A change of state occurring in a system is accompanied by 60.7 kJ of heat that is transferred to the surroundings at a constant pressure and at a constant temperature of 300 K. Calculate the ΔS(surroundings) for the process.

If a cube of ice at 0 °C is placed outside on a warm summer day, the ice will melt spontaneously. What are the signs of ΔrH, ΔrS, and ΔrG for the process?

For a chemical reaction, if ΔrG° = 0, then _____.

Which of the following is correct for the condensation of gaseous oxygen at -188 °C? (The normal boiling point of oxygen is -183 °C.)

For the reaction given below, ?H0 = -1516 kJ at 25°C and ?S0 = -432.8 J/K at 25°C. This reaction is spontaneous _____. SiH4(g) + 2 O2(g) ?SiO2(s) + 2 H2O (ℓ) ( \ell ) (ℓ)

At what temperatures will a reaction be spontaneous if ΔrH° = +117 kJ and ΔrS° = -35 J/K?

While diluting concentrated sulfuric acid with water, the temperature of the solution increases rapidly. What are the signs of ΔrH, ΔrS, and ΔrG for the process?

At what temperatures will a reaction be spontaneous if ΔrH° = +62.4 kJ and ΔrS° = +301 J/K?

Which of the following is true of a reaction that is product-favored?

ΔG° = 0 for a reaction indicates that _____.

The dissolution of ammonium nitrate occurs spontaneously in water at 25 °C. As ammonium nitrate dissolves, the temperature of the water decreases. What are the signs of ΔrH, ΔrS, and ΔrG for this process?

A 100 mL sample of water is placed in a coffee cup calorimeter. When 1.0 g of an ionic solid is added, the temperature decreases from 21.5 °C to 20.8 °C as the solid dissolves. For the dissolving of the solid, _____.

If ΔrG° > 0 for a reaction at all temperatures, then ΔrH° is _____ and ΔrS° is _____.

A flask containing helium gas is released into a closed room. Which of the following ideas concerning entropy is/are true?

Hydrogen gas is prepared by electrolysis of water according to the reaction below. 2 H2O( ℓ\ellℓ ) ? 2 H2(g) + O2(g) Predict the signs of ?rH and ?rS.

For a reaction, ΔrH° = -208.8 kJ and ΔrS° = -308.2 J/K. At what temperature will ΔrG° = 0.00 kJ?

If a chemical reaction is exothermic but not spontaneous, which of the following must be true?

Calculate ΔG° at 298 K for the reaction below. I₂(g) + Br₂(g) → 2IBr(g) Given: Δ_rH° = -11.6 kJ/mol-rxn; Δ_rS° = 12 J/K⋅mol-rxn at 298 K.

If a cube of ice at 0 °C is placed outside on a warm summer day, the ice will melt spontaneously. What are the signs of Δ_rH, Δ_rS, and Δ_rG for the process?

For a chemical reaction, if Δ_rG° = 0, then _____.

For the reaction given below, ?H⁰ = -1516 kJ at 25°C and ?S⁰ = -432.8 J/K at 25°C. This reaction is spontaneous _____. SiH₄(g) + 2 O₂(g) ?SiO₂(s) + 2 H₂O $( \ell )$

At what temperatures will a reaction be spontaneous if Δ_rH° = +117 kJ and Δ_rS° = -35 J/K?

At what temperatures will a reaction be spontaneous if Δ_rH° = +62.4 kJ and Δ_rS° = +301 J/K?

The dissolution of ammonium nitrate occurs spontaneously in water at 25 °C. As ammonium nitrate dissolves, the temperature of the water decreases. What are the signs of Δ_rH, Δ_rS, and Δ_rG for this process?

If Δ_rG° > 0 for a reaction at all temperatures, then Δ_rH° is _____ and Δ_rS° is _____.

Hydrogen gas is prepared by electrolysis of water according to the reaction below. 2 H₂O( $\ell$ ) ? 2 H₂(g) + O₂(g) Predict the signs of ?_rH and ?_rS.

For a reaction, Δ_rH° = -208.8 kJ and Δ_rS° = -308.2 J/K. At what temperature will Δ_rG° = 0.00 kJ?