Q: For the reaction $$aA + bB \rightarrow cC + dD$$, the standard enthalpy change $$\Delta H^\circ_{rxn}$$ can be calculated using which of the following equations?

$$\Delta H^\circ_{rxn} = \Sigma m \Delta H^\circ_f(\text{Products}) - \Sigma n \Delta H^\circ_f(\text{Reactants})$$

Question 1

Which of the following statements best describes the concept of entropy ($$S) in $$thermodynamics?

Accepted Answer

Entropy is a measure of the degree of chaos or disorder in a system, with higher entropy indicating greater disorder.

Question 2

A chemical reaction has $$\Delta H = -120 kJ/$$mol and $$\Delta S = -200 J/K$$·mol. At what temperature (in Kelvin) will the reaction change from spontaneous to non-spontaneous?

Accepted Answer

$$T = 600 K$$

Question 3

Which of the following processes results in a positive change in entropy ($$\Delta S \u003e 0$$)?

Accepted Answer

Dissolving sodium chloride in water.

Question 4

Given the reaction: $$2H_2O(l) \rightarrow 2H_2(g) + O_2$(g)$$, and $$\Delta G^\circ = 474.4 kJ/$$mol at $$25^\circC$$, what is the equilibrium constant $$K at $$this temperature? (Use $$R = 8.314 J/K$$·mol)

Accepted Answer

$$K = 8.3 	imes 10$$^{-84}$$

Question 5

Which of the following best explains why the entropy of the universe increases for a spontaneous process?

Accepted Answer

Because spontaneous processes result in an increase in the total disorder (entropy) of the system and surroundings.

Question 6

For the reaction $$aA + bB \rightarrow cC + dD$$, the standard enthalpy change $$\Delta H^\circ_{rxn}$$ can be calculated using which of the following equations?

Accepted Answer

$$\Delta H^\circ_{rxn} = \Sigma m \Delta H^\circ_f(	ext{Products}) - \Sigma n \Delta H^\circ_f(	ext{Reactants})$$

Question 7

A reaction has $$\Delta H = +50 kJ/$$mol and $$\Delta S = +200 J/K$$·mol. Which statement is true about the spontaneity of this reaction?

Accepted Answer

It is spontaneous only at high temperatures.

Question 8

Which of the following is the correct expression for the Gibbs free energy change under non-standard conditions?

Accepted Answer

$$\Delta G = \Delta G^\circ + RT \ln Q$$

Question 9

Which of the following best describes the significance of a negative $$\Delta G$$ for a chemical reaction?

Accepted Answer

The reaction is spontaneous under the given conditions.

Question 10

Consider the following reaction at $$25^\circC$$: $$C_6H_{12}O_6$(s) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l). $$Which of the following statements about the entropy change ($$\Delta S) $$for this reaction is correct?

Accepted Answer

$$\Delta S is $$positive because the number of gas molecules increases.

Question 11

Which of the following is true at equilibrium for a chemical reaction?

Accepted Answer

$$\Delta G = 0$$

Question 12

A reaction has $$\Delta H = -200 kJ/$$mol and $$\Delta S = -300 J/K$$·mol. At $$T = 298 K$$, what is the value of $$\Delta G$$?

Accepted Answer

$$\Delta G = -110.6 kJ/$$mol

Question 13

Which of the following best describes the relationship between $$\Delta G$$ and the equilibrium constant $$K$$?

Accepted Answer

$$\Delta G^\circ = -RT \ln K$$

Question 14

Which of the following phase changes is associated with the largest increase in entropy?

Accepted Answer

Sublimation of dry ice (solid CO$$_2) to CO$_2$$ gas

Question 15

If a reaction is exothermic ($$\Delta H  0$$), what can be said about its spontaneity?

Accepted Answer

It is spontaneous at all temperatures.

Question 16

Which of the following is NOT a correct statement about standard enthalpy of formation ($$\Delta H_f^\circ$$)?

Accepted Answer

It is always a negative value for all compounds.

Question 17

A reaction at $$25^\circC $$has $$\Delta G^\circ = -40 kJ/$$mol. What is the value of the equilibrium constant $$K$$? (Use $$R = 8.314 J/K$$·mol)

Accepted Answer

$$K \approx 7.5 	imes 10^6$

Question 18

Which of the following is a correct qualitative prediction for the entropy change ($$\Delta S) in $$the reaction: $$C_2H_6(g) + O_2$(g) \rightarrow 2CO_2(g) + 3H_2O(g)$$?

Accepted Answer

$$\Delta S is $$positive because the number of gas molecules increases.

Question 19

Which of the following best describes the effect of temperature on the spontaneity of a reaction with $$\Delta H \u003e 0$$ and $$\Delta S \u003c 0$$?

Accepted Answer

The reaction is non-spontaneous at all temperatures.

Thermodynamics: Spontaneous Change, Entropy & Free Energy (CHEM 120, Chapter 14)

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