Kaplan MCAT General Chemistry Review

Enthalpy is a measure of the potential energy of a system found in the intermolecular interactions and bonds of molecules. Hess’s law states that the total change in the potential energy of a system is equal to the changes in potential energies of the individual steps (reactions) of the process.

Entropy is a measure of the degree to which energy has been spread out through a system or between a system and its surroundings. It is a ratio of heat transferred per unit Kelvin. Systems reach maximum possible entropy (maximum possible energy dispersal) only at equilibrium.

Gibbs free energy is a calculation involving both enthalpy and entropy values for a system. The change in Gibbs function determines whether a process is spontaneous or nonspontaneous. When the change in Gibbs function is negative, the process is spontaneous, but when the change in Gibbs function is positive, the process is nonspontaneous.

Temperature-dependent processes are those that change between spontaneous and nonspontaneous at a certain temperature. For example, water freezes spontaneously only at temperatures below 273 K and boils spontaneously only at temperatures above 373 K.

The larger and more positive the value of the equilibrium constant is, the more spontaneous a reaction will be (that is, the more negative the change in Gibbs function for the system as it moves from its initial position to its equilibrium).

EQUATIONS TO REMEMBER

Practice Questions

1. Consider the cooling of an ideal gas in a closed system. This process is illustrated in the pressure-volume graph shown in Figure 1.

Figure 1

Based on the information in Figure 1, the process is

A. adiabatic.

B. isobaric.

C. isothermal.

D. Two of the above

2. A reaction has a positive entropy and enthalpy. What can be inferred about the progress of this reaction from this information

A. The reaction is spontaneous.

B. The reaction is nonspontaneous.

C. The reaction is at equilibrium.

D. There is not enough information to determine if the reaction is spontaneous.

3. Pure sodium metal spontaneously combusts upon contact with room temperature water. What is true about the equilibrium constant of this combustion reaction at 25°C

A. K_eq < 1.

B. K_eq > 1.

C. K_eq = 1.

D. There is not enough information to determine the equilibrium constant.

4. Which of the following processes has the most exothermic heat of reaction

A. Combustion of ethane

B. Combustion of propane

C. Combustion of n-butane

D. Combustion of isobutane

5. Methanol reacts with acetic acid to form methyl acetate and water, as shown in the following table in the presence of an acid catalyst.

CH₃OH () + CH₃COOH (aq) CH₃COOCH₃ (aq) + H₂O ()

Type of Bond Bond Disassociation Energy (kJ/mol) C—C348C—H413C = O805O—H464C—O360

What is the heat of formation of methyl acetate in kJ/mol

A. 0 kJ/mol

B. 464 kJ/mol

C. 824 kJ/mol

D. 1,288 kJ/mol

6. At standard temperature and pressure, a chemical process is at equilibrium. What is the free energy of reaction (G) for this process?

A. G > 0.

B. G < 0.

C. G = 0.

D. There is not enough information to determine the free energy of reaction.

7. For a certain chemical process, G° = -4.955 kJ/mol. What is the equilibrium constant K_eq for this reaction?

A. K_eq = 0.13

B. K_eq = 7.4

C. K_eq = 8.9

D. K_eq = 100

8. Consider the chemical reaction in the vessel depicted in the following diagram.

A. The reaction is spontaneous.

B. The reaction is nonspontaneous.

C. The reaction is at equilibrium.

D. There is not enough information to determine if the reaction is spontaneous.

Suppose G_rxn° = -2,000 kJ/mol for a chemical reaction. At 300 K, what is the change in Gibb’s free energy, G

A. G = -2,000 kJ/mol + (300 K) (8.314 Jmol^-1K^-1)ln(Q).

B. G = -2,000 kJ/mol - (300 K) (8.314 Jmol^-1K^-1)ln(Q).

C. G = -2,000 kJ/mol + (300 K) (8.314 Jmol^-1K^-1)log(Q).

D. G = -2,000 kJ/mol - (300 K) (8.314 Jmol^-1K^-1)log(Q).

10. An ideal gas undergoes a reversible expansion at constant pressure. Which of the following terms could describe this expansion?

I. Adiabatic

II. Isothermal

III. Isobaric

A. I only

B. I and II only

C. I and III only

D. I, II, and III

11. A chemical reaction has a negative enthalpy and a negative entropy. Which of the following terms describes the energy of this reaction?

A. Exothermic

B. Endothermic

C. Endergonic

D. Exergonic

12. Consider the chemical reaction in the vessel pictured in the following figure.

What can we say about the entropy of this reaction?

A. S > 0

B. S < 0

C. S = 0

D. There is not enough information in the picture to determine S.

13. Which of the following statements is true of a spontaneous process?

A. G > 0 and K_eq > Q

B. G > 0 and K_eq < Q

C. G < 0 and K_eq > Q

D. G < 0 and K_eq < Q

14. Which of the following devices would be the most appropriate to use to measure the heat capacity of a liquid

A. Thermometer

B. Calorimeter

C. Barometer

D. Volumetric flask

15. Which of the following equations does not state a law of thermodynamics?