Physical Chemistry : Batteries

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Batteries

A battery converts __________ energy to __________ energy.

Possible Answers:

electrical . . . chemical

chemical . . . electrical

chemical . . . kinetic

electrical . . . kinetic

Correct answer:

chemical . . . electrical

Explanation:

Electronic devices such as cell phones utilize batteries for power. A discharging battery converts the chemical energy associated with redox reactions to electrical energy that can be utilized to power the device. Kinetic energy is the energy associated with motion and is irrelevant to this question.

Example Question #2 : Batteries

Which of the following statements about a charging battery is true? 

Possible Answers:

The battery behaves as a galvanic cell

The Gibbs free energy of the reaction in the battery is positive

Oxidation occurs at the cathode and reduction occurs at anode

The reaction occurring is exothermic

Correct answer:

The Gibbs free energy of the reaction in the battery is positive

Explanation:

A charging battery behaves as an electrolytic cell. The process involves a nonspontaneous reaction; therefore, energy input is needed for charging a battery. This energy is provided by the electrical outlets that we use to charge our electronic devices, such as cell phones. Since it is nonspontaneous, the reaction occurring during charging has a positive Gibbs free energy.

Oxidation always occurs at the anode and reduction always occurs at the cathode, regardless of the type of cell. As mentioned previously, the charging battery requires energy input and behaves as an electrolytic cell. Exothermic reactions are characterized by a negative change in enthalpy. A reaction happening in an electrolytic cell is nonspontaneous. This means that the Gibbs free energy is always negative; however, other thermodynamic variables such as enthalpy and entropy can be either positive or negative. 

Example Question #1 : Batteries

A charging battery is acting as a(n) __________ cell and a discharging battery is acting as a(n) __________ cell. 

Possible Answers:

galvanic . . . galvanic

electrolytic . . . galvanic

galvanic . . . electrolytic

electrolytic . . . electrolytic

Correct answer:

electrolytic . . . galvanic

Explanation:

A charging battery consumes energy from a power source like an electrical outlet, whereas a discharging battery releases energy and powers a device; therefore, a charging battery behaves as an electrolytic cell whereas a discharging battery behaves as a galvanic cell. 

Example Question #2 : Batteries

Lithium batteries consist of lithium compounds and manganese dioxide. The reduction potential for lithium is , whereas the reduction potential for manganese dioxide is . Which of the following reaction(s) occur(s) when a lithium battery is discharging?

I. Oxidation of manganese

II. Reduction of lithium

III. Oxidation of lithium

Possible Answers:

II and III

III only

I and II

I only

Correct answer:

III only

Explanation:

A discharging battery behaves like a galvanic cell; therefore, it is characterized by a spontaneous reaction. This means that the reaction happening in a discharging battery has a positive standard reaction potential. Given the information in the question, we will get a positive standard reaction potential if the lithium atom is oxidized (half reaction potential = ) and the manganese ion is reduced (half reaction potential = ). The total standard reaction potential is the sum of the half reaction potentials, . The oxidation of manganese and the reduction of lithium will yield a negative potential, not characteristic of a discharging battery. Note that one reaction has to be oxidation and one reaction has to be reduction.

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