Kaplan MCAT General Chemistry Review

—Balance the charges in each half-reaction.

—Add the half-reactions.

—Confirm balance of mass and charge in redox reaction.

There are two basic types of electrochemical cells: galvanic (voltaic) cells and electrolytic cells. Concentration cells are a special type of galvanic cell.

Galvanic cells have spontaneous redox reactions, generate current, and supply energy. The G is negative, and the E_cell is positive.

Concentration cells have spontaneous redox reactions, generate current, and supply energy. Current is dependent upon ion concentration gradient, not the difference in reduction potential between two different electrodes. The G° and the E°_cell are both 0, because the current ceases when the concentrations of the ion are equal in both compartments.

Electrolytic cells have nonspontaneous redox reactions, require external voltage to generate current, and consume energy. The G is positive, and the E_cell is negative.

Reduction potential, measured in volts (V), is a measure of a chemical species’ tendency to be reduced (gain electrons) relative to the standard hydrogen electrode (SHE). The higher the reduction potential, the greater the tendency to gain electrons and be reduced.

The standard emf of a cell is the difference between the standard reduction potential of the cathode minus the standard reduction potential of the anode.

The Nernst equation is useful for calculating E_cell for all ion concentrations.

EQUATIONS TO REMEMBER

Practice Questions

1. An electrolytic cell is filled with water. Which of the following will move toward the cathode of such a cell?

I. H⁺ ions

II. O^2- ions

III. Electrons

A. I only

B. II only

C. I and III

D. II and III

2. Consider the following data:

Reaction Reduction Potential Hg²⁺ + 2 e^– Hg+0.85 VCu⁺ + e^– Cu+0.52 VZn²⁺ + 2 e^– Zn–0.76 VAl³⁺ + 3 e^– Al–1.66 V

The anode of a certain galvanic cell is composed of copper. Which of the metals from the data table can be used at the cathode?

A. Hg

B. Al

C. Zn

D. None of the above

3. Consider the following equation:

3 Na (s) + H₃N (aq) Na₃N (s) + H₂ (g)

Which species acts as an oxidizing agent?

A. H⁺

B. H₂N

C. Na

D. Na⁺

4. How many electrons are involved in the following unbalanced reaction?

Cr₂O₇^2- + H⁺ + e^- Cr²⁺ + H₂O

A. 2

B. 8

C. 12

D. 16

Questions 5 and 6 refer to the following two half-reactions:

O₂ + 4 H⁺ + 4 e^- 2 H₂O +1.23 V (Reaction 1)

PbO₂ + 4 H⁺ + 2 e^- Pb²⁺ + 2 H₂O +1.46 V (Reaction 2)

5. If the two half-reactions combine to form a spontaneous system, what is the net balanced equation of the full reaction?

A. 2 PbO₂ + 4 H⁺ 2 Pb²⁺ + O₂ + 2 H₂O

B. PbO₂ Pb²⁺ + O₂ + 2 e^-

C. Pb²⁺ + O₂ + 2 H₂O 2 PbO₂ + 4 H⁺

D. Pb²⁺ + O₂ + 2 e^- PbO₂

6. Find the standard potential of the following reaction:

Pb²⁺ + O₂ + 2 H₂O 2 PbO₂ + 4 H⁺

A. +0.23 V

B. -0.23 V

C. -1.69 V

D. +2.69 V

7. Rusting occurs due to the oxidation-reduction reaction of iron with environmental oxygen:

4 Fe (s) + 3 O₂ (g) 2 Fe₂O₃ (s)

Some metals, such as copper, are unlikely to react with oxygen. Which of the following best explains this observation?

A. Iron has a more positive reduction potential, making it more likely to donate electrons to oxygen.

B. Iron has a more positive reduction potential, making it more likely to accept electrons from oxygen.

C. Iron has a less positive reduction potential, making it more likely to donate electrons to oxygen.

D. Iron has a less positive reduction potential, making it more likely to accept electrons from oxygen.

8. Lithium aluminum hydride (LiAlH₄) is often used in laboratories because of its tendency to donate a hydride ion. Which of the following properties does LiAlH₄ exhibit?

A. Strong reducing agent

B. Strong oxidizing agent

C. Strong acid

D. Strong base

9. If the value of E°_cell is known, what other data is needed to calculate G?

A. Equilibrium constant

B. Reaction quotient

C. Temperature of the system

D. Number of moles of electrons transferred

10. Which of the following compounds is least likely to be found in the salt bridge of a galvanic cell?

A. NaCl

B. SO₃

C. MgSO₃

D. NH₄NO₃

11. What is the oxidation number of chlorine in NaClO?

A. -1

B. 0

C. +1

D. +2

12. Which of the following is most likely to increase the rate of an electrolytic reaction?

A. Increasing the resistance in the circuit

B. Increasing the volume of electrolyte

C. Increasing the current

D. Increasing the pH

13. The following electronic configurations represent elements in their neutral form. Which element is the strongest oxidizing agent?

A. 1s²2s²p⁶3s²p⁶4s²

B. 1s²2s²p⁶3s²p⁶4s²3d⁵

C. 1s²2s²p⁶3s²p⁶4s²3d¹⁰4p¹

D. 1s²2s²p⁶3s²p⁶4s²3d¹⁰4p⁵

Small Group Questions

1. How can a halogen have a positive oxidation number?

2. When adding standard potentials, why are we instructed not to multiply them by molar coefficients?

Explanations to Practice Questions

1. C