KEY2CHEM

Redox Reactions in Electrochemical Cells

An electrochemical cell contains a redox reaction where the half reactions are physically separated. The redox reaction is either spontaneous (galvanic cell,$$E^\circ > 0$$) or nonspontaneous (electrolytic cell, $$E^\circ < 0$$).

Example 1.

Which statement about galvanic cells is correct?

A. Electrons are spontaneously transferred from the anode to the cathode.

B. Electrons are spontaneously transferred from the cathode to the anode.

C. Electrons are nonspontaneously transferred from the anode to the cathode.

Solution

A. Electrons are spontaneously transferred from the anode to the cathode.

A galvanic cell contains a spontaneous redox reaction, where electrons in the oxidation half reaction (anode) are transferred through an external circuit to the reduction half reaction (cathode).

Example 2.

Sodium metal can be formed by passing electrical current through molten sodium chloride in the following electrolytic cell:

$$\require{mhchem}\ce{NaCl(l) -> Na(l) + 1/2 Cl2(g)}$$

Which statement about this electrolytic cell is true?

A. Sodium is formed at the anode.

B. Chlorine is formed at the anode.

C. The reaction generates electrical current.

Solution

B. Chlorine is formed at the anode.

The sodium ion is reduced, so sodium will form at the cathode. The chloride ion is oxidized, so chlorine will form at the anode. The redox reaction in an electrolytic cell is driven by an externally applied electrical potential (is nonspontaneous).

Example 3.

A galvanic cell is formed from the following half reactions. Which statement about the galvanic cell formed is true?

$$\require{mhchem}\ce{Al^3+(aq) + 3 e- -> Al(s) \;\;\;\;E^{\circ}} = -1.66 \;V$$

$$\require{mhchem}\ce{Ni^2+(aq) + 2 e- -> Ni(s) \;\;\;\;E^{\circ}} = -0.25 \;V$$

A. $$\require{mhchem}\ce{Ni(s)}$$ is oxidized to $$\require{mhchem}\ce{Ni^2+(aq)}$$.

B. $$\require{mhchem}\ce{Al^3+(aq)}$$ is reduced to $$\require{mhchem}\ce{Al(s)}$$.

C. Electrons flow from $$\require{mhchem}\ce{Al(s)}$$ to $$\require{mhchem}\ce{Ni(s)}$$through the external circuit.

Solution

C. Electrons flow from $$\require{mhchem}\ce{Al(s)}$$ to $$\require{mhchem}\ce{Ni(s)}$$ through the external circuit.

The cell potential in a galvanic cell is positive, leading to the overall reaction $$\require{mhchem}\ce{2 Al(s) + 3 Ni^2+(aq) -> 2 Al^3+(aq) + 2 Ni(s)}$$. $$\require{mhchem}\ce{Al(s)}$$ is oxidized to $$\require{mhchem}\ce{Al^3+(aq)}$$, and $$\require{mhchem}\ce{Ni^2+(aq)}$$ is reduced to $$\require{mhchem}\ce{Ni(s)}$$ in this spontaneous redox reaction.