Question

Given the following cell notations, determine the species oxidized, species reduced, and the oxidizing agent and reducing agent, without writing the balanced reactions

a. \({\rm{Mg}}(s)\left| {{\rm{M}}{{\rm{g}}^{2 + }}(aq) || {\rm{C}}{{\rm{u}}^{2 + }}(aq)} \right|{\rm{Cu}}(s)\)

b.\({\rm{Ni}}(s)\left| {{\rm{N}}{{\rm{i}}^{2 + }}(aq) || {\rm{A}}{{\rm{g}}^ + }(aq)} \right|{\rm{Ag}}(s)\)

Short answer

a. Mg is oxidized and it acts as reducing agent and \({\rm{Cu}}\) is reduced and it acts an oxidizing agent.

b. Ni is oxidized and it acts as reducing agent and \({\rm{Ag}}\) is reduced and it acts an oxidizing agent.

Step-by-step solution

Define oxidation and reduction

• The oxidation-reduction reaction is also known as a redox reaction. In this reaction, one reactant is oxidized and other is reduced. In balancing an oxidation-reduction reaction, they must be first divided into two half reactions: one is oxidation reaction and other is reduction reaction.
• The balancing of redox reaction is complicated as compared to simple balancing. It is necessary to determine the half reactions of reactants undergoing oxidation and reduction. On adding the two half reactions, net total equation can be obtained. This method of balancing redox reaction is known as half equation method.

a) Determine balanced reaction for each pair of half reactions in an acidic solution.  

Consider the following cell notation.

\(Mg(s)\left| {M{g^{2 + }}(aq)} \right|\left| {C{u^{2 + }}(aq)} \right|Cu(s)\)

Since half-cell \({\rm{Mg}}({\rm{s}})\mid {\rm{M}}{{\rm{g}}^{2 + }}({\rm{aq}})\) left side of the cell notation, it represents anode or oxidation half-cell. As oxidation state of \({\rm{Mg}}\) increases from 0 to \( + 2\), so it is getting oxidized and acts as reducing agent.

Since Half-cell \({\rm{C}}{{\rm{u}}^{2 + }}({\rm{aq}})\mid {\rm{Cu}}({\rm{s}})\) lies on the right side of cell notation. Hence it represents cathode or reduction half-cell. As oxidation state of \({\rm{C}}{{\rm{u}}^2} + \) decreases from \( + 2\) to 0, so it is getting reduced and acts as oxidizing agent. Mg is oxidized and it acts as reducing agent and \({\rm{Cu}}\) is reduced and it acts an oxidizing agent.

b) Determine balanced reaction for each pair of half reactions in an acidic solution.

Consider the following cell notation

\(Ni(s)\left| {N{i^{2 + }}(aq)} \right|\left| {A{g^ + }(aq)} \right|Ag(s)\)

Since half-cell Ni(s) \(\mid {\rm{N}}{{\rm{i}}^{2 + }}\) (aq) left side of the cell notation, it represents anode or oxidation half-cell. As oxidation state of \({\rm{N}}{{\rm{i}}^2} + \) increases from 0 to \( + 2\), so it is getting oxidized and acts as reducing agent. Since Half-cell \({\rm{A}}{{\rm{g}}^ + }({\rm{aq}})\mid {\rm{Ag}}({\rm{s}})\) lies on the right side of cell notation. Hence it represents cathode or reduction half cell. As oxidation state of \({\rm{Ag}} + \) decreases from \( + 1\) to 0 , so it is getting reduced and acts as oxidizing agent. Ni is oxidized and it acts as reducing agent and \({\rm{Ag}}\) is reduced and it acts an oxidizing agent.