Kaplan MCAT General Chemistry Review

When the electrodes are connected to each other by a conductive material, charge will begin to flow as the result of a redox reaction that is taking place between the two half-cells. The redox reaction in a galvanic cell is spontaneous, and therefore the change in Gibbs function for the reaction is negative (–G). As the spontaneous redox reaction proceeds (toward equilibrium, which we’ll discuss next), the movement of charge (electrons) results in a conversion of electric potential energy into kinetic energy (the electrons are moving, after all, and small as they are, electrons do have mass). By separating the reduction and oxidation half-reactions into two compartments, we are able to harness this energy and use it to do work by connecting various electrical machines or devices into the circuit between the two electrodes. If you’ve ever stuck a 9-volt battery (the rectangular type with the + and - ends on the same side) to your tongue, you know full well the reality of energy transfer by redox reaction. If you’ve never stuck a 9-volt battery to your tongue, we recommend that you consider this an experience you can live without. Seriously, don’t try it.

Figure 11.1

In the Daniell cell, a zinc bar is placed in an aqueous ZnSO₄ solution, and a copper bar is placed in an aqueous CuSO₄ solution. The anode of this cell is the zinc bar where Zn (s) is oxidized to Zn²⁺ (aq). The cathode is the copper bar, and it is the site of the reduction of Cu²⁺ (aq) to Cu (s). The half-cell reactions are written as follows:

Zn (s) Zn²⁺ (aq) + 2 e^- (anode)

Cu²⁺ (aq) + 2 e^- Cu (s) (cathode)

If the two half-cells were not separated, the Cu²⁺ ions would react directly with the zinc bar, and no useful electrical work would be obtained. Since the solutions (and electrodes) are physically separated, they must be connected to complete the circuit. Without connection, the electrons from the zinc oxidation half-reaction would not be able to get to the copper ions; thus, a wire (or other conductor) is necessary. However, if only a wire were provided for this electron flow, the reaction would soon cease, because an excess negative charge would build up in the solution surrounding the cathode and an excess positive charge would build up in the solution surrounding the anode. Eventually, the excessive charge accumulation would provide a counter voltage large enough to prevent the redox reaction from taking place, and the current would cease. This charge gradient is dissipated by the presence of a salt bridge, which permits the exchange of cations and anions. The salt bridge contains an inert electrolyte, usually KCl or NH₄NO₃, whose ions will not react with the electrodes or with the ions in solution. At the same time that the anions from the salt bridge (e.g., Cl^-) diffuse into the ZnSO₄ solution to balance out the charge of the newly created Zn²⁺ ions, the cations of the salt bridge (e.g., K⁺) flow into the CuSO₄ solution to balance out the charge of the SO₄^2- ions left in solution when the Cu²⁺ ions are reduced to Cu and precipitate out of solution (“plate out”) onto the copper cathode.

Key Concept

The purpose of the salt bridge is to exchange anions and cations to balance, or dissipate, newly generated charges.

During the course of the reaction, electrons flow from the zinc bar (anode) through the wire and the voltmeter (if one is connected) toward the copper bar (cathode). The anions (Cl^-) flow externally (via the salt bridge) into the ZnSO₄, and the cations (K⁺) flow into the CuSO₄. This flow depletes the salt bridge and, along with the finite quantity of Cu²⁺ in the solution, accounts for the relatively short lifetime of the cell.

A cell diagram is a shorthand notation representing the reactions in an electrochemical cell. A cell diagram for the Daniell cell is as follows:

Zn (s) | Zn²⁺ (xM SO₄^2- ) || Cu²⁺ (yM SO₄^2-) | Cu (s)

MCAT Expertise

Know the shorthand notation for cells because it is possible that on Test Day they will not spell it all out for us.

The following rules are used in constructing a cell diagram:

1. The reactants and products are always listed from left to right in this form:

anode | anode solution || cathode solution | cathode

2. A single vertical line indicates a phase boundary.

3. A double vertical line indicates the presence of a salt bridge or some other type of barrier.

ELECTROLYTIC CELLS