Electrochemistry - Galvanic/Voltaic Cells

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Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Galvanic Voltaic Cells of Electrochemistry. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
The standard cell potential of a redox reaction is the sum of the standard oxidation and reduction potentials of the individual reactions occurring in that system. Looking at the equation ΔG° = -nFE°cell we noted that a redox reaction with a positive cell potential has a negative change in Gibbs free energy and is spontaneous. Galvanic cells, otherwise known as voltaic cells, are electrochemical systems that take advantage of the spontaneous nature of these redox reactions to generate current. In other words, the spontaneous release of chemical energy is harnessed to produce electrical energy.
Design of a Galvanic Cell
A redox reaction system is composed of an oxidation reaction and a reduction reaction in which electrons will pass from the oxidized species to the reduced species. In a galvanic cell, the two species can be separated into half-cells, and by connecting them with a conductive metal wire, electrons will spontaneously flow from one species to the other. The current generated by electrons flowing through the wire can be used to power electrical devices.
Consider a galvanic cell composed of the redox reaction between copper and zinc E°cell = +1.10 V as shown below. The cell would be structured so that the oxidation and reduction reactions are separated into two half-cells, connected by a conductive metal wire and a salt bridge. One half-cell of our galvanic cell system is an electrode of zinc metal submerged in solution, and the other half-cell is an electrode of copper metal submerged in solution. The half-cell containing the oxidation reaction is termed the anode, which in this case is the zinc electrode, because the zinc has a lower reduction potential and higher oxidation potential than copper. Within our anode, neutral atoms are found in the metallic electrode and cations are found submerged in solution. Conversely, the half-cell containing the reduction reaction is termed the cathode, which in this case is a submerged electrode composed of electrically neutral copper. Within our cathode half-cell, neutral copper atoms are found in the electrode and anions are found submerged in solution.
Reactions Within the Cathode and Anode
Within the anode and cathode, oxidation and reduction reactions are occurring in equilibrium, respectively. In our system, the oxidation reaction occurring at the anode will cause the breakdown of atoms in the zinc electrode, degrading it over time. The breakdown of anode metal due to oxidation is termed electrode pitting. Electrons resulting from this breakdown will travel through the current carrying wire connecting both half cells, from the anode solution to cathode solution. Within the cathode solution, the electrons will react with cations (here, copper cations) to form neutral molecules of metal that will plate on top of the cathode electrode. In this case, electrons will react with copper cations to form more neutral copper that will attach to the copper electrode. The addition of the products of reduction to the cathode electrode is called electroplating. During pitting, the oxidation at the anode electrode will cause it to shrink, whereas during electroplating, the reduction at the cathode electrode will cause it to grow.
Salt Bridges
Why would electrons naturally tend to move from the anode toward the cathode? In general, the anode of a galvanic cell will accumulate negative charge whereas the cathode of a galvanic cell accumulates positive charge. Electrons are therefore attracted toward the cathode. The source of the anode’s negative charge is a confusing point. During the course of running a galvanic cell, cations are being pitted from the metal of the anode electrode into the solution, which we would predict to have the effect of making the anode solution more positive. Additionally, electroplating at the cathode removes cations from the cathode solution, which we would predict to have the effect of making the cathode solution more negative. In order to compensate for this and create a negative charge at the anode, a salt bridge is fixed between the anode and cathode. A salt bridge is made of an inert salt that will not participate in or interfere with the redox reactions in the cell, for example, KNO3. Potassium is cationic, whereas nitrate is anionic.
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