Kaplan MCAT General Chemistry Review

A concentration cell is a special type of voltaic cell: two half-cells connected by a conductive material, allowing a spontaneous redox reaction to proceed, generating a current and delivering energy. Just like a galvanic cell, the concentration cell houses a redox reaction that has a negative G. The distinguishing characteristic of a concentration cell is in its design: The electrodes are chemically identical. For example, if both electrodes are copper metal (they are chemically identical), they have the same reduction potential so current is generated as a function of a concentration gradient established between the two solutions surrounding the electrodes. The concentration gradient results in a potential difference between the two compartments and drives the movement of electrons in the direction that results in equilibration of the ion gradient. Current will cease when the concentrations of ion species in the half-cells are equal. This implies that the voltage (V) or emf of a concentration cell is zero when the concentrations are equal; the voltage as a function of concentrations can be calculated using the Nernst equation.

ELECTRODE CHARGE DESIGNATION

In a galvanic cell, current is spontaneously generated as electrons are released by the oxidized species at the anode and travel through the conductive material to the cathode, where the reduction occurs. Because the anode of a galvanic cell is the source of electrons, it is considered the negative electrode; the cathode is considered the positive electrode. Electrons, therefore, move from negative (low electric potential) to positive (high electric potential), while the current (by convention, the movement of positive charge) is from positive (high electric potential) to negative (low electric potential).

Conversely, the anode of an electrolytic cell is considered positive, because it is attached to the positive pole (the cathode) of the external voltage source (external battery) and so attracts anions from the solution. The cathode of an electrolytic cell is considered negative, because it is attached to the negative pole (the anode) of the external voltage source and so attracts cations from the solution.

Key Concept

In an electrolytic cell, the anode is positive and the cathode is negative. In a galvanic cell, the anode is negative and the cathode is positive. However, in both types of cells, reduction occurs at the cathode, and oxidation occurs at the anode.

In spite of this difference in designating charge (sign), oxidation takes place at the anode and reduction takes place at the cathode in both types of cells, and electrons always flow through the wire from the anode to the cathode. A simple mnemonic is that the CAThode attracts the CATions and the ANode attracts the ANions. In the Daniell cell, for example, the electrons created at the anode by the oxidation of the elemental zinc travel through the wire to the copper half-cell. There they attract copper(II) cations to the cathode, resulting in the reduction of the copper ions to elemental copper, and cations out of the salt bridge into the compartment. The anode, having lost electrons, attracts anions from the salt bridge into the compartment at the same time the Zn²⁺ ions formed by the oxidation process move away from the anode and toward the cathode.

Realize that this is an important distinction to understand not just for electrochemistry in the Physical Sciences section of Test Day but also for applications of electrochemistry in the Biological Sciences section. This distinction arises, for example, in a variant of electrophoresis, called isoelectric focusing, a technique often used to separate amino acids based on the unique isoelectric point (pI) of each amino acid. The positively charged amino acids (those that are protonated at the pH of the solution) will migrate toward the cathode; negatively charged amino acids (those that are deprotonated at the solution pH) will migrate instead toward the anode.

Reduction Potentials and the Electromotive Force

For galvanic cells, the direction of spontaneous movement of charge is from anode, the site of oxidation, to cathode, the site of reduction. This is simple enough to remember, but it begs the question; How do we determine which electrode species will be oxidized and which will be reduced? The relative tendencies of different chemical species to be reduced have been determined experimentally, using the tendency of the hydrogen ion (H⁺) to be reduced as an arbitrary zero reference point.

REDUCTION POTENTIALS