Electrochemistry is the branch of chemistry that deals with the interconversion of chemical and electrical energy. A galvanic (or voltaic) cell is a chemical system that uses an oxidation–reduction reaction to convert chemical energy into electrical energy (hence it is also known as an electrochemical cell). This process is the opposite of electrolysis (explained in section 23.10), wherein electrical energy is used to bring about chemical changes. The two systems are similar in that both are redox processes; in both, the oxidation takes place at one electrode, the anode, while reduction occurs at the cathode. Figure 23-1 shows a galvanic cell, indicating the half-reactions at the two electrodes. Electrons flow through the external circuit from the anode (Zn) to the cathode (Cu). The overall reaction, which is obtained by adding the anodic and cathodic half-cell reactions, is: . . .Zn(s)+Cu2+(aq) → Zn2+(aq)+Cu(s). . . This cell has a potential of 1.10 V (see next section). The potential energy of electrons at the anode is higher than at the cathode. This difference in potential is the driving force that propels electrons through the external circuit. The cell potential (Ecell) is a measure of the potential difference between the two half-cells. It is also known as the electromotive force (emf) of the cell, or, since it is measured in volts, the cell voltage. An electrochemical cell consists of two half-reactions at different potentials, which are known as electrode potentials. The electrode potential for the oxidation half-reaction is called the oxidation potential. Similarly, for the reduction half-reaction, we have the reduction potential. The potential of a galvanic cell is determined by the concentrations of the species in solution, the partial pressures of any gaseous reactants or products, and the reaction temperature. When the electrochemical measurement is carried out under standard-state conditions, the cell potential is called the standard electrode potential and is given the symbol E0. The standard conditions include a concentration of 1 M, gaseous partial pressure of 1 atm, and a temperature of 25°C. It is impossible to measure the absolute potential value of a single electrode, since every oxidation is accompanied by a reduction. Therefore any measurement is carried out against a reference electrode.
