Voltaic cells can be regarded as made up of two half cells, each composed of an electrode in contact with an electrolyte. For instance, a zinc rod dipped in zinc sulphate solution is a Zn|Zn2+ half cell. In such a system zinc atoms dissolve as zinc ions, leaving a negative charge on the electrode Zn(s) → Zn2+(aq)+2e The solution of zinc continues until the charge build-up is sufficient to prevent further ionization. There is then a potential difference between the zinc rod and its solution. This cannot be measured directly, since measurement would involve making contact with the electrolyte, thereby introducing another half cell (see electrode potential). A rod of copper in copper sulphate solution comprises another half cell. In this case the spontaneous reaction is one in which copper ions in solution take electrons from the electrode and are deposited on the electrode as copper atoms. In this case, the copper acquires a positive charge.
Zn(s) → Zn2+(aq)+2e
The two half cells can be connected by using a porous pot for the liquid junction (as in the Daniell cell) or by using a salt bridge. The resulting cell can then supply current if the electrodes are connected through an external circuit. The cell is written Zn(s)|Zn2+(aq)|Cu2+(aq)|CuE = 1.10 V Here, E is the e.m.f. of the cell equal to the potential of the right-hand electrode minus that of the left-hand electrode for zero current. Note that ‘right’ and ‘left’ refer to the cell as written. Thus, the cell could be written Cu(s)|Cu2+(aq)|Zn2+(aq)|Zn(s)E = –1.10 V The overall reaction for the cell is Zn(s)+Cu2+(aq) → Cu(s)+Zn2+(aq) This is the direction in which the cell reaction occurs for a positive e.m.f.
Zn(s)|Zn2+(aq)|Cu2+(aq)|CuE = 1.10 V
Cu(s)|Cu2+(aq)|Zn2+(aq)|Zn(s)E = –1.10 V
Zn(s)+Cu2+(aq) → Cu(s)+Zn2+(aq)
The cell above is a simple example of a chemical cell; i.e. one in which the e.m.f. is produced by a chemical difference. Concentration cells are cells in which the e.m.f. is caused by a difference of concentration. This may be a difference in concentration of the electrolyte in the two half cells. Alternatively, it may be an electrode concentration difference (e.g. different concentrations of metal in an amalgam, or different pressures of gas in two gas electrodes). Cells are also classified into cells without transport (having a single electrolyte) and with transport (having a liquid junction across which ions are transferred). Various types of voltaic cell exist, used as sources of current, standards of potential, and experimental set-ups for studying electrochemical reactions. See also dry cell; primary cell; secondary cell; lithium battery.
Subjects: Chemistry — Physics.