The electrode potential of two ionic half-reactions is the potential difference measured on their connection in an electrochemical cell. The standard electrode potential of a half-reaction is the potential measured between it and hydrogen in an electrochemical cell at 298K, 1 mol concentration and 1 atm pressure.
Different half-reactions attached through electrochemical cells give different potential readings. For example, on connection, the below half-reactions give a reading of 0.59V:
- (1) Fe3+ + e- -> Fe2+
- (2) Cl2 + 2e- -> 2Cl-
- (1) Fe3+ + e- -> Fe2+
- (3) I2 + 2e- -> 2I-
- (1) Fe3+ + e- -> Fe2+
- (4) 2H+ + 2e- -> H2
Once the standard electrode potentials of two substances are known, it is possible to predict the direction electrons will travel from one substance to another, as well as the potential measured between them. The more positive half-reaction will always accept electrons, whilst the less positive (not necessarily negative) reaction will donate electrons. The voltage measured between the half-reactions is equal to the difference in standard electrode potentials between the substances. For example given our original reaction:
- (1) Fe3+ + e- -> Fe2+ EP = 0.77 (See Table of standard electrode potentials)
- (2) Cl2 + 2e- -> 2Cl- EP = 1.36
- (1) Fe2+ -> Fe3+ + e-
- (2) Cl2 + 2e- -> 2Cl-
- (5) Cl2 + 2e- + 2Fe2+ -> 2Cl- + 2Fe3+ + 2e-
- (6) Cl2 + 2Fe2+ -> 2Cl- + 2Fe3+
The use of standard electrode potentials is not restricted to electrochemical cells - the same predictions we made are equally true of reactions occurring naturally.
- See also: Electrochemical cell -- [Electrochemical potential]]