Writing Balanced Equations for Redox Reactions Tutorial
- Reactions in which electrons are transferred from one species to another are known as redox reactions, or, oxidation-reduction reactions, or, electron-transfer reactions.
- A redox reaction is made up of two half-reactions:
(i) A reduction half-reaction in which one species, X, gains a electrons.
X + ae- → Xa-
(ii) An oxidation half-reaction in which one species, M, loses b electrons.
M → Mb+ + be-
- In a redox reaction, the number of electrons lost by the species being oxidized must balance the number of electrons gained by the species being reduced.
(1) Multiply balanced reduction half-equation by b
(number of electrons lost in oxidation half-equation)
bX + b ae- → bXa- (2) Multiply balanced oxidation half-equation by a
(number of electrons gained in reduction half-equation)
aM → a be- + aMb+ (3) Add the 2 half-equations together
to give the overall redox equation
bX + aM → bXa- + aMb+
- In a balanced redox reaction equation:
(1) The number of atoms of each element must be balanced
(see balancing chemical equations).
(2) The total charge on the ions on the left hand side of the equation will equal the total charge on the ions on the right hand side of the equation.
(3) Electrons will not appear as either a reactant or a product in the equation
(they have been "cancelled out" in the process of balancing the redox reaction equation).
- Redox Reactions are very widely used, for example: to
(i) generate electricity (galvanic cells, fuel cells)
(ii) recharge rechargable batteries
(iv) plate one metal onto another electrolytically in a process called electroplating
(v) quantitatively determine the amount of a substance present (redox titrations, ethanol in alcoholic drinks, concentration of hypochlorite in commercial bleach)