# Writing Balanced Equations for Redox Reactions Tutorial

## Key Concepts

- 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
*red**ox*reaction is made up of two half-reactions:(i) A

*red*uction half-reaction in which one species,*X*, gains*a*electrons.*X*+*a*e^{-}→*X*^{a-}(ii) An

*ox*idation half-reaction in which one species,*M*, loses*b*electrons.*M*→*M*+^{b+}*b*e^{-} - In a
*re**dox*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)**b**X*+ ~~b~~~~ae~~^{-}→ *b**X*^{a-}(2) Multiply balanced *oxidation*half-equation by*a**(number of electrons gained in reduction half-equation)**a**M*→ ~~a~~~~be~~^{-}+ *a**M*^{b+}(3) Add the 2 half-equations together

to give the overall redox equation*b**X*+ *a**M*→ *b**X*^{a-}+ *a**M*^{b+} - 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

(iii) extract of metals from their ores electrolytically (electrowinning, electrorefining, extraction of aluminium, extraction of sodium)

(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)