Key Concepts and Summary
Redox reactions are defined by changes in reactant oxidation numbers, and those most relevant to electrochemistry involve actual transfer of electrons. Aqueous phase redox processes often involve water or its characteristic ions, H+ and OH−, as reactants in addition to the oxidant and reductant, and equations representing these reactions can be challenging to balance. The half-reaction method is a systematic approach to balancing such equations that involves separate treatment of the oxidation and reduction half-reactions.
Practice Problems
Balance the equations below assuming they occur in an acidic solution.
(a) $H_2O_2+Sn^{2+}⟶H_2O+Sn^{4+}$
(b) $PbO_2+Hg⟶Hg_2^{2+}+Pb^{2+}$
(c) $Al+Cr_2O_7^{2-}⟶Al^{3+}+Cr^{3+}$
Identify the oxidant and reductant of each reaction of the previous exercise.
Solution
Oxidized: (a) Sn2+; (b) Hg; (c) Al; reduced: (a) H2O2; (b) PbO2; (c) $CrO_7^{2-}$;
oxidizing agent: (a) H2O2; (b) PbO2; (c) $CrO_7^{2-}$;
reducing agent: (a) Sn2+; (b) Hg; (c) Al
Balance the equations below assuming they occur in a basic solution.
(a) $SO_3^{2-}(aq)+Cu(OH)_2(s)⟶SO_4^{2-}(aq)+Cu(OH)(s)$
(b) $O_2(g)+Mn(OH)_2(s)⟶MnO_2(s)$
(c) $NO_3^-(aq)+H_2(g)⟶NO(g)$
(d) $Al(s)+CrO_4^{2-}(aq)⟶Al(OH)_3(s)+Cr(OH)_4^-(aq)$
Identify the oxidant and reductant of each reaction of the previous exercise.
Solution
Oxidized = reducing agent: (a) $SO_3^{2-}$; (b) Mn(OH)2; (c) H2; (d) Al; reduced = oxidizing agent: (a) Cu(OH)2; (b) O2; (c) $NO_3^-$; (d) $CrO_4^{2-}$
Why don’t hydroxide ions appear in equations for half-reactions occurring in acidic solution?
Why don’t hydrogen ions appear in equations for half-reactions occurring in basic solution?
Solution
In basic solution, [OH−] > $1×10^{-7}\;M$ > > [H+]. Hydrogen ion cannot appear as a reactant because its concentration is essentially zero. If it were produced, it would instantly react with the excess hydroxide ion to produce water. Thus, hydrogen ion should not appear as a reactant or product in basic solution.
Why must the charge balance in oxidation-reduction reactions?