In an acidic soil, nitrate ions may undergo reduction to form ammonium ions. Deduce a half-equation for the reaction.

${\text{NO}}_3^ -  + {\text{10}}{{\text{H}}^ + } + {\text{8}}{{\text{e}}^ - } \to {\text{NH}}_4^ +  + {\text{3}}{{\text{H}}_2}{\text{O}}$

correct reactants;

correct products;

correct balancing;

M3 can only be scored if M1 and M2 correct.

The equation caused some difficulty for many students; electrons were omitted on the left-hand side, incorrect products were given and equations were left unbalanced.