4. According to the explanation given in one previous session, in this case we have the following reaction:
Zn + NO3- -> Zn(OH)42- + NH3
Our important informations to have in mind always first:
Single elements = 0 of oxidation number
Zn = 0
Oxygen = 2-
NO3- = overall charge must be -1, and we have 3 oxygens, so a -6 charge, therefore N must give a charge of +5
N (reactant) = 5+
Zn(OH)42- = overall charge -2, the charge for OH is always -1, we have 4 OH, therefore -4 of charge, to give -2 of overall, Zn must have a charge of +2
Zn (product) = 2+
OH = 1-
NH3 = 0
Now for the balancing of the reaction:
4 Zn + NO3- + 7 OH- + 6 H2O -> NH3 + 4 Zn(OH)42-