4. In this question we need to find the oxidation number of each individual element, first, let's write the reaction:
P + Cu2+ -> Cu + H2PO4
Important things to have in mind
Oxidation number of single atoms is = 0
Oxidation number of Oxygen = -2
Oxidation number of Hydrogen = +1
These are informations that help us determine other oxidation numbers, therefore:
P (reactant) = 0
Cu (reactant) = +2 (it has a charge already)
Cu (product) = 0
H2 = 1*2 = 2
O = -2*4 = -8
P (product) = here we have to match the value given from H and O, therefore +2 -8 = -6, but H2PO4 is an anion, having as a final charge -1, therefore P must have a charge that when calculated with -6 it must give -1 as a result, the only value possible is +5, -6 + 5 = -1, and +5 is one of the possible values of oxidation number for P
Now balancing this redox reaction in acidic solution, we will have to add water in this case, and the reaction will be the following:
2 P + 5 Cu2+ + 8 H2O -> 2 H2PO4- + 5 Cu + 12 H+
