Oxidation number of an atom is the charge that atom would have if the compound is composed of ions. In neutral substances that contains atoms of one element the oxidation number of an atom is zero. Thus atoms in O2, Ni2, and aluminium all have oxidation number of zero. In this case, Ni2, the oxidation number of Ni atom is zero, for NiO4-, assuming oxidation number of Ni is x (x ×1) + (-2 × 4) = -1 x = + 7 Therefore, the oxidation number goes from 0 to +7
According to the gas law, the pressure of the gas is related to the temperature when the volume is constant as P1/T1=P2/T2. So the pressure will increase and the container may explode.