Final answer:
In the reaction equation, no changes are specified for Ni because the activities of pure crystalline solids are constant at a value of 1, but the mass of Ni can change during the reaction.
Step-by-step explanation:
In the given chemical equation, no changes are specified for Ni because the activities of pure crystalline solids such as nickel (Ni) are considered to be constant with a value of 1. The changes in the equation generally refer to the concentrations of reactants and products in a chemical reaction. However, this does not apply to solids and pure liquids because their concentrations are not variable; they are always at their maximum and thus do not appear in the expression for the reaction quotient Q or the equilibrium constant K. Nevertheless, the mass of Ni does change even though its activity remains constant.
Therefore, when considering changes in a chemical reaction involving solids, we focus on the change in their mass rather than their concentration or activity in the system.