You are trying to experimentally determine the value for Δ H_{{reaction}}. Before that, you should calculate a theoretical value using the enthalpy of formation for each metal ion and a Hess cycle
A. Perform experimental determination of Δ H_{{reaction}}
B. Calculate theoretical Δ H_{{reaction}} using enthalpies of formation
C. Use a Hess cycle for the theoretical calculation
D. Compare experimental and theoretical values for validation