Learn how to plan an investigation and select the correct variables to control with this experiment to determine the enthalpy change for the reaction MgSO4 (s) + 7H2O(l) → MgSO4•7H2O(s) using Hess cycles. By measuring the enthalpy change of solution for the two solids and using a Hess cycle to calculate the enthalpy change for the reaction, we can gain a better understanding of thermodynamic principles.