The combination of HCl and MgO causes a chemical reaction that gives the products MgCl2 and H2O. The balanced equation for this reaction is 2HCl + MgO -> MgCl2 + H2O.
This formula can be used to measure the enthalpy of the reaction that occurs when magnesium metal is burned (2Mg + O2 -> 2MgO). This reaction results in the magnesium metal burning quickly at high temperature, leaving MgO behind. The rapid reaction is difficult to measure. However, by using Hess's Law, it is possible to combine the enthalpy changes in other reactions that add up to the final reaction.