To find the enthalpy change for the reaction MgO s + 2HCl aq MgCl aq + H g , we can use Hess's Law, which states that the enthalpy change of a reaction is the same whether it occurs in one step or several steps. We can manipulate the given reactions to obtain the desired reaction and then sum their enthalpy changes.First, we need to reverse reaction 3 and multiply it by 2 to obtain 2HO l 2H g + O g with H = 2 * 285.8 kJ/mol = 571.6 kJ/mol.Next, we can add reaction 1 and the modified reaction 3 to obtain the following reaction:Mg s + ½O g + 2HCl aq MgCl aq + 2HO l with H = H + H = -454.9 kJ/mol + 571.6 kJ/mol = 116.7 kJ/mol.Now, we can subtract reaction 2 from the combined reaction 5 to obtain the desired reaction:MgO s + 2HCl aq MgCl aq + H g with H = H - H = 116.7 kJ/mol - -131.3 kJ/mol = 248 kJ/mol.So, the enthalpy change for the reaction MgO s + 2HCl aq MgCl aq + H g is 248 kJ/mol.