Because the target equation has C2H2 as a product, we turn the first equation around; the sign of ∆H is therefore changed. Because the target equation has 2C(s) as a reactant, we multiply the second equation and its ∆H by2. Because the target equation has H2 as a reactant, we keep the third equation as it is. We then add the three equations and their enthalpy changes in accordance with Hess's law: