Hess’s law is very powerful. It allows us to combine equations to generate new chemical reactions whose enthalpy changes can be calculated, rather than directly measured. Besides that, Hess’s law states that when chemical equations are combined algebraically, their enthalpies can be combined in exactly the same way. Below points are to be taken as well: 1) If a chemical reaction is reversed, the sign on ΔH is changed; 2) If a multiple of a chemical reaction is taken, the same multiple of the ΔH is taken as well. As attached is an example of Hess Law problem solving question. By using Hess law , combine the equation algebraically and determine the enthaphy change of the reaction.