“How can we find the change in enthalpy for a chemical reaction without actually performing the reaction?”
Finding the change in enthalpy for a chemical reaction is a rather straightforward procedure, one simply carries forward with the necessary steps and measures the temperature before and after the reaction took place. However, some reactions take an extraordinary long time to perform, or their process is highly volatile. So how can we find the change in enthalpy for such reactions? Well let’s think about it. We know that if we were to take one chemical reaction and reverse it, then the resulting change in enthalpy would reverse in sign. And we know that if we add one element of a chemical equation to the opposite side of an equation containing that element, then they would cancel out. So what if were to take the results of some reactions that we already know, modify them if necessary, and then add them together to fashion the equation of the reaction that we desire? This is the operating principle behind Hess’ law.
To illustrate, let’s examine the reaction Mg(s) + H2O(l) → MgO(s) +H2(g). Since Mg does not react with water, completing this experimentally would be a nightmarish process. However, we can easily obtain the results for Mg(s) +2HCL(aq) → MgCl2(aq) + H2 and MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O. If we were to take the former equation and subtract the latter from it, we would be able to obtain our desired equation. All we need to do is obtain the change in enthalpies for these reactions, and then proceed forward with the mathematics, and next thing you know we would obtain our necessary results!