Final answer:
Hess's law allows for the combination and manipulation of reactions in order to calculate their enthalpy changes.
Step-by-step explanation:
Hess's law allows us to combine reactions algebraically and then combine their enthalpy changes in the same way. It enables us to calculate the enthalpy change of a chemical reaction by manipulating and adding the enthalpy changes of other reactions. For example, if we want to calculate the enthalpy change of the reaction that forms acetylene from carbon and hydrogen, we can use Hess's law to combine other reactions whose enthalpy changes are known and manipulate them to obtain the desired reaction.