During a chemical reaction, energy is transferred in the form of heat. To determine whether a chemical reaction is endothermic or exothermic – whether the reaction absorbs heat or releases heat – we can measure the heat exchange between the chemical reaction and its environment. However, because the exchange of heat cannot be measured directly, scientists measure the change in the temperature of a given reaction, or the enthalpy of a chemical reaction, to reach the same conclusion. With a calculator and a heat of formation table in hand, calculating enthalpies of reaction is simple.
TL;DR (Too Long; Didn't Read)
Tables and Balancing
Determine Products and Reactants
ΔHr = ∑ΔHf(products) − ∑ΔHf(reactants)
Determine which of the compounds are products or reactants in the chemical equation and then plug them into the Hess’s Law equation.