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)

Calculating the enthalpy, or change of total system temperature, of a chemical reaction, allows scientists to determine the amount of energy exchanged between the environment and a given chemical reaction. The enthalpy change for a reaction is equal to the sum of the enthalpies of formation of all the products, minus the sum of the enthalpies of formation of all the reactants.

1. Tables and Balancing

To calculate the enthalpy of a chemical reaction, first balance the chemical equation. When that is done, use a heat of formation table to determine the heat of formation (ΔHf) values for the compounds involved in the equation. Take note of each compound’s heat of formation value.

2. Determine Products and Reactants

According to Hess’s Law, one of the foundations of thermodynamics, the total enthalpy change for a chemical reaction is independent of the route by which that chemical change occurs. In other words, regardless of how many steps are involved in the chemical reaction, the enthalpy change for a reaction is equal to the sum of the enthalpy of formation of all the products, minus the sum of the enthalpy of formation of all the reactants. The equation for enthalpies of reactions can then be expressed as:

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.

3. Determine Number of Moles

On a heat of formation table, the ΔHf value for a given compound is listed in terms of kilojoules (kJ) per mole (mol). Each listed value is the heat of formation of a single unit of the given compound. If calculating the enthalpy of a chemical reaction involving multiple units of a compound, multiply the ΔHf values by the necessary moles. When this is done, you can complete the Hess’s Law equation to calculate the enthalpy of the chemical reaction.