How can thermodynamic properties of reactants and products be used to calculate changes during a chemical reaction?

In thermodynamics, the changes in properties during a chemical reaction are calculated by determining the difference between the sum of the properties of the products and the sum of the properties of the reactants. Because these properties—such as enthalpy, entropy, and free energy—are state functions, their values depend only on the initial and final states of the system, not the path taken.12

For a general reaction, the change in a property ($\Delta X$) is expressed as:
$$\Delta X_{\text{reaction}}=\sum X_{\text{products}}-\sum X_{\text{reactants}}$$

Key applications include:

• Enthalpy of Reaction ($\Delta H$): Calculated as the difference in enthalpy between products and reactants. Hess’s law allows for the determination of these values using standard heats of formation or combustion, even when direct measurement is impractical.34
• Gibbs Free Energy ($\Delta G$): Used to determine the spontaneity of a reaction. Under constant temperature and pressure, a reaction is spontaneous if $\Delta G$ is negative. The standard Gibbs free energy of reaction ($\Delta G^{\circ }$) is also directly related to the equilibrium constant ($K_c$) of the reaction.25

By utilizing extensive tables of thermodynamic data for individual substances, scientists can predict the energy changes and equilibrium states of complex chemical processes.1