What does the enthalpy change of a reaction (Δ H rxn) depend on?

The enthalpy change of a reaction, denoted as Δ H rxn, fundamentally depends on the energies of the bonds that are broken and formed during the reaction process. This is a core principle of thermochemistry, as the enthalpy change measures the total energy difference between the reactants and the products.

When a chemical reaction occurs, bonds within the reactants must be broken (which requires energy), and new bonds are formed in the products (which releases energy). The difference between the total energy required to break the bonds in the reactants and the total energy released from forming the new bonds in the products results in the overall change in enthalpy for the reaction. Therefore, the specific energies of these bonds directly influence the Δ H rxn.

Other factors, such as temperature changes of the system and the concentrations of reactants, affect the rate or direction of a reaction but do not inherently change the calculated enthalpy change for the reaction itself. Additionally, catalysts can speed up a reaction without altering the overall energy changes involved, meaning they do not affect the Δ H rxn directly either. Thus, the energies of bonds broken and formed are the primary consideration in determining the enthalpy change of a reaction.