What critical concept does Δ H rxn rely on for its calculation?

The calculation of Δ H rxn, or the change in enthalpy for a reaction, fundamentally relies on the energy associated with the bonds that are broken and formed during that reaction. When reactants undergo a chemical change, the bonds holding the reactants together must be broken, which requires energy input. Conversely, when products are formed, new bonds are created, releasing energy.

The difference between the energy required to break the bonds in the reactants and the energy released from forming bonds in the products determines the overall change in enthalpy of the reaction. This concept is rooted in bond energies, which indicate how much energy is needed to break specific types of bonds and how much energy is released when those bonds are formed.

While factors such as concentrations, volume of gas produced, and temperature changes can influence reaction rates and equilibrium, they are not directly critical for calculating the change in enthalpy itself. Instead, the focus for Δ H rxn is on the bond energies involved and the net energy change resulting from the reaction.