heat of reaction

Meaning: The heat of reaction is the heat absorbed or released when a reaction proceeds as written. At constant pressure it equals the enthalpy change, ΔH. Negative ΔH is exothermic (heat released); positive ΔH is endothermic (heat absorbed). Enthalpy is a state function, so ΔH depends on the start and end states, not the path (Hess's law). ΔH scales with the coefficients and flips sign when the reaction is reversed.

Equation: ΔH°rxn = ΣnΔH°f(products) − ΣmΔH°f(reactants), where n and m are coefficients (mol) and ΔH°f is the standard enthalpy of formation (J/mol). From bond energies: ΔH ≈ Σ(bonds broken) − Σ(bonds formed). Calorimetry: q = mcΔT, with m in kg, c in J/(kg·K), and ΔT in K.

On the MCAT: Expect ΔH calculated from a table of ΔH°f or bond energies, a Hess's law problem combining steps, a coffee-cup calorimeter where the water's temperature change gives q, or an energy diagram comparing reactant and product levels.

Common confusion: If the surrounding water warms, the reaction released heat, so ΔH of the reaction is negative. A negative ΔH alone does not guarantee spontaneity; ΔG = ΔH − TΔS decides. A catalyst changes the activation energy, not ΔH.