exothermic

Meaning: An exothermic process gives off net energy as heat. The products are lower in enthalpy than the reactants, because the energy released in forming new bonds is greater than the energy absorbed in breaking old ones. The surroundings gain heat, so a calorimeter or solvent warms up. Combustion, neutralization, condensation and freezing are examples.

Equation: ΔH = H(products) − H(reactants) < 0, where ΔH is the enthalpy change (J, or J/mol) and H is enthalpy (J). At constant pressure, q = ΔH, where q is the heat absorbed by the system (J), so q is negative. Spontaneity is judged by ΔG = ΔH − TΔS, where T is temperature (K) and ΔS is the entropy change (J/K).

On the MCAT: Expect an energy diagram with products drawn below reactants, or a calorimetry setup where the water temperature rises and q = mcΔT is used to find the heat released. Equilibrium questions treat heat as a product: raising the temperature of an exothermic reaction shifts it toward reactants and lowers K, while cooling does the opposite.

Common confusion: Students equate exothermic with spontaneous or with fast. An exothermic reaction with a large negative ΔS can be nonspontaneous at high T, and it still needs activation energy, so it may be slow. Also keep the sign convention straight: q is negative for the system and positive for the surroundings.