Thermochemistry

Thermochemistry applies the first law of thermodynamics — energy is conserved — to chemical change. Reactions that release heat to the surroundings are exothermic and have a negative enthalpy change (ΔH < 0); reactions that absorb heat are endothermic with ΔH > 0. Because most laboratory reactions run at constant pressure, enthalpy is the convenient bookkeeping quantity, and heat flow is measured by calorimetry using q = mcΔT, where c is the specific heat capacity.

Enthalpy is a state function, so its change depends only on the initial and final states and not on the path taken. That is the basis of Hess's law: the enthalpy change of an overall reaction equals the sum of the enthalpy changes of any set of steps that add up to it. In practice you compute ΔH from standard enthalpies of formation as the sum for products minus the sum for reactants, or estimate it from bond energies, since breaking bonds costs energy and forming bonds releases it.

Enthalpy alone does not determine whether a reaction proceeds. Entropy (S) measures the dispersal of energy and matter, and the Gibbs free energy change ties the two together: ΔG = ΔH − TΔS. A negative ΔG means the process is spontaneous in the forward direction; a positive ΔG means it is not; and at ΔG = 0 the system is at equilibrium. Because temperature multiplies the entropy term, some reactions switch between spontaneous and nonspontaneous as temperature changes. Free energy also links to the equilibrium constant, which is why thermochemistry underlies bioenergetics — ATP hydrolysis providing the free energy that drives otherwise unfavorable cellular reactions.

The MCAT tests thermochemistry across both the chemistry and physics section and the biochemistry content. Expect calculations using Hess's law and enthalpies of formation, sign conventions for exothermic and endothermic processes, and questions that require predicting spontaneity from ΔH, ΔS, and temperature, then applying the result to coupled biological reactions.