Chemical Energetics (Enthalpy & Hess's Law)

Enthalpy change

The enthalpy change $\Delta H$ is the heat energy change at constant pressure. Exothermic reactions release energy ($\Delta H$ negative); endothermic absorb it ($\Delta H$ positive). Standard conditions ($^{\ominus}$): $298\,\text{K}$, $100\,\text{kPa}$, standard states, $1\,\text{mol dm}^{-3}$ solutions.

Key definitions (all "of one mole, under standard conditions"):

$\Delta H_f^{\ominus}$ — formation: 1 mol of compound from its elements.
$\Delta H_c^{\ominus}$ — combustion: 1 mol of substance burned completely in oxygen.