Thermochemical equations must specify: (1) Physical states of all species (s, l, g, aq). (2) The enthalpy change for the equation as written. (3) The temperature (usually 298 K). Key rules: if the equation is reversed, delta_H changes sign. If multiplied by n, delta_H is multiplied by n. If two equations are added, their delta_H values add. Fractional coefficients are allowed (e.g., 1/2 N2 + 3/2 H2 -> NH3). delta_H depends on physical states: H2O(g) vs H2O(l) differ by delta_H_vaporisation (44 kJ/mol at 25 C). Always check the state of water in combustion problems.