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Hess's law: total enthalpy change is path-independent (consequence of H being a state function). Three methods to calculate delta_H: (1) Formation enthalpies: delta_H = sum(n * delta_Hf products) - sum(n * delta_Hf reactants). Elements in standard state have delta_Hf = 0. (2) Bond energies: delta_H = sum(BE bonds broken) - sum(BE bonds formed). Breaking bonds requires energy (+), forming bonds releases energy (-). This gives approximate results because mean bond energies are averages. (3) Combustion enthalpies: delta_H = sum(n * delta_Hc reactants) - sum(n * delta_Hc products). Note the reversal compared to formation! The sign convention difference is the most common source of errors: formation uses products minus reactants, while bond energy and combustion use reactants minus products (in different senses). Thermochemical equation rules: reversing changes the sign, multiplying scales delta_H, adding equations adds delta_H values. Fractional coefficients are allowed. Always specify physical states — H2O(g) vs H2O(l) differ by 44 kJ/mol.