$word_{count}$: 210

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.