Thermochemistry & Hess's Law

Energy in chemical reactions — exothermic vs endothermic

Thermochemistry studies energy changes in chemical reactions. Enthalpy (H) is heat flow at constant pressure. Exothermic reactions release heat (ΔH < 0): combustion, neutralization, most phase changes from gas → liquid → solid. Endothermic reactions absorb heat (ΔH > 0): photosynthesis, dissolving NH₄NO₃, most decompositions. Bond energy approach: breaking bonds requires energy (endothermic), forming bonds releases energy (exothermic). ΔH = energy in (break bonds) - energy out (form bonds). Hess's Law: ΔH for a reaction equals the sum of ΔH for any series of steps with the same overall equation — enthalpy is a state function (path-independent). Standard enthalpies of formation (ΔHf°) are measured for 1 mol from elements in standard state.