Heat of Reaction
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Tabulated enthalpies of reaction are quoted per mole, so the total heat is simply the moles reacted times ΔH. By convention ΔH is negative for exothermic reactions — burning 2 mol of methane with ΔH = −890 kJ/mol releases 1780 kJ, the negative q signalling heat flowing out of the system. Endothermic reactions carry positive ΔH and soak heat in, like the ammonium nitrate cold packs in a first-aid kit.
Heat of Reaction
Where
- = Heat released or absorbed
- = Amount of substance
- = Molar enthalpy change
Missing one of these? Work it out first, then come back
- Heat released or absorbed — Latent Heat, Sensible Heat (Q = mcΔT)
- Amount of substance — Moles from Mass (n = m/M), Particles from Moles (Avogadro's Number)
- Molar enthalpy change — Gibbs Free Energy Change (ΔG = ΔH − TΔS), Hess's Law (Three-Step Sum)