Chapter 8 · Quantities in Chemical Reactions
8.7Enthalpy: A Measure of the Heat Evolved or Absorbed in a Reaction
12 min · two checks
Predict
A reaction has ΔH = −890 kJ per mole of methane burned. Does the surroundings warm or cool?
The idea
- Interpret the sign of ΔH.
- Scale reaction heat with a mole ratio, just like mass.
Enthalpy change, ΔH, is the heat exchanged at constant pressure, reported for the equation as written. Negative ΔH is exothermic: the system loses enthalpy and the surroundings gain heat. Positive ΔH is endothermic. The value is tied to the coefficients. If CH₄ + 2 O₂ → CO₂ + 2 H₂O has ΔH = −890 kJ, that is the heat for one mole of methane, not for one gram, and not for a different balancing of the same chemistry. Double the equation and you double ΔH.
Stoichiometry of heat uses ΔH as a conversion factor. For that methane equation, 1 mol CH₄ = −890 kJ, where the minus sign records direction. The heat released when 2.00 mol burn is 1780 kJ. Report both the magnitude and whether heat is released or absorbed. Per-gram values require one more step through molar mass. Do not attach ΔH to an unbalanced equation.
Keep these
- ΔH < 0 exothermic; ΔH > 0 endothermic. The sign is the system’s.
- ΔH scales with the coefficients.
- Use ΔH like a mole conversion factor to find heat from amount.
Worked path
CH₄ + 2 O₂ → CO₂ + 2 H₂O, ΔH = −890 kJ. How much heat is released when 8.0 g of methane burn? Molar mass 16.0 g/mol.
Observe
8.0 g is 0.50 mol CH₄. The reaction is exothermic.