6.6 Introduction to Enthalpy of Reaction

What is enthalpy of reaction?

Enthalpy (H) is a state function related to a system's internal energy plus pressure-volume work. The enthalpy of reaction (ΔH_rxn) is the heat absorbed or released during a chemical reaction at constant pressure. It's typically calculated per mole of reaction as written, using stoichiometric coefficients from the balanced equation.

Exothermic vs. endothermic

If ΔH is negative, the reaction is exothermic — heat flows from system to surroundings, and products have lower enthalpy than reactants. If ΔH is positive, the reaction is endothermic — heat flows into the system, and products have higher enthalpy than reactants. This sign convention is essential for interpreting energy diagrams.

Scaling with stoichiometry

ΔH_rxn values scale directly with the amount of substance reacting. If you double the moles of reactants, you double the ΔH. This proportionality lets chemists calculate the heat released or absorbed for any given mass or moles of reactant, connecting thermochemistry to stoichiometry calculations.

Key terms

Enthalpy (H)

A state function representing internal energy plus pressure-volume work; used to track heat flow at constant pressure.

Enthalpy of reaction (ΔH_rxn)

The heat absorbed or released during a reaction at constant pressure, per mole of reaction as written.

Exothermic reaction

A reaction with negative ΔH; heat flows from the system to the surroundings; products have lower enthalpy than reactants.

Endothermic reaction

A reaction with positive ΔH; heat flows into the system; products have higher enthalpy than reactants.

Stoichiometric scaling of ΔH

ΔH_rxn is proportional to the amount of reactants/products; doubling the equation doubles ΔH.

Try an AP-style question

2H₂ + O₂ → 2H₂O has ΔH = −572 kJ. What is ΔH per mole of H₂O formed?

1. −572 kJ
2. +286 kJ
3. −286 kJ
4. −1144 kJ