Hess's Law

AQA A-Level Chemistry

Hess’s Law

If a reaction takes place by more than one route, the enthalpy change of the reaction is the same, regardless of the route taken.

Enthalpy of Formation

∆_rH = ∑∆_fH (products) - ∑∆_fH (reactants)

Enthalpy of Combustion

∆_rH = ∑∆_cH (reactants) - ∑∆_cH (products)

Bond Enthalpies

∆_rH = ∑ bonds broken (reactants) - ∑ bonds made (products)

Bond Enthalpy Definition

The enthalpy change when one mole of bonds in gaseous molecules are broken under standard conditions.