Hess's Law of Constant Heat Summation
Definition and meaning of Hess's Law of Constant Heat Summation in chemistry.
Hess's law of constant heat summation states that the total enthalpy change of a chemical reaction is the same whether the reaction occurs in a single step or through a series of steps, as long as the initial and final states are identical.
In more detail
This holds because enthalpy is a state function: its value depends only on the initial and final states of a system, not on the pathway taken between them. As a result, thermochemical equations can be added, subtracted, reversed, or multiplied like algebraic equations to obtain the enthalpy change of a reaction that is difficult or unsafe to measure directly. Hess's law is the theoretical foundation for calculating standard enthalpies of reaction from tabulated standard enthalpies of formation or combustion.
Key facts
| Field | Physical Chemistry |
|---|---|
| Proposed by | Germain Hess, 1840 |
| Underlying principle | Enthalpy (H) is a state function |
| Main use | Calculating ΔH for reactions not directly measurable |
The enthalpy of formation of CO2(g) (ΔH = −393.5 kJ/mol) can be obtained by summing two measurable steps: C(s) + ½O2(g) → CO(g), ΔH1 = −110.5 kJ/mol, and CO(g) + ½O2(g) → CO2(g), ΔH2 = −283.0 kJ/mol. Adding ΔH1 + ΔH2 gives −393.5 kJ/mol, matching the direct combustion value.
Frequently asked questions
Why does Hess's law work?
Because enthalpy is a state function, the total enthalpy change of a reaction depends only on the initial and final states, not on the path or number of steps, so intermediate reaction enthalpies can be combined algebraically to give the same overall value.
How is Hess's law applied in practice?
Known thermochemical equations, such as standard enthalpies of formation or combustion, are added or reversed so their intermediates cancel, yielding the enthalpy change of the target reaction without measuring it directly.