6.9 Hess's Law

Hess's Law basics

Hess's Law states that the overall enthalpy change (ΔH) for a reaction is the same regardless of the number of steps taken to get from reactants to products. Since enthalpy is a state function, ΔH depends only on initial and final states, not the pathway. This lets chemists calculate ΔH for reactions that are hard to measure directly by combining known reactions.

Manipulating equations

To combine reactions, you may reverse an equation (flip the sign of ΔH), or multiply an equation by a coefficient (multiply ΔH by the same factor). Add the adjusted equations so intermediate species cancel, leaving the target reaction. Sum the adjusted ΔH values to get the overall ΔH.

Using standard enthalpies

Hess's Law also underlies the formula ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants), using standard enthalpies of formation. This treats formation reactions as the intermediate steps: decomposing reactants to elements, then forming products, summing enthalpy changes along the way.

Key terms

Hess's Law

The total ΔH for a reaction is the same no matter how many steps the reaction is carried out in, because enthalpy is a state function.

State function

A property (like enthalpy) that depends only on the initial and final states of a system, not the path taken.

Reversing a reaction (Hess's Law)

Flips the sign of ΔH for that reaction.

Multiplying a reaction (Hess's Law)

ΔH must be multiplied by the same factor as the equation's coefficients.

ΔH°rxn formula using formation enthalpies

ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants)

Try an AP-style question

A → B has ΔH = −100 kJ, and B → C has ΔH = +40 kJ. What is ΔH for A → C?

1. +60 kJ
2. +140 kJ
3. −60 kJ
4. −140 kJ