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Hess's Law

Simple Explanation

Hess's law states that the total enthalpy change for a reaction is the same no matter what route is taken from reactants to products β€” whether the reaction happens in one step or several.

Why Do We Need It?

Hess's law lets chemists calculate the enthalpy change of a reaction that would be difficult, dangerous, or impossible to measure directly, by combining the enthalpy changes of other reactions that ARE easy to measure.

Formula

Hess's Law

Ξ”H(overall) = Ξ”H₁ + Ξ”Hβ‚‚ + Ξ”H₃ + …

The overall enthalpy change for a reaction equals the sum of the enthalpy changes for any set of steps that add up to the same overall reaction.

Ξ”H(overall)
β€” enthalpy change for the reaction as a whole, in kJ/mol
Ξ”H₁, Ξ”Hβ‚‚, …
β€” enthalpy changes of each individual step in an alternative pathway between the same starting reactants and final products

When to use it: Whenever a reaction's enthalpy change cannot be measured directly, but can be built from other reactions whose enthalpy changes are already known.

Worked Example

Verify Hess's law with carbon's two-step combustion

Carbon burns directly to COβ‚‚ with Ξ”H = βˆ’393.5 kJ/mol. It can also burn in two steps: C + Β½Oβ‚‚ β†’ CO, Ξ”H₁ = βˆ’110.5 kJ/mol, then CO + Β½Oβ‚‚ β†’ COβ‚‚, Ξ”Hβ‚‚ = βˆ’283.0 kJ/mol. Show that Hess's law holds.

    Why Does This Work?

    Enthalpy is a state function β€” it depends only on the starting and ending states (the identity of the reactants and products), not on the pathway taken between them. Since energy is conserved overall, any route between the same starting and ending points must release or absorb the exact same total energy.

    Real-Life Example

    Finding the enthalpy of an unmeasurable reaction

    The direct formation of carbon monoxide (C + Β½Oβ‚‚ β†’ CO) is nearly impossible to measure directly, because carbon almost always burns all the way to COβ‚‚.

    Chemists instead measure the enthalpy of C β†’ COβ‚‚ directly and CO β†’ COβ‚‚ directly (both easy to measure precisely), then use Hess's law to calculate the otherwise unmeasurable C β†’ CO enthalpy change by subtraction.

    Practice

    Why does Hess's law work?

    Medium

    Common mistake

    Assuming Hess's law only applies to two-step pathways β€” it applies to any number of steps, and also requires you to reverse the sign of Ξ”H if you have to flip a reaction's direction, or multiply Ξ”H by a factor if you scale a reaction's coefficients.

    Quick Review

    • Hess's law: total Ξ”H is the same regardless of the reaction pathway.
    • This works because enthalpy is a state function.
    • It lets you calculate hard-to-measure Ξ”H values from easier ones.
    ← Back to Energy Changes in Chemical ReactionsNext: Calculating Enthalpy Changes Using Hess's Law β†’