Energetics & electrochemistry · SL/HL

Hess / formation enthalpy

Apply products minus reactants to formation enthalpies.

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Number formats & units

Use decimal dots in datasets. Single values also accept decimal commas and scientific notation such as 1.2e-3 or 1.2 × 10⁻³.

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Example

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\Delta H^\circ_{\mathrm{reaction}}=\sum_{\mathrm{products}}\nu_i\Delta H^\circ_{\mathrm f,i}-\sum_{\mathrm{reactants}}\nu_i\Delta H^\circ_{\mathrm f,i}

What the variables mean

Symbol	Meaning
$\nu$	Stoichiometric coefficient.
$\Delta H_{\mathrm f}^{\circ}$	Standard enthalpy of formation.

74.8 kJ mol⁻¹

Follow the example steps

Include coefficients in products minus reactants.

A → 2B: ΔH = −80 − (−100)

ΔH = 2 × (−80) − (−100) = −60 kJ mol⁻¹

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Method notes

This is one application of Hess’s law. Check that physical states and thermochemical data correspond to the reaction being analysed.