High school11 min

Reaction enthalpy: enthalpies of formation, Hess's law, calorimetry

Calculate a standard reaction enthalpy from enthalpies of formation, combine equations using Hess's law and work up a calorimeter measurement.

The essentials

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Reaction enthalpy

At constant pressure, the heat exchanged is Qp=ΔH=Hproducts−HreactantsQ_p = \Delta H = H_{\text{products}} - H_{\text{reactants}}. ΔH<0\Delta H < 0: exothermic. ΔH>0\Delta H > 0: endothermic. Unit: kJ/mol\text{kJ/mol}, meaning per mole of reaction as the equation is written. ΔH∘\Delta H^\circ is the standard value: at 1 bar1\ \text{bar}, tabulated at 298 K298\ \text{K} (25 ∘C25\ ^\circ\text{C}).
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Standard enthalpy of formation

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Products minus reactants

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Hess's law

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Calorimetry

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