Thermochemistry: Enthalpy, Hess & Kirchhoff

Module 3 · 3 of 8

🧮Thermochemistry: Enthalpy, Hess & Kirchhoff

Reaction enthalpies from formation data and temperature

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Thermochemistry: Enthalpy, Hess & Kirchhoff

Reaction enthalpies from formation data and temperature

Because enthalpy is a state function, the heat of any reaction can be assembled from tabulated formation enthalpies — Hess's law. Kirchhoff's law then extends a single 298 K value to any temperature using heat capacities.

Difficulty

intermediate

Time

~30 min

Equations

Practice Qs

Learning Objectives

✓Use standard formation enthalpies to compute ΔH°rxn (Hess's law)
✓Explain why Hess's law works (enthalpy is a state function)
✓Apply Kirchhoff's law to convert ΔH between temperatures
✓Distinguish exothermic from endothermic by the sign of ΔH
✓Relate bond enthalpies to approximate reaction enthalpies

Prerequisites

Enthalpy & the first lawHeat capacity Cp

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