Thermochemistry and Calorimetry

Thermochemistry quantifies the heat absorbed or released by chemical reactions, and calorimetry is the experimental art of measuring that heat to obtain reaction enthalpies and heat capacities.

Definition

Thermochemistry is the study of the heat evolved or absorbed in chemical reactions and phase changes, and calorimetry is the set of experimental methods that measure these heats and the heat capacities of substances.

Scope

This topic covers the heat changes accompanying chemical reactions: standard enthalpies of formation, combustion, and reaction; Hess's law of constant heat summation; the temperature dependence of reaction enthalpy through Kirchhoff's law; and bond enthalpies. It covers the calorimetric techniques used to measure these quantities, including bomb and constant-pressure calorimetry and differential scanning calorimetry, together with heat capacity measurement. The statistical-mechanical prediction of heat capacities is treated under statistical thermodynamics.

Core questions

• How does Hess's law allow reaction enthalpies to be combined from tabulated formation enthalpies?
• How does Kirchhoff's law describe the temperature dependence of reaction enthalpy?
• How do bomb and constant-pressure calorimeters measure internal energy and enthalpy changes?
• How are bond enthalpies used to estimate reaction enthalpies?

Key concepts

• Standard enthalpies of formation and reaction
• Hess's law of constant heat summation
• Kirchhoff's law and heat capacity
• Bomb and constant-pressure calorimetry
• Bond enthalpies

Key theories

Hess's law

Because enthalpy is a state function, the enthalpy change of a reaction is the same regardless of the pathway taken, so reaction enthalpies can be added and subtracted from tabulated standard formation enthalpies.

Kirchhoff's law of temperature dependence

The variation of a reaction enthalpy with temperature is governed by the difference in heat capacities between products and reactants, allowing enthalpies measured at one temperature to be corrected to another.

Clinical relevance

Thermochemistry and calorimetry provide the fuel values and reaction heats needed for energy and combustion engineering, the thermal data for process and reactor safety, the stability and folding energetics measured by differential scanning and isothermal titration calorimetry in biochemistry, and the nutritional energy content of foods.

History

Calorimetry began with Lavoisier and Laplace's ice calorimeter in the 1780s; Hess established the additivity of reaction heats in 1840, and Berthelot's nineteenth-century bomb calorimetry made systematic measurement of combustion and formation enthalpies a foundation of physical chemistry.

Key figures

• Germain Henri Hess
• Antoine Lavoisier
• Marcellin Berthelot

Seminal works

• atkins2018
• mcquarrie1997

Frequently asked questions

What is the difference between a bomb calorimeter and a constant-pressure calorimeter?

A bomb calorimeter holds the reaction at constant volume and measures the internal energy change, while a constant-pressure calorimeter, open to the atmosphere, measures the enthalpy change directly; the two are related by the work done against pressure as gases expand or contract.

Why are enthalpies of formation defined relative to the elements?

Absolute enthalpies cannot be measured, so a reference is needed; assigning zero formation enthalpy to elements in their standard states provides a consistent baseline from which the formation enthalpy of any compound, and hence any reaction enthalpy, can be computed.