Thermodynamics studies the energy changes accompanying physical and chemical processes in macroscopic systems, considering only initial and final equilibrium states, not the rate or mechanism of change. The system, surroundings and boundary are defined, and systems are classified as open, closed or isolated by their exchange of matter and energy. The internal energy \(U\) is a state function whose change in an adiabatic process equals the work done; combined with heat it gives the first law of thermodynamics, \(\Delta U=q+w\), an expression of energy conservation. Pressure-volume work is examined for reversible, irreversible and free expansions. Enthalpy is defined as \(H=U+pV\), with the heat absorbed at constant pressure equal to \(\Delta H\), related to \(\Delta U\) by \(\Delta H=\Delta U+\Delta n_g RT\), and with \(C_p-C_v=R\) for an ideal gas. Calorimetry (bomb and constant-pressure) measures these quantities. Standard reaction, formation, combustion, atomisation, bond, lattice (via the Born-Haber cycle) and solution enthalpies are introduced, together with Hess's law of constant heat summation. Spontaneity requires more than a fall in enthalpy: the entropy \(S\), a measure of disorder, and the second and third laws are introduced, and Gibbs energy \(G=H-TS\) provides the criterion \(\Delta G<0\) for a spontaneous change. Finally, \(\Delta G^{\circ}=-RT\ln K\) links Gibbs energy to chemical equilibrium.
System, surroundings and types of systemsInternal energy and the first lawPressure-volume workEnthalpy and heat capacityCalorimetryReaction, formation and bond enthalpiesHess's law and Born-Haber cycleEntropy, spontaneity and Gibbs energyGibbs energy and equilibrium
Key terms
Internal energy
The total energy of a system, a state function whose change equals the adiabatic work done on the system.
Enthalpy
A state function defined as \(H=U+pV\); its change equals the heat absorbed at constant pressure.
State function
A property such as \(U\), \(H\), \(p\), \(V\) or \(T\) whose value depends only on the state of the system, not on the path taken.
Entropy
A state function measuring the degree of randomness or disorder; for a reversible process \(\Delta S=q_{rev}/T\).
Gibbs energy
The function \(G=H-TS\); a negative change \(\Delta G\) indicates a spontaneous process.
Hess's law
The total enthalpy change of a reaction is the same whether it occurs in one step or several steps.
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The first law of thermodynamics is expressed as:
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