Thermodynamic Laws in Chemistry

The laws of thermodynamics, applied to chemical systems, fix how energy is conserved in reactions, why some reactions proceed spontaneously, and how entropies approach a definite limit at absolute zero.

Definition

The thermodynamic laws in chemistry are the first, second, and third laws of thermodynamics specialized to chemical change, governing reaction energetics, the direction of spontaneous reactions, and the absolute entropies of substances.

Scope

This topic covers the chemical application of the four laws: the zeroth law and temperature; the first law expressed through internal energy, enthalpy, and reaction heats; the second law expressed through entropy and the Clausius inequality as the criterion of spontaneity; and the third law, which assigns absolute entropies and underlies the calculation of equilibrium constants from calorimetric data. The detailed treatment of free energy criteria, chemical potential, and equilibrium is developed in sibling topics.

Core questions

• How does the first law relate reaction heat at constant volume to that at constant pressure through internal energy and enthalpy?
• Why does the entropy of an isolated system not decrease, and how does this define spontaneity?
• How does the third law allow absolute entropies to be tabulated and used in chemical calculations?
• How are state functions distinguished from path-dependent quantities such as heat and work?

Key concepts

• Internal energy and enthalpy of reaction
• Entropy and the Clausius inequality
• Reversible and irreversible processes
• Absolute entropy and the third law
• State functions versus path functions

Key theories

First law for chemical systems

The change in internal energy of a reacting system equals the heat absorbed minus the work done; at constant pressure this is captured by the enthalpy, making reaction enthalpy a measurable state function independent of path.

Second and third laws as criteria and reference

The second law makes entropy production the universal criterion for spontaneous chemical change, while the third law fixes the entropy of a perfect crystal at zero at absolute zero, providing absolute entropies for reaction calculations.

Clinical relevance

These laws provide the bookkeeping for all chemical energetics, from the heat released by combustion and the feasibility of industrial syntheses to the entropy-driven processes of dissolution, mixing, and biological self-assembly.

History

The chemical use of thermodynamics grew from Hess's 1840 law of constant heat summation and the mid-nineteenth-century formulation of the first and second laws by Clausius and Kelvin; Nernst's heat theorem of 1906, the basis of the third law, completed the framework by making absolute entropies accessible.

Key figures

• Rudolf Clausius
• Walther Nernst
• Germain Henri Hess

Seminal works

• atkins2018
• mcquarrie1997

Frequently asked questions

Why do chemists usually work with enthalpy rather than internal energy?

Most reactions are run at constant atmospheric pressure rather than constant volume, and at constant pressure the heat exchanged equals the enthalpy change, so enthalpy is the directly measurable and most convenient energy function for chemistry.

Can the entropy of a chemical system decrease?

Yes, locally. A system can lose entropy, as when a gas condenses or a solid crystallizes, provided the entropy of the surroundings increases by at least as much so that the total entropy of system plus surroundings does not decrease.