Chemical Thermodynamics

Chemical Thermodynamics

Chemical Thermodynamics is the branch of thermodynamics that studies the interrelation of heat and work with chemical reactions or physical changes of state within chemical systems. It provides the framework to predict whether a reaction will occur spontaneously and to what extent it proceeds.

Basic Concepts

Chemical thermodynamics deals with the energy changes and equilibrium conditions in chemical reactions, focusing on variables such as enthalpy, entropy, Gibbs free energy, and equilibrium constants.

Key thermodynamic quantities include:

• Internal Energy ($U$)
• Enthalpy ($H$)
• Entropy ($S$)
• Gibbs Free Energy ($G$)
• Helmholtz Free Energy ($F$)

Laws of Thermodynamics in Chemistry

• First Law: Conservation of energy in chemical processes. The change in internal energy $\mathrm{\Delta }U$ is equal to the heat added to the system plus the work done on the system:

$\mathrm{\Delta }U=q+w$

where $q$ is heat and $w$ is work.

• Second Law: Entropy of an isolated system always increases or remains constant:

$\mathrm{\Delta }{S}_{\text{total}}\ge 0$

This law defines the direction of spontaneous chemical reactions.

Gibbs Free Energy and Reaction Spontaneity

The Gibbs free energy change ($\mathrm{\Delta }G$) at constant temperature and pressure determines spontaneity:

$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$

where:

• $\mathrm{\Delta }H$ = enthalpy change
• $T$ = absolute temperature
• $\mathrm{\Delta }S$ = entropy change

A reaction is:

• Spontaneous if $\mathrm{\Delta }G<0$
• At equilibrium if $\mathrm{\Delta }G=0$
• Non-spontaneous if $\mathrm{\Delta }G>0$

Equilibrium Constant and Standard Gibbs Energy

The relationship between the standard Gibbs free energy change ($\mathrm{\Delta }{G}^{\circ }$) and the equilibrium constant ($K$) is:

$\mathrm{\Delta }{G}^{\circ }=-RT\ln K$

where $R$ is the gas constant and $T$ is the temperature in Kelvin.

This allows prediction of equilibrium position from thermodynamic data.

Applications

• Predicting reaction direction and extent
• Calculating equilibrium constants
• Understanding phase changes and solution chemistry
• Designing industrial chemical processes
• Studying biological energetics

References

• Atkins, P., & de Paula, J. (2010). Physical Chemistry. Oxford University Press.
• Laidler, K. J. (1996). Chemical Kinetics. Harper & Row.