Physical chemistry

Reaction kinetics, equilibrium, and the thermodynamics of chemical change

What is physical chemistry?

Physical chemistry sits at the boundary between chemistry and physics — it uses the physical principles covered elsewhere in this encyclopedia (energy, heat, motion) to explain why chemical reactions happen, how fast they happen, and how far they go.

Energy and chemical reactions

Every chemical reaction involves an energy change: some reactions release energy (exothermic reactions, like burning fuel), while others require an input of energy to proceed (endothermic reactions, like photosynthesis, which uses light energy). This connects directly to thermodynamics, covered in the Thermodynamics article — the same energy-conservation principles that govern engines and heat flow also govern chemical reactions.

Reaction rates

Not all reactions happen at the same speed. Some, like an explosion, are essentially instantaneous; others, like iron rusting, take months or years. The study of how fast reactions proceed, and what factors control that speed — temperature, concentration, and the presence of a catalyst — is called chemical kinetics. Raising the temperature almost always speeds up a reaction, since faster-moving molecules collide more often and more forcefully.

Chemical equilibrium

Many reactions don't run to completion in one direction — they reach a state called equilibrium, where the forward and reverse reactions are both still happening, but at exactly equal rates, so the overall amounts of reactants and products stop changing. This isn't a static, frozen state; it's a dynamic balance, and it can be shifted by changing conditions like temperature, pressure, or concentration.

Why physical chemistry matters

Physical chemistry provides the quantitative foundation for designing industrial chemical processes, understanding batteries and fuel cells, predicting how fast a drug will break down in the body, and much more — anywhere a chemical reaction's speed, energy, or extent needs to be predicted or controlled.