Rate of Reaction & Collision Theory

Chemical kinetics is the branch of chemistry concerned with how fast reactions proceed and with the microscopic events that control that speed. Thermodynamics tells us whether a reaction is feasible, but it says nothing about the timescale. A mixture of hydrogen and oxygen is thermodynamically primed to form water, yet the gases can sit together indefinitely at room temperature. Kinetics explains this apparent contradiction and gives us the tools to speed reactions up or slow them down.

The rate of reaction is defined as the change in concentration of a reactant or product per unit time. For a reactant, concentration falls over time, so we write the rate as the negative of the gradient; for a product, concentration rises. The usual units are $\text{mol dm}^{-3}\text{ s}^{-1}$. Because stoi