Rate is change per unit time. The reaction rate is the change in concentration per unit time.

The rate of a reaction can be expressed in units of mol L^–1 s^–1.

Consider a chemical reaction of the type

Reactant → Product

Initially, at t = 0, only the reactant is present. As the reaction progresses and time elapses, the reactant concentration decreases during the reaction, so Δ [reactant] is negative. The product increases in concentration, and Δ [product] is positive.

Rate and reaction stoichiometry

Because the amounts of products and reactants are related by stoichiometry, any substance in the reaction can be used to express the rate of reaction. In the case of the decomposition of N₂O₅ to NO₂ and O₂, we gave the rate in terms of the rate of formation of oxygen, Δ[O₂]/Δt. However, we can also express it in terms of the rate of decomposition of N₂O₅.

Note the negative sign. It always occurs in a rate expression for a reactant in order to indicate a decrease in concentration. Thus, because [N₂O₅] decreases, Δ[N₂O₅] is negative.

The rate of decomposition of N₂O₅ and the rate of formation of oxygen are easily related. Two moles of N₂O₅ decompose for each mole of oxygen formed, so the rate of decomposition of N₂O₅ is twice the rate of formation of oxygen. To equate the rates, you must divide the rate of decomposition of N₂O₅ by 2 (its coefficient in the balanced chemical equation).

In general, for the reaction:

The rate of reaction is given by:

where r is the rate of reaction, A and B are reactants, C and D are products, and a, b, c, d are stoichiometric coefficients.