Reaction order

Reaction order is the exponent of a reactant concentration in an experimentally determined rate law. The overall order is the sum of the individual exponents.

Rate law: An equation relating reaction rate to reactant concentrations and, often, a rate constant. Reaction order is read from the concentration exponents in this equation.
Reaction rate: The change in concentration of a reactant or product per unit time. Reaction order describes how this measured quantity responds to concentration changes.
Zero-order reaction: A reaction whose rate is independent of the concentration of the specified reactant. It illustrates an order of zero, even while reactant is being consumed.
Molecularity: The number of reacting particles participating in a single elementary reaction step. Unlike overall reaction order, molecularity is assigned to an elementary step from its event.
Initial rates method: An experimental method that infers a rate law by comparing early reaction rates at different starting concentrations. Changing one starting concentration at a time helps determine its order.
Concentration: The amount of a substance present in a defined volume. Rate-law exponents specify how concentration changes affect the rate.
First-order reaction: A reaction whose rate is proportional to the concentration of a specified reactant. Its concentration exponent is one, producing a characteristic half-life.
Reaction molecularity: The count of reactant entities involved in an elementary chemical event. It is sometimes confused with reaction order, but applies only to an elementary step.
Integrated rate law: An equation relating reactant concentration to elapsed time for a specified rate law. Concentration-versus-time data can test the order suggested by rate measurements.
Rate constant: A proportionality factor in a rate law whose value depends on temperature and other conditions. Its units depend on the overall reaction order.

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