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How is the order of a chemical reaction determined from experimental data?

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A graph showing a potential energy curve for a chemical reaction, with the y-axis labeled potential energy and the x-axis labeled progress of reaction. The curve rises from the initial state (reactants) to a peak labeled transition state (activated complex), indicating activation energy, then drops to the final state (products). — chemical kinetics In the Arrhenius equation, the activation energy (E) represents the minimum amount of energy required to transform reactants into products in a chemical reaction. On a potential energy curve, the value of the activation energy is equivalent to the difference in potential energy between particles in an intermediate configuration (known as the activated complex, or transition state) and particles of reactants in their initial state. The activation energy thus can be visualized as a barrier that must be overcome by reactants before products can be formed.

The order of a chemical reaction is an experimental quantity determined by observing how the reaction rate changes in response to variations in the concentrations of the reacting substances.12

To determine the order with respect to a specific reactant, researchers typically hold the concentrations of all other reactants constant while varying the concentration of the reactant in question. By measuring how the reaction rate changes as this concentration is adjusted, scientists can establish the power to which that reactant's concentration is raised in the rate expression.21

The overall order of the reaction is the sum of these individual exponents. Experimental methods used to track these rates include:31

Sampling and Analysis: Removing samples at intervals to measure the concentration of starting materials or products.2
Real-time Monitoring: Using techniques like light absorption to automatically record changes in the mixture.2
Relaxation Procedures: Rapidly perturbing a system at equilibrium and observing its return to an equilibrium state.2

While the kinetic order is a useful classification tool, it is important to note that it may not always perfectly represent the number of molecules involved in the rate-limiting transition state, particularly if solvent molecules are involved or if the reaction occurs through complex, stepwise mechanisms.24