Order of a reaction

The order of a reaction describes the relationship between the rate of a chemical reaction and the concentration of the species involved in it.

As mentioned in the article on differential forms of rate laws, the rate of a reaction is not always directly proportional to the concentration of a reactant, e.g. for the reaction, NO_2+CO\rightarrow NO+CO_2 :


The rate equation, except for one that describes an elementary reaction, cannot be predicted from the reaction’s stoichiometric equation and has to be determined experimentally.

In general, for a reaction: aA + bB + cC + dD… → mM + nN + oO + pP… , a proposed but not experimentally verified rate law is:


where the exponents i, j, k and l are called the order of the reaction with respect to A, B, C and D respectively and are not related to the stoichiometric coefficients of the reaction.

Consider the reaction: aA + bB + cC → mM + nN + oO. If experimental results reveal that the rate law is


we say that the reaction is second order with respect to A, zero order with respect to B and first order with respect to C. We can also say that the reaction is overall third order (2+0+1). The order with respect to a reactant usually ranges from zero to three. It can be a whole number or a fraction.

Next, let’s look at the acid-catalysed hydrolysis of methyl ethanoate, CH3COOCH3 +H2O CH3COOH + CH3OH, with an overall second order rate law of:

rate=k[CH_3COOCH_3][H_2O]\; \; \; \; \; \; \; \; 11

If water is present in large excess, its concentration remains constant throughout the reaction and eq11 becomes:


where k’ = k[H2O] .

We call such a scenario, where a second (or higher order) rate law reduces to a first order rate law due to the concentration of one (or more) of the reactants being constant throughout the reaction, a pseudo-first order reaction, with the rate law being pseudo-first order.

To determine the order, the rate constant and therefore the rate of a reaction, we need to know how to monitor the progress of a reaction and subsequently analyse the data obtained.


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