Determining Order of Reaction

For a chemical reaction, the rate law expresses the relationship between the reaction rate and reactant concentration. The exponents of the reactant concentrations influence the reaction rate individually and are called reaction orders.

The reaction order is determined experimentally by employing the method of initial rates, where a chemical reaction is repeated multiple times with varying reactant concentrations to measure the initial reaction rates.

An increase in the reactant concentration, producing a linearly-proportional increase in the reaction rate, characterizes a first-order reaction. If doubling the reactant concentration quadruples the initial rate, then a second-order reaction is observed.

However, if the change of reactant concentrations does not affect the initial rate values, then a zero-order reaction is observed.

When the obtained data of initial rates present an indiscernible relationship between the change in initial reactant concentration and the corresponding rate, a ratio of rate laws is calculated.

Here, any two concentration values of the reactant and their corresponding reaction rates are used to determine the reaction order.

But what about reactions with multiple reactants? First, using the method of initial rates, the reaction order of each reactant is determined individually.

Next, the individual reaction orders are expressed as exponents to their respective reactant concentrations to formulate the rate law. Lastly, the summation of individual exponents from the rate law determines the overall reaction order.

While the reaction order depicts the reaction rate’s dependence on the reactant concentration, a direct measure of the relative reaction speed is indicated by the rate constant.

The rate constant k, is the proportionality coefficient relating the reaction rate to the product of reactant concentrations.

The unit of a rate constant depends on the overall reaction order and can be determined by rearranging the rate law to solve for the rate constant. For a zero-order reaction, k has the unit molarity per seconds. The unit for a first-order reaction is 1/s, and for a second-order reaction it is 1/M·s.

A large rate constant indicates a fast reaction. Conversely, a smaller rate constant indicates a slow reaction. A value of zero signifies the absence of any chemical reaction.