A straightforward graphical method

In general, the aim is to find just how the rate depends on concentration, e.g. is it proportional to [reactant], [reactant]2, [ reactant]1/2, [reactant]0 etc. If graphs of rate versus [reactant], [reactant]2, [reactant]1/2, [reactant]0 etc. are drawn, then the one which is linear gives the order. 

An interesting point. Why is it not necessary to draw the graph of rate versus [reactant]0 to show that the reactions A and B in Table 3.1 are not zero order  

Answer. If the reaction were zero order then the rate would be independent of [reactant]. This is clearly not so. 

The value of k can be found from the graph. Since the rate = [reactant]2, a graph of rate versus [reactant]2 is linear with slope = k, and intercept = zero. The gradient of the line gives k with units mol-1 dm3 min-1. 

These are completely systematic, and eliminate the necessity of making guesses as to possible orders. They give the order and rate constant direct from one graph. 

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