15. The Rates of Chemical Reactions   Previous PageNext Page
       Order of a Reaction

Solution. The reaction is a first-order decomposition, because the rate depends on the first power of N205 concentration.

Notice that the order of a reaction is an experimentally measured quantity, which does not depend on how you write the equation for the reaction. Writing the reaction with a coefficient 2 in front of the N205 term in the above example does not make the reaction second order, any more than writing the same reaction as

 


would make it first order. To determine the order of a chemical reaction, you must carry out real experiments.

Example.The reduction of NO with H2,
 

is observed to depend on the NO and H2 concentrations in the following way:
 

with a dependence on the first power of the H2 concentration rather than [H2]2 as the balanced equation might suggest. What is the order of the overall reaction, and what is its order with respect to each of the reacting substances?

 
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