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

Solution. The reaction is third order overall, is second order in NO concentration and is first order in H2 concentration. Observe once again that the simple answer that you might expect from looking at the coefficients of NO and H2 in the balanced equation does not correspond with the experimental facts.

Example. The reaction of chlorine with chloroform to yield carbon tetrachloride and hydrogen chloride is

and the observed rate expression for production of HCl is

What is the overall order of reaction, and what is the order with respect to each reactant?

Solution. The reaction is first order in chloroform concentration, half order in chlorine concentration, and one-and-a-half order overall. Note that there is no reason that the order cannot be fractional in one or more concentrations.

The reason that the coefficients in the balanced equation and in the rate law generally do not agree is that the actual chemical reaction mechanism usually is not a simultaneous collision of as many molecules as the coefficients indicate. Instead, the overall mechanism is built up from a series of smaller steps.


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