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### Order of a Reaction

Posted: Thu Mar 07, 2019 5:32 pm
Why would a reaction involving [A][B] creating some product [P] not necessarily be a second order reaction even though two different molecules must collide for the reaction to occur? Is a reaction only second order if the concentration of the reactants are raised to the power of 2?

### Re: Order of a Reaction

Posted: Thu Mar 07, 2019 6:23 pm
A reaction involving [A][B] is not necessarily a second order reaction since the rate of the reaction might not be dependent on the concentration of one of the reactants (such as when they're already in excess compared to the other reactant).

The sum of the powers of the reactant concentrations have to add up to two in the rate law equation for the reaction to be second order.

### Re: Order of a Reaction

Posted: Fri Mar 08, 2019 9:37 am
To add to the previous answer, if a reaction rate is not dependent on the concentration of a certain reactant, that means it is to the power 0. This, along with the fact that [A] and [B] can be to technically any power, means [A][B] is not necessarily second order.

### Re: Order of a Reaction

Posted: Fri Mar 08, 2019 8:15 pm
Adding on to the answers, to find the order one must plot the ln A, one over A, and just A (A being concentration of a reactant) against time to determine if it is first, second, or zero order. For the overall order, add the orders of each reactant to get the order for the entire reaction.