Postby ch1995 » Fri Feb 21, 2014 9:13 pm
Do you mean "what happens to the rate of a second order reaction as the concentration doubles?" If so, you would have to look at the order of each reactant:
Rate = k[A][B] and Rate = k[A]2 are both second order reaction. In the first case, A and B are both first order and in the second, A is second order.
If a reactant is first order, then when its concentration is doubled, the rate of the reaction doubles; if the concentration is tripled, the rate triples, etc.If a reactant is second order, then when its concentration is doubled, the rate of the reaction quadruples (22 = 4); if the concentration is tripled, the rate increases by a factor of 9 (32 = 9)
(If a reactant is third order, then when its concentration is doubled, the rate of reaction increases by a factor of 8 (23 = 8), etc.)
If a reactant is 0 order, then changing its concentration does not affect the rate of reaction.
Chemistry Chemical Kinetics Rate Law
Second Order
#"rate" = k["A"]^2#
The rate is proportional to the square of the concentration.
If you double the concentration, you multiply the rate by four. If you triple the concentration, you multiply the rate by nine.
If you halve the concentration, you divide the rate by four, and so on.
Since concentration changes during an experiment, we must measure the initial rate of the reaction, before the concentration has had a chance to decrease.
We set up an experiment and measure the rate. Then we do another experiment in which we change only the concentration of component A. Let's say we double the concentration of A.
If the rate did not change, the reaction was zero order in A.
If the rate doubles, the reaction is first order in A.
If the rate quadruples, the reaction is second order in A.