Given: A + 2B → 2C + D

Question

This reaction is first order with respect to reactant A and second order with respect to reactant B. If the concentration of A is doubled and the concentration of B is halved the rate of the reaction would by a factor of

Select one:

a. 

decrease, 2

b. 

increase, 4

c. 

increase, 2

d. 

decrease, 4

Answer 1

If the reaction is 1st order with respect to A and 2nd order with respect to B, then the rate equation is:

Rate = k[A]¹[B]², thus if we imagine the original concentration of A is 1M and of B is 1M, their new concentrations will be [A] = 2M and [B] = 0.5M. For the original: Rate = k[1][1]² = k, for the second situation Rate = k[2][0.5]² = 0.5k. Therefore the new reaction rate is one half of the original reaction rate. i.e. it would decrease by a factor of 2.