Question

Consider the rate constants below. which would correspond to the highest order reaction? k = 8.79 x 102 ms–1 k = 4.46 x 10–1 m–1s–1 k = 2.35 x 106 s–1 k = 1.88 x 10–3 m–2s–1

Answer 1

When the order n of the reaction rate is determined by this formula:

rate = K * [A]^n

when we have the unit of rate M S^-1 

and the unit of [A] M

So:

1- for k = 8.79 x 10^2 M S^-1 :

when the unit of K here is M S^-1

by substitution in rate formula:

M S^-1 = M S^-1 * M ^n

∴ M^n = 1

∴M^0 = 1 

∴ this reaction rate is zero order reaction


2- when k = 4.46 x 10^-1  and has unit M-1 S-1:

by substitution in rate formula:

M S^-1 = M-1 S-1  * M^n

∴M^2 = M^n

∴n = 2 so this is the second order reaction

3- when K 2.35 x 10^6 and it's unit is S-1 :

So by substitution in rate formula:

M S^-1 = S^-1 * M^n

∴ M^n = M

∴ n = 1 

So this is the first order reaction

4- when k = 1.88 x 10^-3 and its unit is M-2 S-1 

by substitution in rate formula:

M S^-1 = M^-2 S^-1 * M^n

∴ M^n = M^3

∴ n = 3 

∴ this is a third order reaction 

from 1 & 2 & 3 & 4 

so we can see that the correct answer is (4), K= 1.88 x 10^-3 M-2 S-1 is the highest order reaction.