For the reaction X2 + Y + Z→ XY + XZ it is found that doubling the concentration of X2 dou bles the reaction rate, tripling the concentration of Y triples the rate, and doubling the concentration of Z has no effect. (a) What is the rate law for this reac tion? (b) Why is it that the change in the concentra tion of Z has no effect on the rate? (c) Suggest a mechanism for the reaction that is consistent with the rate law.

Question

For the reaction X2 + Y + Z→ XY + XZ it is found that doubling the concentration of X2 dou bles the reaction rate, tripling the concentration of Y triples the rate, and doubling the concentration of Z has no effect. (a) What is the rate law for this reac tion? (b) Why is it that the change in the concentra tion of Z has no effect on the rate? (c) Suggest a mechanism for the reaction that is consistent with the rate law.

Answer 1

Doubling (X2) doubles rate so it is first order in X2 and rate = k(X2)^1

Tripling (Y) triples rate so it is first order in Y and rate = k(Y)^1
Doubline (Z) has no effect so it is zero order in Z and rate = k(Z)^0 (note that k(Z)^0 = is k*1 no matter what Z is.
So rate law for the reaction is rate = k(X2)(Y)(1) = k(X2)(Y)

b. Z may be a catalyst OR it may be in a multistep reaction where it combines with another molecule and that duo then combines with a third and releases the Z component thus Z doesn't change.
c. I'll leave the speculation of the mechanism to you to work on.