Question

A reaction mechanism is defined as the sequence of reaction steps that define the pathway from reactants to products. Each step in a mechanism is an elementary reaction, which describes a single molecular event of usually one or two molecules interacting.
The rate law for an overall reaction is the rate law for the slowest step in the mechanism, which is directly related to the stoichiometric coefficients of the reactants.

The exception to this rule occurs when the slowest step contains intermediates. In these cases, the slowest step is usually preceded by an equilibrium step, which can be used to substitute for the intermediates in the overall rate law.

Part A

What is the rate law for the following mechanism in terms of the overall rate constant k?

Step1:Step2:A+BB+C⇌→CD(fast)(slow)
Express your answer in terms of k and the necessary concentrations (e.g., k*[A]^3*[D]).

Part B

Consider the reaction

2X2Y2+Z2⇌2X2Y2Z

which has a rate law of

rate= k[X2Y2][Z2]

Select a possible mechanism for the reaction.

Consider the reaction

which has a rate law of

Select a possible mechanism for the reaction.

Step1:Step2:Step3:Z2→Z+Z (slow)X2Y2+Z→X2Y2Z (fast)X2Y2+Z→X2Y2Z (fast)
Step1:Step2:X2Y2+Z2→X2Y2Z+Z (slow)X2Y2+Z→X2Y2Z (fast)
Step1:Step2:X2Y2+Z2→X2Y2Z2 (slow)X2Y2Z2→X2Y2Z+Z (fast)
D Step1:Step2:2X2Y2⇌X4Y4 (fast)X4Y4+Z2→2X2Y2Z (slow)
E Step1:2X2Y2+Z2→2X2Y2 (slow)

Answer 1

Answer: (a) K *[A][B]^2

(b) The answer is B

Explanation:

A)

Step1:A+B<--> C (fast)

Step2: B+C→D(slow)

Rate depends on slowest step.

so,

rate = k' [B][C] ...eqn 1

But C is intermediate.so use step 1

Since 1st step is an equilibrium,

Kc = [C] /[A][B]

so,

[C] = Kc [A][B] ...eqn 2

put eqn 2 in eqn 1

rate = k' *[B] * Kc [A][B]

= k'Kc*[A][B]^2

= K *[A][B]^2 {writing k'Kc = K}

Answer: K *[A][B]^2

B)

Answer is B

Since rate depends on slowest step.

if slowest step is:

X2Y2+Z2→X2Y2Z+Z

then only,

rate= k[X2Y2][Z2]

Answer: B