RATES OF REACTION

Question

Consider the overall reaction involving three elements as reactants and compound as the product: X+2Y+2Z ---> XY₂Z₂.

When a series of reactions is performed with different initial concentrations of reactants, the results are as follows:

1. Doubling the concentration of X has no effect on the overall rate
2. Doubling the concentration of Y multiplies the overall rate by 4.
3. Doubling the concentration of Z doubles the overall rate.

A possible three step mechanism is as follows:

Step 1: X + Z ---> XZ

Step 2: 2Y + Z ---> Y₂Z

Step 3: ?

State:

1. The rate law for this system
2. The possible missing step three in this mechanism.
3. The rate determining step

Answer 1

When a series of reactions is performed with different initial concentrations of reactants, the results are as follows:

1. 
   
2. 
   
3. 
   

A possible three step mechanism is as follows:

Step 1: X + Z ---> XZ

Step 2: 2Y + Z ---> Y₂Z

Step 3: ?

State:

1. The rate law for this system
2. The possible missing step three in this mechanism.
3. The rate determining step

Doubling the concentration of X has no effect on the overall rate

(rate / rate) = ( [X] / [X] )^a

(1/1)=(2/1)^a

1=2^a

a=0

[X] is zero order

Doubling the concentration of Y multiplies the overall rate by 4.

(rate / rate) = ( [Y] / [Y] )^x

(4/1)=(2/1)^x

4=2^x

x=2

Y is second order

Doubling the concentration of Z doubles the overall rate.

(rate / rate) = ( [Z] / [Z] )^x

(2/1)=(2/1)^x

2=2^x

x=1

[Z] is 1st order

The rate law for this system:

Given this, the rate law is:

rate=k[Y]²[Z]

The possible missing step three in this mechanism.:

Step 1: X + Z ---> XZ

Step 2: 2Y + Z ---> Y₂Z

Step 3: ?

OVERALL: X+2Y+2Z ---> XY₂Z₂.

Fill in the blanks, basically

Step 1: X + Z ---> XZ

Step 2: 2Y + Z ---> Y₂Z

Step 3: XZ+Y₂Z --> XY₂Z₂

OVERALL: X+2Y+2Z ---> XY₂Z₂.

The rate determining step

Well, the rate determining step is the slow step and has the same rate law as the rate law of the overall reaction. The rate law of the overall reaction was determined to be:

rate=k[Y]²[Z]

And step 2 is:

Step 2: 2Y + Z ---> Y₂Z

So the orders of the reactants are the coefficients for ELEMENTARY steps, and therefore step 2 must be rate-determining, and step 2 must be the slowest step with the largest activation energy (E_A).