Chapter 10 Self Test

Finding the Mechanism

The following data were obtained for the oxidation of CO over a catalyst. All rates are initial rates

The initial rate was found to be independent of CO₂.  

a)   Suggest a rate law consistent with the data

Hint 1  Sketch versus C_O2.

Hint 2  Sketch versus C_CO at low concentration.

Hint 3  Sketch versus C_CO at high concentration.

Full Solution What is the rate law?

b)   Suggest mechanisms consistent with the rate law

Hint 1 What do the trends suggest?

Hint 2 What is the mechanism?

Full Solution

Hint 1

Use only the data points for which the concentration of CO is the same.

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Hint 2: Low concentration

For constant CO₂

We see at low concentration the rate is linear in CO and

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Hint 3: High concentration

At high concentrations the rate decreases with increasing concentration

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Full Solution: Part A

Combining

For fixed concentration of O₂ and low concentration of CO and

High concentration

Consequently the rate law

Satisfies the data

Back to Part b

Hint 1

From the denominator of the rate law, we see that CO is adsorbed and that O₂ is either weakly adsorbed (K_O2P_O2^1/2 << 1) or not adsorbed at all.  

The 1/2 order with respect to oxygen suggests dissociative adsorption.  

Because the initial rate is independent of CO2, it is either not adsorbed on the surface or weakly adsorbed.  

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Hint 2

The following mechanism is consistent with the rate law

Molecular Adsorption                               

Dissociative Adsorption                                

Surface Reaction                                                    

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Full Solution: Part B

The following mechanism is consistent with the rate law

Molecular Adsorption                               

Dissociative Adsorption                                

Surface Reaction                                                    

Assume surface reaction limits

            1)                                     

            2)                                       

            3)                    

            4)                                          

            5)                           

We see that oxygen is weakly adsorbed (very small) such that

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