Rates Solution Page 7

Mechanism Evaluation

(I)

1. NO2 + F2 = NO2F + F (slow)

2. F + NO2 = NO2F (fast)

The elementary steps add up

The order based on the slowest step is consistent with the rate law. Hence the mechanism is acceptable

(II)

1. NO2 + NO2 = NO + NO3 (slow)

2. NO3 + F2 =  NO2F + OF (fast)

3. NO + OF = NO2F (fast)

The elementary steps add up

The order is consistent with the rate law but the RDS(Rate determining step) does not include a required reactant F2 for the reaction

4. (CH3)3CCl + OH = (CH3)3COH + Cl

A suggested mechanism which agrees with the rate law data is

  1. (CH3)3CCl = (CH3)3C+ + Cl-1 (slow)
  • (CH3)3C+ + OH-1® (CH3)3COH (fast)

  • (a) Which step is the rate determining step? Explain

    The slowest step is the rate determining step so equation 1 in question 4 is the rate determining step.

    (b) What is the rate law expression? Rate = k [(CH3)3CCl][OH]

    (c) What is the effect on the rate of the reaction if

    (1) The concentration of (CH3)3CCl is tripled

    The rate of the reaction will triple

    (1) the concentration of OH-1 is doubled?

    This will not affect the rate of the reaction as the order is zero with respect to hydroxide ions.

    1. What advantage is there to the tabulation of specific rate constants for a group of reactions at a common temperature? Why is the temperature important?

    Rate of reaction depends on temperature, increasing the temperature increases the rate of reaction. The fraction of molecules possessing energy of activation and greater increases hence the rate of reaction increases. If the number of molecules having energy of activation or greater doubles, the rate of reaction doubles.