Question

The following three step mechanism has been proposed for this reaction:

Step 1: A + B <--> G fast

Step 2: A + G --> E + D slow

Step 3: E + 2C --> 2D + F fast

Determine the rate law predicted by the mechanism. Identify any intermediates shown. The experiemental rate law was determined to be Rate = [A]²[B]. Is this mechanism valid?

Answer 1

The experimental rate law satisfies the proposed mechanism since ,

1) rate of a reaction is decided by the slowest step- ie.

A + G <--------------> E + D

2 ) G happens to act as an intermediate compound , undergoing subsequent reaction with the reactant A to yield

the product D via the slowest step ,

A + G <-----------------> E + D

3 ) The third step , however , is again a fast step which is based on the assumption that E is another intermediate produced to react with the reactant B giving out another product F along with D ( being formed in the second - slowest step , also). Further, since this step is a fast step the concentration of reactants taking part in this step do not appear in the experimental rate law.( ie . Rate = [ A ]² [ B ] )

Thus , G and E are the intermediates formed according to proposed mechanism.