Question

answer all questions
1 multiple choice 2 points
consider the three - step mechanism for the reaction of a through intermediates b and c to produce d shown below.
a→b eₐ = 15 kcal/mol, δh° = 13 kcal/mol
b→c eₐ = 10 kcal/mol, δh° = - 6 kcal/mol
c→d eₐ = 2 kcal/mol, δh° = - 20 kcal/mol
which of the three steps is rate - limiting?
the reaction of c to d.
the reaction of b to c.
the reaction of a to b.
the most exothermic step is rate - limiting
all three steps occur at the same rate; there is no rate - limiting step.

Explanation:

Step1: Recall rate - limiting step concept

The rate - limiting step of a multi - step reaction is the step with the highest activation energy ($E_a$).

Step2: Compare activation energies

For $A
ightarrow B$, $E_a = 15$ kcal/mol; for $B
ightarrow C$, $E_a=10$ kcal/mol; for $C
ightarrow D$, $E_a = 2$ kcal/mol.

Step3: Determine rate - limiting step

Since 15 kcal/mol (for $A
ightarrow B$) is the highest activation energy among the three steps, the reaction of A to B is the rate - limiting step.

Answer:

C. The reaction of A to B.