In the following mechanism, which is the rate-determining step?
step 1: h2(g)+icl(g)→hi(g)+hcl(g) (slow)
step 2: hi(g)+icl(g)→i2(g)+hcl(g) (fast)
a. step 1: (slow)
b. step 2: (fast)
c. step 1
d. step 2

Slow step is the rate determining step.

Explanation:

In general a reaction occurs in two or more elementary steps. Rate of reaction of each step is different. In chemical kinetics, rate of overall reaction is decided by rate of slowest step.

Consider the given reaction,

The given reaction has two elementary steps.

Step 1: H2(g)+ICl(g)→HI(g)+HCl(g) (slow)  

Step 2: HI(g)+ICl(g)→I2(g)+HCl(g) (fast)

Step 2 is fast and completes in no time. Whereas step 1 is slow. As reaction cannot go to completion without step 1, so rate of the reaction is decided by rate of the slowest step.