Question

What is the answer? With an explanation, please.

Answer 1

The essence of these problems is determining the rate determining step: which the rate law is derived from.

Consider,

`"Rate" = k[NOCl]^2`

In the first mechanism, the slow step is the RDS, and there is only one mole of `NOCl`. So, that is wrong.

In the second mechanism, the slow step is the RDS, and there are two moles of `NOCl`. This may be a mechanism for this reaction.

In the third mechanism, we'll need to do some analysis,

`NOCl rightleftharpoons NO + Cl` fast, eq.

This isn't the RDS, but it makes the intermediate in the RDS (the slow step after). So let's substitute,

`k_1[NOCl] = k_(-1)[NO][Cl]`
`=> [Cl] = (k_1[NOCl])/(k_(-1)[NO])`

for the intermediate in the RDS and see what we come up with.

Consider,

`NOCl + Cl to NO + Cl_2` slow (RDS)

where,

`"Rate" = (k_1[NOCl])/(k_(-1)[NO]) * k_2[NOCl] = (k_1k_2[NOCl]^2)/(k_(-1)[NO])`

Hence, the only reliable mechanism we have that matches that rate law is `d.` or the second mechanism.