Phosgene (Cl2CO), a poison gas used in World War I , is formed by the reaction of Cl2 and CO. The proposed mechanism for the reaction is Cl2⇌2Cl(fast, equilibrium)

Cl+CO⇌ClCO(fast, equilibrium)

ClCO+Cl2→Cl2CO+Cl(slow)

What rate law is consistent with this mechanism?

Rate = k [ClCO][Cl2]

Explanation:

Cl2⇌2Cl(fast,equilibrium)

Cl+CO⇌ClCO(fast,equilibrium)

ClCO+Cl2→Cl2CO+Cl(slow)

The rate determining step from which the rate aw is obtained from is the slowest step of a chemical reaction. In this reaction, the slowest step is;

ClCO+Cl2 → Cl2CO+Cl (slow)

The rate equation for the step is;

Rate = k [ClCO][Cl2]

ClCO is an intermediate and would eventually get cancelled out from the overall rate equation.