Chemical Kinetics Problem Seen Before

Chemistry Level 3

The reaction of formation of phosghene from $\ce{CO}$ and $\ce{Cl2}$ is $\ce{CO}+\ce{Cl2}\rightarrow \ce{COCl2}$ . The proposed mechanism is as follows:

$\text{(i) }\ce{Cl2}\xrightarrow [ K_{ 1 } ]{ \xleftarrow { K_{-1} } } \ce{2Cl}\quad(\text{fast})$

$\text{(ii) }\ce{Cl}+\ce{CO}\xrightarrow [ K_{ 2 } ]{ \xleftarrow { K_{-2} } } \ce{COCl}\quad(\text{fast})$

$\text{(iii) }\ce{COCl}+\ce{Cl2}\xrightarrow { { K }_{ 3 } } \ce{COCl2}+\ce{Cl}\quad(\text{slow})$

What is the order of reaction?

1 1.5 2 2.5 3

1 solution

Aamir Faisal Ansari
Jul 16, 2015

For first reaction at equilibrium, $k_1=\frac{[Cl]^2}{[Cl_2]}$ Obtainig value for [Cl] from above equation as $[Cl]={k_{1}}^{1/2}[Cl_2]^{1/2}$ Similarly for second reaction at equilibrium, $k_2=\frac{[COCl]}{[Cl][CO]}$ putting value of [Cl] in this equation and solving for $[COCl_2]$ gives $[COCl]={k_{1}}^{1/2}k_2[Cl_2]^{1/2}[CO]$ Rate expression can be written from last reaction as it is slowest and rate determinig step. $-r=k_3[Cl_2][COCl]$ Putting value of [COCl] as obtained in above equation which gives $-r={k_{1}}^{1/2}k_2k_3[Cl_2]^{3/2}[CO]$ Hence overall order of reaction is 3/2+1=5/2.

why cant v take conc of cl at 2 step

Vishwanth Kb - 5 years, 4 months ago

why you consider co as it is intermediate that cant represent the rate of reaction

Jatin Kumbhani - 3 years, 11 months ago

CO is not an intermediate Jatin.

Meister Albrecht - 4 months, 2 weeks ago

If it's in the main rxn, it is not an intermediate. Cl and COCl are intermediates but Cl2 and CO aren't

Meister Albrecht - 4 months, 2 weeks ago

Chemical Kinetics Problem Seen Before

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