Four mechanisms have been proposed to rationalize this transformation. Write the

Question

Four mechanisms have been proposed to rationalize this transformation. Write the rate law for each proposed mechanism

Mechanism A

H2 (g)   + 2 ICl (g) ------------> 2 HCl (g) + I2 (g)

Mechanism B

H2 (g) + ICl (g) ------------> HCl (g) + HI (g)      (SLOW)

ICl (g) + HI (g) -----------> HCl (g)   +    I2 (g)     (FAST)

Mechanism C

H2 (g)   + ICl (g)< ---------->   HCl (g)   + HI (g)   (FAST)- k1 on forward and reserve arrow

ICl (g) + HI (g) ------------> HCl (g) + I2 (g)     (SLOW)- k2 on arrow

Mechanism D

H2 (g) + ICl (g) ---------> HClI (g) +    H (g) (SLOW) - k1 on arrow

H (g) + ICl (g) <-----------> HCl (g) + I (g) (FAST) - k2 on forward and reverse arrow

HClI (g) <--------------> HCl (g) + I (g)   (FAST) - k3 on forward and reverse arrow

I (g) + I (g) <-----------> I2 (g) (FAST) - k4 on forward and reverse arrow

Which of the above mechanism(s) is/are consistent with the experimentally observed rate law? (rate = k[H2][ICl] )

Mechanism A

H2 (g)   + 2 ICl (g) ------------> 2 HCl (g) + I2 (g)

Mechanism B

H2 (g) + ICl (g) ------------> HCl (g) + HI (g)      (SLOW)

ICl (g) + HI (g) -----------> HCl (g)   +    I2 (g)     (FAST)

Mechanism C

H2 (g)   + ICl (g)< ---------->   HCl (g)   + HI (g)   (FAST)- k1 on forward and reserve arrow

ICl (g) + HI (g) ------------> HCl (g) + I2 (g)     (SLOW)- k2 on arrow

Mechanism D

H2 (g) + ICl (g) ---------> HClI (g) +    H (g) (SLOW) - k1 on arrow

H (g) + ICl (g) <-----------> HCl (g) + I (g) (FAST) - k2 on forward and reverse arrow

HClI (g) <--------------> HCl (g) + I (g)   (FAST) - k3 on forward and reverse arrow

I (g) + I (g) <-----------> I2 (g) (FAST) - k4 on forward and reverse arrow

Explanation / Answer

Rate law can be written for slow step of the process.

A) rate = k [H2][ICl]^2

B) Rate = k [H2][ICl]

C) Rate = -k2[ICl][HI]

D) Rate = -k1[H2][ICl]

Mechanism B is is consistent with the experimentally observed rate law.

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