Question

Mechanism of a hypothetical reaction [2017]
X₂ + Y₂ → 2XY is given below :
(i) X₂ → X + X(fast)
(ii) X + Y₂ ⇋ XY + Y (slow)
(iii) X + Y → XY (fast)
The overall order of the reaction will be :
(a) 2 (b) 0
(c) 1.5 (d) 1

Answer 1

Answer:

Mechanism of a hypothetical reaction [2017]

X₂ + Y₂ → 2XY is given below :

(i) X₂ → X + X(fast)

(ii) X + Y₂ ⇋ XY + Y (slow)

(iii) X + Y → XY (fast)

ans:(c) 1.5

Answer 2

The overall order of the reaction is c)1.5.

• Three reactions are given in the mechanism.
• We know, rate of the reaction depends on the slowest step in the mechanism.
• Therefore, the rate law can be written using the slowest step                            (ii) X + Y₂ ⇋ XY + Y (slow)   as            
• r = K [X][Y₂]
• Here we see, Y2 is a reactant , but X is an intermediate so  it cant be used to write rate law directly.
• Now, in the reaction X₂ → 2X (fast)  we see 2X is formed.
• Therefore 1/2X₂ → X (fast) can be used.
• This implies  r = K [$X2^{1/2}$][Y2]
• Now we know, order of a reaction is sum of the stoichiometric coefficients in the rate law.
• Therfore, the order of the reaction is 1+1/2 = 1+0.5 = 1.5