Question

The reaction described by H²(g) + I²(g) yields 2HI(g)

has an experimentally determined rate law of
rate of reaction= k [H²] [I²]
some proposed mechanisms for this reaction are:
Mechanism a.
1. H²(g) + I²(g) yields (k1 on top of the arrow) 2HI (g) (one step reaction)
Mechanism b.
(1) I²(g) equilbrium arrow (k1 on top of the arrow, and k-1 on the bottom of the arrow ) 2I (g) (fast equlibrium )
(2) H²(g) + 2I(g) yields (k2 on top of the arrow) 2HI(g) (slow)
Mechanism c.
(1) I²(g) equlibrium arrow (k1 on top of the arrow, k-1 on the bottom of the arrow) 2I(g)
(2) I(g) +H²(g) equlibrium arrow (k2 on the top and k-2 on the bottom of the equilibrium arrow) yields H²I(g) (Fast equlibrium)
(3) H²I(g) yields ( k3 on top of the arrow) HI(g) + I(g) (slow)
a) which of these mechanisms are consistent with the observed rate law?
1) mechanism a
2) mechanism b
3) mechanism c
b) in 1967 J.H Sullivan showed that this reaction was dramtically catalyzed by light when the energy of the light was sufficent to break the I-I bond in an I2 molecule. Which mechanism(s) are consistent with both the rate law and this additional observation?
1) Mechanism a
2) Mechanism b
3) mechanism c

Answer 1

The rate law for the reaction H²(g) + I²(g) → 2HI(g) is consistent with Mechanism a. Mechanism a and Mechanism c are consistent with both the rate law and the additional observation that the reaction is catalyzed by light.

Step-by-step explanation:

The proposed mechanisms for the reaction H²(g) + I²(g) → 2HI(g) are:

a) Mechanism a:
1. H²(g) + I²(g) → (k1) 2HI(g) (one-step reaction)

b) Mechanism b:
(1) I²(g) ⇌ (k1) 2I(g) (fast equilibrium)
(2) H²(g) + 2I(g) → (k2) 2HI(g) (slow)

c) Mechanism c:
(1) I²(g) ⇌ (k1) 2I(g)
(2) I(g) + H²(g) ⇌ (k2) H²I(g) (fast equilibrium)
(3) H²I(g) → (k3) HI(g) + I(g) (slow)

a) The rate law for the overall reaction is rate = k [H²] [I²]. Mechanism a is consistent with the observed rate law.

b) Mechanism a and Mechanism c are consistent with both the rate law and the additional observation that the reaction is dramatically catalyzed by light when the energy of the light is sufficient to break the I-I bond in an I₂ molecule.