Consider the overall reaction, which is experimentally observed to be second ord

Question

Consider the overall reaction, which is experimentally observed to be second order in X and first order in Y: X + Y rightarrow XY Does the reaction occur in a single step in which X and Y collide? Explain your answer. Demonstrate whether or not this twostep mechanism is valid. 2X X_2 Fast X_2 + Y rightarrow XY + X Slow Consider the gas phase reaction: H_2 (g) + I_2 (g) rightarrow 2 HI (g) The reaction was experimentally determined to be first order in H_2 and find order in I_2. Consider the proposed mechanisms. Proposed mechanisms I: H_2 (g) + I_2 (g) rightarrow 2 HI (g) single step Proposed mechanisms II: I_2 (g) 2 I(g) Fast H_2 (g) + 2 I(g) rightarrow 2 HI(g) Slow Show that both proposed mechanism are valid.

Explanation / Answer

2) Proposed Mechanism-I

H2 (g) + I2 (g) ----> 2HI (g)

Proposed Mechanism: Single-Step

Rate = k [H2][I2]

Rate Law agrees with experiment

Proposed Mechanism II- It is a 2-Step Mechanism.

Step1.

1. Iodine Dissociates (unimolecular process)

I2   < ----> 2I         Fast rate

Reactive Intermediate I is formed.K1 is forward rate and – K1 is reverse rate

Step2. Iodine atoms combine with H2

2I + H2 --------à 2HI       K2 slow rate

Law from Mechanism

Slow Step determines the rate:

Rate = k2 [H2][I]2

From Step 1-

At equilibrium Forward Rate = Reverse Rate k1 [I2] = k-1[I]2

So, [I]2 = (k1/k-1)[I2]

Substitute the rate = k2k1/k-1[H2][I2]

Finally

Rate = k[H2][I2] which agrees with experiment.

So both proposed mechanisms I and II are valid.

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