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

1. The given reaction which is second order in X and first order in Y

a. Is not single step reaction. Two molecules of X combines to form X2 which then reacts with another molecule of Y to form the product.

b. Knwoing the order of reaction is twice to X2 and one to Y, the two step mechanism with the second step involving both the reactants as a slow step satisfies this need.

2. For the given reaction,

The rate law suggests the reaction is first order with respect to both the reactants. Thus the two step mechanism satifies this criteria and is thus valid.

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