Chemistry Practice Problems Reaction Mechanism Practice Problems Solution: Consider the gas-phase reaction: H2(g) + I2(g) → 2...

Solution: Consider the gas-phase reaction: H2(g) + I2(g) → 2 HI(g)The reaction was experimentally determined to be first order in H2 and first order in I2. Consider the proposed mechanisms.Proposed mechanism I: H2(g) + I2(g) → 2 HI(g) Single stepProposed mechanism II:I2(g) → 2 I(g)          FastH2(g) + 2 I(g) → 2 HI(g)          SlowIs the second of the proposed mechanisms valid?

Problem

Consider the gas-phase reaction: H2(g) + I2(g) → 2 HI(g)
The reaction was experimentally determined to be first order in H2 and first order in I2. Consider the proposed mechanisms.
Proposed mechanism I: H2(g) + I2(g) → 2 HI(g) Single step
Proposed mechanism II:
I2(g) → 2 I(g)          Fast
H2(g) + 2 I(g) → 2 HI(g)          Slow

Is the second of the proposed mechanisms valid?

Solution

A reaction mechanism is a step-by-step sequence of elementary reactions by which overall chemical change occurs. Information about a reaction mechanism is often obtained through the use of chemical kinetics.


To solve this problem, we will go through the following steps:

  • Write a rate law for the first step of the mechanism II.
  • Eliminate any intermediates from this rate law.
  • Inspect its order to determine if its correct.


First, let’s see if the two equations in mechanism II add up to a balanced chemical equation.


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