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 stepProposed mechanism II:I2(g) → 2 I(g)          FastH2(g) + 2 I(g) → 2 HI(g)          SlowIs the second of the proposed mechanisms valid?

FREE Expert 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.


87% (91 ratings)
View Complete Written Solution
Problem Details

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?

Frequently Asked Questions

What scientific concept do you need to know in order to solve this problem?

Our tutors have indicated that to solve this problem you will need to apply the Reaction Mechanism concept. If you need more Reaction Mechanism practice, you can also practice Reaction Mechanism practice problems.