The reaction H₂ + 2ICl → I₂ + 2HCl is found to be first order in H₂ and first order in ICl…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The reaction is found to be first order in and first order in ICl. Two mechanisms are proposed. Mechanism 1: (slow), then (fast). Mechanism 2: (slow), then (fast). Which mechanism agrees with the experiment? Do both add up to the overall equation?
Need a reminder? The key ideaShow
Molecularity is the number of particles that collide in an elementary step, and it is 1, 2 or 3. For an elementary step the rate law follows from the equation. For a complex reaction, the rate law is that of the slow step, the rate-determining step.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Do the steps add up? Mechanism 1: adding gives . The cancels. Mechanism 2: adding gives . The cancels. Both add up to the overall equation.
Step 2. The slow step of mechanism 1. and collide, so . This is first order in each, as measured.
Step 3. The slow step of mechanism 2. Two molecules of collide, so . This is second order in and zero order in , which does not agree.
Answer: mechanism 1 agrees with the measured rate law. Mechanism 2 does not. Both have an overall order of 2, so the total order alone could not tell them apart. We needed the order in each reactant.
Watch out. A match does not prove mechanism 1. It only shows that mechanism 1 is possible.
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