Chemistry · Chapter 12

Mechanisms and Catalysis: The Slow Step Rules

Walk through the two steps of NO2 + CO → NO + CO2. Then raise and lower the hills and see which step sets the pace, and what a catalyst buys you.

The mechanism, one event at a time use the step buttons

Step 1, slow: NO2 + NO2 → NO3 + NO
Step 2, fast: NO3 + CO → NO2 + CO2
Overall: NO2 + CO → NO + CO2

Energy along the path solid is bare, dashed is catalyzed

Before the collisionTwo NO2 molecules drift in. CO waits its turn.
What is happening
Nothing yet
Two NO2 molecules are about to meet. This first meeting is the slow step, so it alone decides how fast the whole reaction runs.

Shape the hills

Which step rules, and how fast

Rate-determining step
step 1
Highest point above start
55 kJ/mol
Peak of step 1
55 kJ/mol
Peak of step 2
34 kJ/mol
Relative rate, bare
2.7 x 10^-10
Speed-up from the catalyst
1.0 times
rate = k[NO2]2Step 1 is slow and bimolecular in NO2, so CO never enters the rate law.