What a learner can do afterwards
- Labels initiation, propagation, branching and termination steps in a written mechanism
- Derives a chain-reaction rate law using the steady-state approximation on the carrier
- Defines chain length and calculates it from the rates of propagation and initiation
- Explains the pressure limits of the hydrogen and oxygen reaction in terms of branching against wall termination
1 · Read
Some mechanisms regenerate their own reactive intermediate, so one starting event drives many product cycles. Initiation breeds the first carriers. Propagation spends one carrier and grows a replacement. Branching grows more than one. Termination buries carriers for good.
In the hydrogen-bromine chain, splitting Br2 starts the chain. Each propagation step spends one radical and makes another, so the carrier count stays level. Steady state sets each carrier net rate to zero, solves for it, and plugs it back, so the final law lists only stable species.
To get the rate law you apply the steady-state approximation to the carrier. Chain length tells you how many cycles each start buys: divide the propagation rate by the initiation rate. If propagation runs at 300 units and initiation at 3, the chain length is 100.
The hydrogen-oxygen mixture explodes between a lower and an upper pressure limit. At low pressure, carriers hit the wall and die, so termination wins. At higher pressure, gas collisions let branching multiply carriers faster than the wall can bury them. Just above the upper limit, three-body collisions in the gas remove carriers faster than branching makes them. A third limit at still higher pressure brings explosion back.
Label each step, steady state the carrier, and read explosions as branching racing termination.
The rewritten readings were written for this dataset.
2 · Watch
En anglaisTake it off screen
Where it sits
Optional connections
This opens up
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.