Chains that copy their own carriers

Science · Chemistry · ages 20-21

Name ______________________ Date ____________

A step spends one radical and makes two radicals. What is it?
- Branching
- Initiation
- Termination
Which step breeds the first carriers of a chain?
- Propagation
- Termination
- Initiation
A termination step buries carriers for good.
Circle one: True False
Propagation runs at 240 units and initiation at 6 units. What is the chain length? Type the number.
Answer: ______________
In the hydrogen-bromine chain, Br plus H2 gives HBr plus H. What role does this step play?
- Initiation, because a new molecule appears
- Propagation, because one radical swaps for another
- Termination, because HBr is a stable product
Why does the hydrogen-oxygen mixture refuse to explode at very low pressure?
- Branching multiplies carriers too fast to control
- Carriers die on the wall before branching can multiply them
- Initiation stops because the gases separate
A student labels two radicals joining to one stable molecule as propagation. What is wrong?
- Two carriers vanish with no replacement, so it is termination
- Stable products can never appear in propagation
- Radicals are forbidden from meeting each other
Why does the final rate law for the hydrogen-bromine chain have no carrier concentration in it?
- Because steady state solves and plugs back
- Because initiation is always the fastest step
- Because termination removes all the products

Answer key

For grown-ups. Fold this page away before handing over the rest.

Branching · Making more carriers than it spends is the mark of branching.
Initiation · Initiation creates carriers where none existed before.
True · Termination removes carriers without growing replacements.
40 · Chain length is propagation rate divided by initiation rate.
Propagation, because one radical swaps for another · One radical enters and one radical leaves, so the carrier count stays level.
Carriers die on the wall before branching can multiply them · Wall collisions terminate carriers faster than gas collisions branch them.
Two carriers vanish with no replacement, so it is termination · Propagation must grow a replacement carrier, and none grows here.
Because steady state solves and plugs back · Carriers are too short-lived to measure, so steady state writes them with stable species and plugs them back.