Stop 11 of 12 · The path
Energy & rates — hot, cold, fast, slow
Breaking bonds takes energy in; forming bonds gives it out. Whether it happens, and how fast, are two different questions.
Bench tool
Rate lab: heat it, crush it
Drop a tablet in water — how fast does it fizz?
Heat and surface both raise collisions — but neither changes whether the reaction gives out or takes in energy.
How it works
- Energy first. Overall release → favoured to happen; overall absorb → needs steady input.
- Rate second. Heat, surface, concentration, catalysts. More collisions, harder collisions, faster.
- Catalysts lower the hill. Same start, same end, quicker middle. Enzymes are living catalysts.
Worked examples
Hand warmer. Iron rusting fast in a pouch — exothermic, controlled.
Sugar cube vs powder. Same mass, powder burns faster — more surface.
Fridge slows rot. Cold means slower collisions — same chemistry, lazier.
Where it goes wrong
Fast ≠ favoured. Diamond should become graphite (favoured) but takes aeons (slow). Rate and energy answer different questions.
Try it at home: tablet race
Hot vs cold. Drop half a vitamin-C tablet into cold water and another half into adult-poured warm water. The warm cup fizzes out first — heat speeds reactions with harder, more frequent collisions.
For teachers
Learners confuse heat with temperature and think catalysts get used up — probe by asking where the warmth of burning went. Quickest check: ask which way energy moved in a cold-pack vs a match.