Rate of reaction
Rate measures how fast reactants are used up or products are formed.
Rate = change in amount / time taken.
Common units: g/s, cm^3/s, mol/dm^3/s.
Collision theory
For a reaction to occur, particles must:
- Collide with each other.
- Collide with enough energy (the activation energy, E_a).
- Collide with the correct orientation.
Anything that increases the frequency of successful collisions speeds up the reaction.
Factors that change rate
| Factor | Effect | Reason |
|---|---|---|
| Higher concentration (or pressure for gases) | Faster | More particles per unit volume so collisions are more frequent |
| Higher temperature | Faster | Particles move faster and a greater fraction have energy >= E_a |
| Smaller pieces / larger surface area | Faster | More particles exposed at the surface so collisions are more frequent |
| Adding a catalyst | Faster | Provides an alternative pathway with a lower activation energy; not used up |
Measuring rate
- Gas given off: collect in a gas syringe, measure volume against time.
- Mass loss: place flask on a balance, plot mass against time.
- Colour change / cloudiness: measure time for a cross to disappear under a beaker.
A graph of product against time has a steep gradient at the start (fast) and levels off when a reactant is used up.
WJEC exam tip
When asked to explain why a higher temperature speeds up a reaction, you must mention BOTH: particles move faster (more collisions per second) AND a larger fraction have energy above E_a (collisions are more energetic). One of these two alone usually scores M1 only.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science-leaves