Topic 3 of 15Pearson EdExcel

Rates of Reaction

Factors affecting how fast reactions occur and collision theory

Rate of reaction = how quickly reactants are converted to products. Measured by amount of product formed (or reactant used) per unit time.

Collision theory: Reactions occur when particles collide with **sufficient energy** (activation energy). More successful collisions = faster rate.

Factors affecting rate:

Temperature — higher T → particles move faster → more frequent AND more energetic collisions → faster rate

Concentration (or pressure for gases) — more particles per volume → more frequent collisions

Surface area — smaller particles expose more surface → more collisions possible

Catalyst — provides alternative pathway with lower activation energy → more particles have enough energy. Not consumed.

Measuring rate:

Gas collection (e.g. marble chips + HCl → CO₂) — measure volume collected over time

Mass loss — measure decrease in mass as gas escapes

Turbidity — reaction produces a precipitate, light transmission decreases

Graphs: Initial gradient = initial rate. Flat line = reaction complete.

Key Points to Remember

1Reactions need successful collisions with enough activation energy
2Higher temperature: faster particles, more frequent/energetic collisions
3Catalyst: lower activation energy, not consumed
4Measure rate by gas volume, mass loss, or turbidity

Pakistan Example

Roti Rising — Yeast as a Biological Catalyst

When a daal chawal cook uses yeast to leaven bread, the yeast enzymes (biological catalysts) break down glucose to produce CO₂ gas. The dough rises faster in a warm kitchen — higher temperature increases yeast activity. In industrial bread production in Karachi, temperature is precisely controlled. Grinding spices finer (more surface area) for Pakistani curries is another real-world application of surface area increasing reaction rate.

Quick Revision Infographic

Chemistry — Quick Revision