Rate of reaction / Kadar tindak balas
Rate of reaction tests collision theory and how concentration, temperature, surface area, pressure and catalysts change rate, plus interpretation of rate curves. You must separate rate from yield and equilibrium.
Core
- Rate = change in amount of reactant or product per unit time; it is the gradient of a graph.
- Collision theory: particles must collide with energy at least equal to the activation energy and with correct orientation.
- Higher concentration or pressure: more particles per volume, so more frequent collisions.
- Higher temperature: more particles have energy at least equal to the activation energy, and collisions are more frequent and energetic.
- Smaller particle size: larger surface area, so more collisions.
- A catalyst provides an alternative pathway with lower activation energy; it is not consumed and does not change yield.
Worked example
Equal masses of powdered and lump calcium carbonate react with excess 1 mol dm⁻³ HCl. The powdered form reacts faster: the same mass has a larger exposed surface area, so more acid particles contact the carbonate per second and more successful collisions occur. The final volume of CO₂ is the same because the amount of CaCO₃ is the same and the acid is in excess. Rate increases; yield does not. Reasoning: surface area affects collision frequency, not total stoichiometry.
Traps
A catalyst does not simply increase the number of collisions; it lowers activation energy so more collisions are effective. A temperature increase does not increase the final amount of product. Rate decreases over time because reactant concentration falls, not because the catalyst is used up. On a gas-collection graph, a steeper initial gradient means a faster rate; the same plateau means the same amount of gas.