Rate of a reaction and the factors affecting it
Rate is the change in concentration per unit time. It depends on the reactants, concentration, temperature, pressure, a catalyst and surface area.
Fast and slow reactions
A cracker bursts in a moment. Milk sets into curd overnight. An iron gate rusts over months. All are reactions, but their rates (speeds) differ. As a reaction goes on, the concentration (amount per litre) of the reactants falls and that of the products rises. The rate of reaction is how fast this concentration changes with time.
Worked example: average rate
The concentration of a reactant falls from 0.8 mol L⁻¹ to 0.4 mol L⁻¹ in 20 s. Find the average rate of the reaction.
- Formula: average rate = change in concentration ÷ time taken
- Change in concentration = 0.8 − 0.4 = 0.4 mol L⁻¹
- Rate = 0.4 mol L⁻¹ ÷ 20 s
- = 0.02 mol L⁻¹ s⁻¹
Average rate = 0.02 mol L⁻¹ s⁻¹.
Factors affecting the rate
| Factor | Effect | Example |
|---|---|---|
| Nature of reactants | Some substances react faster than others | Sodium reacts vigorously with cold water; iron reacts very slowly |
| Concentration | More concentrated reactants react faster | Zinc gives hydrogen faster with concentrated HCl than with dilute HCl |
| Temperature | Higher temperature, faster reaction | Food spoils faster in summer; a fridge slows it down |
| Pressure | For gases, higher pressure brings molecules closer, so they react faster | Gas reactions in industry are run at high pressure |
| Catalyst | A substance that changes the rate without being used up | MnO₂ speeds up 2KClO₃ → 2KCl + 3O₂ |
| Surface area | Powders react faster than lumps | Powdered marble reacts with acid faster than marble chips |
Activity: marble chips vs marble powder
- 1Pour the same amount of dilute hydrochloric acid into two test tubes, A and B.
- 2Add a few marble chips to A and the same mass of marble powder to B at the same moment.
- 3Observe: B fizzes much faster than A.The gas is carbon dioxide.
- 4CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
- 5★Conclusion: a larger surface area exposes more particles to the acid, so the reaction is faster.
★ key step
We keep milk and vegetables in a refrigerator because low temperature slows the reactions that spoil food. A pressure cooker cooks dal faster at its higher temperature, and finely chopped vegetables cook faster because of their larger surface area.
2 marks: “Define rate of a reaction. Give its unit.” The change in concentration of a reactant or product per unit time. Unit: mol L⁻¹ s⁻¹.
2 marks: “A student finds that crushed marble fizzes faster in dilute HCl than marble chips of the same mass. Explain why.” Powder has a larger surface area, so more particles are exposed to the acid and react at the same time.
4 marks: “Choose any four factors that change how fast a reaction goes. Explain each with an example.” Concentration (strong vs dilute HCl with zinc), temperature (food in summer), catalyst (MnO₂ with KClO₃), surface area (powder vs chips).
Hot, Crowded, Crushed and Helped reactions run faster: temperature, concentration, surface area, catalyst.