What is the rate of a reaction? Explain the factors that affect the rate of a reaction.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define with formula and unit. Rate = change in concentration of a reactant or product ÷ time taken. Unit: mol L⁻¹ s⁻¹.
2 · Nature of reactants. Some substances react faster: sodium reacts vigorously with water, iron slowly.
3 · Concentration. More concentrated reactants react faster: zinc with concentrated HCl gives hydrogen faster than with dilute HCl.
4 · Temperature. Higher temperature increases the rate: food spoils faster in summer.
5 · Pressure and catalyst.
Pressure: for gases, more pressure brings molecules closer and speeds the reaction
Catalyst: MnO₂ speeds up 2KClO₃ → 2KCl + 3O₂ and is not used up
6 · Surface area, with a sketch. [Two test tubes of dilute HCl: A with marble chips (slow bubbles), B with marble powder (fast bubbles), labelled] Powder reacts faster.
7 · Real-life link. Refrigerators slow food spoilage; pressure cookers and finely chopped vegetables speed up cooking.
8 · Close in one line. Thus the rate can be controlled by changing these six factors.
Scoring tip: Give one example for every factor. The marble chips and powder sketch is quick and earns credit.
