Controlling rate in everyday life
The same ideas that control rate in the laboratory — surface area, concentration, temperature and catalysts — are used every day to make useful reactions faster and unwanted reactions slower.
Speeding reactions up
- Cooking — food is cut into small pieces (larger surface area) and heated (higher temperature) so it cooks faster. A pressure cooker raises pressure and temperature to cook food more quickly.
- Industry — catalysts make important reactions economical. Iron catalyses the Haber process for ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g). Vanadium(V) oxide, V2O5, catalyses the Contact process for sulphuric acid.
Slowing reactions down
- Refrigeration — a low temperature slows the reactions that spoil food, so it stays fresh longer.
- Vacuum packing — removing air lowers the concentration of oxygen, slowing oxidation of food.
- Preservatives — chemicals added to food slow the growth of microorganisms.
Key idea
To make a reaction faster: increase surface area, concentration, pressure or temperature, or add a catalyst. To make it slower: do the opposite — chiefly lower the temperature or reduce the concentration of a reactant such as oxygen.
Worked example
Milk kept at 4 °C in a fridge stays fresh for about 7 days, while the same milk left at 30 °C spoils within a day. The lower temperature slows the spoiling reactions, giving roughly seven times the storage life for a drop of about 26 °C.
Remember
- Small pieces + heat = faster cooking (surface area and temperature).
- Cold + no air = slower spoiling (temperature and oxygen concentration).
- Industrial catalysts (Fe, V2O5) cut cost and energy use.