Physics and chemistry Year 8: Reaction rate and the factors that change it
Physics and chemistry, Year 8 (Portugal): How fast a reagent disappears or a product appears, and how concentration, temperature, state of division and a catalyst speed a reaction up or slow it down.
Fast and slow reactions
The rate of a chemical reaction tells how fast a reactant disappears or a product appears. A fast reaction, like an effervescent tablet or fireworks, uses up reactant in seconds; a slow one, like rust forming on a nail, takes weeks or months. It is measured by following a quantity over time: a mass that decreases, a volume of gas that grows, a colour that changes.
Four factors
For substances to react, their particles must collide. The more effective collisions per second, the faster the reaction. Higher concentration: more particles per volume, so more collisions. Higher temperature: faster particles, so more frequent and stronger collisions. A finer state of division (a powder instead of a lump): more surface exposed. A catalyst is a substance that speeds the reaction up without being used up in it.
Reading a table of gas volumes
An effervescent tablet releases gas into an upturned measuring cylinder: 0 s, 0 mL; 30 s, 24 mL; 60 s, 40 mL; 90 s, 50 mL. Average rate = change in volume ÷ time. From 0 to 30 s: 24 ÷ 30 = 0.80 mL/s. From 30 to 60 s: 16 ÷ 30 ≈ 0.53 mL/s. From 60 to 90 s: 10 ÷ 30 ≈ 0.33 mL/s. The rate falls as the reactant is used up. With the tablet crushed, the same volume would appear sooner: a finer state of division.
Faster is not «more product»
Many people think that heating, or adding a catalyst, makes a reaction «yield more». It does not: the most product you can get depends on how much reactant there is. The factors only change how long the reaction takes. A catalyst is not a reactant, is not used up and does not change the amount of product; it only speeds things up. A whole tablet and a crushed one release the same final volume of gas, but the crushed one releases it faster.
From the fridge to the exhaust pipe
We keep food in the fridge to slow down the reactions that spoil it: lower temperature, slower reactions. Flour dust floating in the air can ignite violently because so much surface is exposed, which is why mills and grain silos control it. A car's catalytic converter uses catalysts to speed up the conversion of pollutant gases, such as carbon monoxide, into less harmful ones, without being used up.
Test what you learned
Which measurement tells you the rate of a reaction?
- a) How fast a reactant is used up or a product appears, for example mL of gas per second
- b) The total amount of product at the end
- c) The temperature at the start
Correct answer: a) How fast a reactant is used up or a product appears, for example mL of gas per second — Right: rate is the change of a quantity divided by the time it took.
A reaction releases gas: 0 s, 0 mL; 20 s, 30 mL; 40 s, 50 mL. What is the average rate between 20 s and 40 s?
- a) 1.25 mL/s
- b) 2.5 mL/s
- c) 1.0 mL/s
- d) 20 mL/s
Correct answer: c) 1.0 mL/s — Right: (50 − 30) mL ÷ (40 − 20) s = 20 ÷ 20 = 1.0 mL/s. It is slower than in the first 20 s (1.5 mL/s).
Two identical effervescent tablets react in the same amount of water: one whole, one crushed. What is true?
- a) The crushed one releases more gas in total
- b) Both release the same total volume of gas, but the crushed one releases it faster
- c) The whole one is faster, because it is compact
Correct answer: b) Both release the same total volume of gas, but the crushed one releases it faster — Right: the reactant sets how much; the state of division only changes how fast.
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