Chemical changes · GCSE Chemistry
Rates of reaction
Collision theory, concentration, pressure, surface area, temperature, catalysts, rate graphs and reversible reactions.
Rate depends on how often successful collisions happen. Concentration, pressure and surface area change the frequency. Temperature and catalysts also change the energy needed for a collision to count.
The important bits
What you need to know
- 1
A successful collision has enough energy to overcome the activation energy and the particles must be correctly oriented. Rate increases when successful collisions happen more often.
- 2
Increasing concentration (solutions) or pressure (gases) packs more particles into the same volume, so collision frequency rises and rate rises.
- 3
Increasing surface area — powder instead of a lump — exposes more particles, so collision frequency with the other reactant rises.
- 4
Increasing temperature makes particles move faster: more collisions, and a greater proportion have energy above the activation energy, so rate rises sharply.
- 5
A catalyst provides an alternative reaction pathway with a lower activation energy. It is not used up. Enzymes are biological catalysts.
- 6
Rate can be measured as change in mass of a gas-producing reaction, volume of gas collected, or time for a precipitate to obscure a cross. Rate = quantity of reactant used or product formed ÷ time.
- 7
A rate graph is steepest at the start, when concentrations are highest, then flattens when a reactant is used up. Reversible reactions can reach equilibrium, where forward and reverse rates are equal and concentrations are constant (Higher / Triple).
Go deeper
Collision theory in one sentence per factor
Write a cause, then the collision effect, then the rate. Higher concentration: more particles per unit volume, more frequent collisions, faster rate. Higher pressure for gases: the same story. Larger surface area: more particles exposed, more frequent collisions. Higher temperature: particles have more kinetic energy, collide more often, and more collisions exceed the activation energy. Catalyst: alternative path, lower activation energy, so a larger fraction of collisions are successful even at the same temperature. If you omit “successful” or “activation energy” on a temperature or catalyst question, you drop a mark. Frequency alone is not enough for those two.
Go deeper
Graphs tell you when the reaction is finished
The slope is the rate. A steeper line at the start of experiment A than experiment B means A was faster then. When the line is horizontal, the reaction has stopped because at least one reactant is used up. If two graphs finish at the same volume of gas, they produced the same amount of product — often the same mass of marble, with acid in excess — even if one was faster. Catalysts do not change the final amount of product; they change how quickly you get there. In required practicals, control volume and concentration of acid, temperature, and mass of carbonate, and change only surface area or concentration. Repeat and average.
See the idea in action
Two flasks contain the same mass of calcium carbonate and the same volume of hydrochloric acid. Flask A uses chips; flask B uses powder. The mass lost as carbon dioxide is recorded. Flask B’s graph is steeper at the start because powder has a larger surface area, so more frequent successful collisions. Both graphs level off at the same mass loss if the carbonate is limiting and fully reacts. A catalyst would also steepen the line without changing that final mass.
Exam technique
Turn knowledge into marks
For each factor, mention particles, collisions and — for temperature or catalysts — activation energy. On graphs, distinguish a faster rate (steeper slope) from more product (higher final volume or mass change). Do not say a catalyst is used up.
Common mistakes
Do not give these marks away
- 01
Writing that a catalyst is used up, or that it increases the temperature.
- 02
Saying temperature increases rate only because particles collide more often, without the activation-energy point.
- 03
Reading a higher final gas volume as a faster rate rather than more product.
How does a catalyst increase the rate of a reaction?
AIt is used up to provide extra energy
BIt provides an alternative pathway with a lower activation energy
CIt increases the concentration of the products
DIt raises the temperature of the surroundings
Show the answer
It provides an alternative pathway with a lower activation energy. More collisions then have enough energy to react, so successful collisions are more frequent. The catalyst is not used up and does not change the products.
Quick questions
If this is the bit you searched
What is activation energy?
The minimum energy colliding particles must have for a reaction to occur. Catalysts lower it by providing an alternative pathway.
Why does grinding a solid speed up its reaction?
Powder has a larger surface area, so more particles are exposed and successful collisions with the other reactant happen more often.