Factors affecting rateIB MYP Chemistry: Revision notes
Section 1
Five factors that change the rate
The rate of a reaction depends on five main factors:
- Concentration of reactants in solution
- Temperature
- Surface area of solid reactants
- Pressure of gaseous reactants
- Presence of a catalyst
In an investigation, you change one of these (the independent variable) and control the others so that the test is fair.
Section 2
Concentration and pressure
A higher concentration means more particles in the same volume. The particles are closer together, so they collide more frequently, and the rate increases.
For gases, increasing the pressure squeezes the same number of particles into a smaller volume. This has the same effect as increasing concentration: more frequent collisions and a faster rate.
Pressure only affects reactions that involve gases.
Section 3
Temperature
Raising the temperature gives the particles more kinetic energy, so they move faster.
- They collide more frequently.
- A greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful.
The second effect matters most. As a rough rule, the rate of many reactions roughly doubles for every 10 °C rise.
Section 4
Surface area
Reactions of a solid happen only at its surface. Breaking a solid into smaller pieces increases the surface area for the same mass.
More particles are exposed to the other reactant, so there are more frequent collisions and the rate increases. Powder reacts much faster than lumps. Dust explosions in flour mills happen because the huge surface area makes the reaction extremely fast.
Example: 1.0 g of zinc powder reacts faster than 1.0 g of zinc granules.
Surface area is not the same as size. Smaller pieces have a bigger surface area for the same total mass.
Section 5
Catalysts
A catalyst speeds up a reaction but is not used up, so it is unchanged at the end and can be reused.
It works by giving the reaction an alternative route with a lower activation energy, so more collisions are successful.
Examples: iron in the Haber process; manganese(IV) oxide for the decomposition of hydrogen peroxide. Each catalyst works for specific reactions.
Section 6
Effects on rate graphs
On a graph of product against time:
- A faster rate gives a steeper line, which levels off sooner.
- If the amount of reactant is the same, all lines end at the same final volume; only the time taken differs.
- If there is more reactant (for example more moles of acid), the line ends higher.
Example: zinc powder gives a steeper line than granules but finishes at the same volume, because the mass of zinc is the same.
A catalyst or higher temperature changes how fast you reach the final volume, not how much is made.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Factors affecting rate
- A student in Lima reacts 1.0 g of zinc granules with 50 cm³ of excess dilute hydrochloric acid and measures the volume of hydrogen made over time. She then repeats the experiment with 1.0 g of zinc powder, keeping everything else the same.Explain why zinc powder reacts faster than zinc granules.2 marks
- Ammonia for fertiliser is made in the Haber process, in which nitrogen gas reacts with hydrogen gas at high temperature and high pressure in the presence of an iron catalyst. Engineers at the plant are always looking for ways to make ammonia faster.State two ways, other than adding a catalyst, of increasing the rate of the reaction between nitrogen and hydrogen.2 marks
- An engineer in Seoul tests an effervescent tablet that fizzes when it is dropped into 200 cm³ of water. She times how long the tablet takes to stop fizzing at three temperatures. At 15 °C it takes 120 s, at 25 °C it takes 60 s and at 35 °C it takes 30 s.Describe the effect of temperature on the rate of reaction, using the data to support your answer.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).