Rates of Reaction Factors
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Rate of Reaction?
The rate of reaction is how quickly reactants are converted into products.
rate of reaction = (amount of reactant used (or product formed)/time taken)
A fast reaction (e.g. an explosion) is over in a fraction of a second; a slow reaction (e.g. rusting) can take years.
Four Factors That Change the Rate
The rate of a reaction can be increased by:
- increasing the concentration of a reactant in solution
- increasing the surface area of a solid reactant (smaller pieces)
- increasing the temperature
- adding a catalyst
Why each factor speeds up a reaction is explained fully, using collision theory, later in this topic.
Worked Example: Slowing a Reaction Down
- Question: an experiment measuring the rate of reaction between 1 g of solid X and 100 cm³ of solution Y happens too quickly to take measurements. Which change would slow the reaction down: adding a catalyst, decreasing the concentration of Y, decreasing the particle size of X, or increasing the temperature?
- Answer: decrease the concentration of solution Y - this is the only option that reduces the rate. Adding a catalyst, decreasing particle size (increasing surface area), and increasing temperature would all make the reaction faster, not slower.
Catalysts
A catalyst is a substance that increases the rate of a reaction and is chemically unchanged at the end of the reaction.
- Not used up - can be used again and again
- Only a small amount is needed
- Does not change the products formed or the overall yield
- Different reactions generally need different catalysts
Worked Example: Defining "Catalyst"
- Question: describe what is meant by the term catalyst. [2]
- Answer: a substance that speeds up the rate of a reaction [1], without being used up (chemically unchanged at the end of the reaction) [1]
Common Mistakes
- ❌ Saying a catalyst is "used up slowly"
- ✓ a catalyst is not used up at all - the same mass is present at the end as at the start
- ❌ Saying a catalyst increases the amount of product made
- ✓ a catalyst only changes how fast the reaction reaches completion, not the final yield
Collision Theory (Supplement)
Collision theory explains reaction rates at a particle level. For a reaction to happen, particles must collide with each other with energy ≥ activation energy (Ea) - the minimum energy colliding particles must have for a reaction to occur. A collision that meets this condition is a successful collision; most collisions are unsuccessful.
| Quantity | What it means |
|---|---|
| Number of particles per unit volume | how crowded the particles are - set by concentration |
| Frequency of collisions | how often particles collide with each other |
| Kinetic energy of particles | how fast particles are moving - set by temperature |
| Activation energy, Ea | the energy threshold a collision must clear to be successful |
rate of reaction is proportional to the number of successful collisions per second
Common Mistakes
- ❌ Thinking every collision leads to a reaction
- ✓ most collisions fail because the energy is below Ea
Explaining Each Factor Using Collision Theory (Supplement)
Exam answers should explain the effect of a factor in terms of collisions: state what happens to the particles, explain the effect on collision frequency and/or energy, then link this to the rate.
| Factor increased | Effect on particles | Effect on collisions | Why rate increases |
|---|---|---|---|
| Concentration | more particles in the same volume | particles are closer together - more frequent collisions | more collisions overall → more successful collisions per second |
| Surface area (solids) | more surface exposed | more reactant particles available at the surface | more collisions possible → more successful collisions per second |
| Temperature | particles move faster (more kinetic energy) | more frequent collisions, AND a greater proportion have energy ≥ Ea | more successful collisions per second - this is why rate is so sensitive to temperature |
| Catalyst | reaction follows an alternative pathway with lower Ea | collision frequency is unchanged | a greater proportion of collisions now have energy ≥ the new, lower Ea |
Key distinction: concentration and surface area only increase collision frequency. Temperature increases both frequency and the proportion of particles with sufficient energy - which is why a 10°C rise can roughly double the rate. A catalyst does not change collision frequency at all; it only increases the proportion of collisions with enough energy.
Worked Example: Explaining the Effect of Temperature
- Question: use collision theory to explain why increasing temperature increases the rate of a reaction. [3]
- Answer: particles have more kinetic energy and move faster [1]; this causes more frequent collisions, and a greater proportion of particles now have energy greater than or equal to the activation energy [1]; so there are more successful collisions per second, and the rate increases [1]
Worked Example: Explaining the Effect of Surface Area
- Question: powdered zinc reacts faster with dilute acid than the same mass of zinc lumps. Explain why. [3]
- Answer: powdered zinc has a much larger surface area than lumps of the same mass [1]; more zinc particles are exposed and available for collisions with acid particles, so collisions are more frequent [1]; more successful collisions per second means a faster rate of reaction [1]
Common Mistakes
- ❌ "Higher concentration gives particles more energy"
- ✓ concentration does not change particle energy - it changes how many particles are packed into a given volume, so collisions are more frequent
- ❌ "A catalyst gives particles more energy"
- ✓ a catalyst does not add energy to particles - it lowers the activation energy needed, so more of the existing particles already qualify
Investigating the Rate of a Reaction
Change in mass: used when a gas escapes from the reaction flask. The flask sits on a balance, and mass is recorded at regular time intervals - mass decreases as gas is lost. Use a plug of cotton wool in the flask neck (not a sealed bung) so gas can escape while spray/spitting is contained. Example: marble chips + acid → CO₂ escapes, mass decreases.
Formation of a gas (gas volume): the volume of gas collected is recorded at regular time intervals, using a gas syringe (gas pushes the plunger out; volume is read directly from the graduated barrel) or an inverted, graduated measuring cylinder collecting gas over water.
Volume of gas collected is read from the syringe at regular time intervals and plotted against time
Fair Testing: Control Variables
When investigating the effect of ONE factor, every other factor must be kept constant.
Interactive revision notes, videos and practice questions load below.