Choose the measuring method that best matches the observable change.
3. Plotting and Interpreting Rate Graphs
For ordinary level, you should be able to draw and interpret simple reaction-rate graphs.
The steeper the graph at the start, the faster the reaction.
A steeper slope means a faster reaction.
When the graph levels off, the reaction has stopped or finished.
A graph that reaches the same final value more quickly represents a faster rate, not more product overall.
4. Factors Affecting Reaction Rate
Concentration
Higher concentration means more particles per unit volume, so collisions happen more often.
Particle Size / Surface Area
Smaller pieces expose more surface, so more particles are available for collision.
Temperature
Higher temperature gives particles more kinetic energy and increases successful collisions.
Nature of Reactants
Some substances react naturally faster than others because of their bonding and structure.
Catalysts
Catalysts increase reaction rate without being used up.
Pressure (gases)
For gases, higher pressure crowds particles together and increases collision frequency.
Examiner Tip: When explaining a rate change, use the phrase more frequent successful collisions.
5. Collision Theory and Activation Energy
Collision theory explains why reactions happen and why some reactions are faster than others.
For a reaction to occur, particles must collide with enough energy and with the correct orientation.
Requirement for a successful collision
Why it matters
Enough energy
Particles must have energy equal to or greater than the activation energy.
Correct orientation
The correct parts of the particles must meet to break old bonds and form new bonds.
Sufficient frequency of collisions
More collisions per second means more chances for successful collisions.
Activation energy = minimum energy needed for a reaction to start
Common Mistake: Activation energy is not the energy released by the reaction. It is the minimum energy needed to begin the reaction.
6. Explaining the Factors Using Collision Theory
Concentration
More particles in the same volume means more frequent collisions, so the rate increases.
Particle Size / Surface Area
Breaking a solid into smaller pieces increases its surface area. More particles are exposed, so there are more successful collisions.
Temperature
At higher temperature, particles move faster and collide more often. Also, a greater fraction of particles have enough energy to overcome the activation energy.
Catalysts
A catalyst provides an alternative pathway with lower activation energy, so more collisions are successful.
7. Catalysts
A catalyst is a substance that changes the rate of a reaction without being used up.
Catalyst = speeds up reaction + not used up + lower activation energy pathway
Key fact
Meaning
Not used up
The catalyst is still present at the end.
Specific
Many catalysts only work for certain reactions.
Lowers activation energy
More particles can react successfully.
Examiner Trap: A catalyst does not change the final amount of product available at completion. It only helps the reaction reach completion faster.
8. Catalytic Converters, Catalyst Poisons and Enzymes
Catalytic Converters
Catalytic converters are fitted in car exhaust systems. They use catalysts such as platinum, palladium and rhodium to convert harmful gases into less harmful gases.
Harmful gas
Problem
Converted into
Carbon monoxide (CO)
Poisonous gas
Carbon dioxide (CO₂)
Nitrogen oxides (NOx)
Air pollution / acid rain
Nitrogen (N₂)
Unburnt hydrocarbons
Smog / pollution
Carbon dioxide and water
Environmental benefit: Catalytic converters reduce harmful exhaust emissions and improve air quality.
Catalyst Poisons
A catalyst poison is a substance that reduces or destroys the action of a catalyst by blocking active sites or interfering with the catalyst surface.
Example: Lead compounds used in petrol used to poison catalysts in catalytic converters, which is one reason unleaded petrol became important.
Enzymes as Catalysts
Enzymes are catalysts produced by living cells. They speed up biological reactions.
Catalase speeds up the breakdown of hydrogen peroxide into water and oxygen.
Amylase helps break down starch into simpler sugars.
9. Social and Applied Chemistry Links
Dust Explosions
Fine powders such as flour dust, coal dust or grain dust can burn very rapidly in air because they have a very large surface area. This can cause a dust explosion.
Key idea: The smaller the particle size, the larger the surface area, so the faster the reaction can occur.
Demonstration: Oxidation of Methanol
A hot platinum or nichrome wire can catalyse the oxidation of methanol. This demonstration shows that a catalyst can allow oxidation to proceed rapidly without the catalyst being used up.
Demonstration: Oxidation of Potassium Sodium Tartrate
Hydrogen peroxide can oxidise potassium sodium tartrate more quickly when catalysed by cobalt(II) salts. This demonstrates the effect of catalysts on reaction rate.
10. Mandatory Experiment 6.1
Monitoring the rate of production of oxygen from hydrogen peroxide using manganese dioxide
2H₂O₂(aq) → 2H₂O(l) + O₂(g)
Manganese dioxide (MnO₂) acts as the catalyst.
Aim
To monitor the rate of oxygen production from hydrogen peroxide.
Apparatus
Conical flask, bung, delivery tube, gas syringe or measuring cylinder, stopwatch, hydrogen peroxide solution, manganese dioxide.
Method
Place hydrogen peroxide in a conical flask.
Add a small amount of manganese dioxide.
Immediately attach the bung and begin timing.
Record the volume of oxygen produced at regular time intervals.
Plot a graph of gas volume against time.
Observation: Oxygen is produced rapidly at first, then more slowly, until the graph levels off.
Why rate slows down: Hydrogen peroxide is used up as the reaction proceeds.
11. Mandatory Experiment 6.2
Studying the effects of concentration and temperature using sodium thiosulfate solution and hydrochloric acid
Na₂S₂O₃ + 2HCl → 2NaCl + SO₂ + S + H₂O
The sulfur formed makes the mixture cloudy, so the time for a cross under the flask to disappear can be measured.
Part A: Effect of Concentration
Place sodium thiosulfate solution in a flask over a marked cross.
Add hydrochloric acid and start the stopwatch immediately.
Stop timing when the cross is no longer visible.
Repeat using different concentrations of sodium thiosulfate.
Result: Higher concentration makes the cross disappear faster because there are more successful collisions.
Part B: Effect of Temperature
Warm the sodium thiosulfate solution to different temperatures.
Repeat the reaction with hydrochloric acid.
Measure the time for the cross to disappear.
Result: Higher temperature shortens the time because particles move faster and more have energy above the activation energy.
12. Worked Examples
Example 1: A reaction produces 60 cm³ of gas in 120 s. Average rate = 60 ÷ 120 = 0.50 cm³ s⁻¹
Example 2: Explain why powder reacts faster than large chips. Answer: Powder has a larger surface area, so more particles are exposed, leading to more frequent successful collisions.
Example 3: Explain why increasing temperature speeds up a reaction. Answer: Particles gain kinetic energy, collide more often, and a greater fraction have enough energy to overcome the activation energy.
13. Examiner Secrets, Mistakes and Traps
Examiner Secret: In graph questions, the word steeper usually links directly to faster rate.
Common Mistake: Saying a catalyst is used up or “gives energy” to the reaction.
Examiner Trap: More gas at the end does not automatically mean a faster rate. Compare how much gas is produced in the same time interval.
14. Final Revision Sheet
Rate of reaction means change per unit time.
Steeper graph = faster reaction.
Successful collisions need enough energy and correct orientation.
Activation energy is the minimum energy needed to start a reaction.
Concentration, particle size and temperature increase rate by increasing successful collisions.
Catalysts lower activation energy and are not used up.
Know catalytic converters, catalyst poisons, dust explosions and two enzyme examples.
Know mandatory experiments 6.1 and 6.2 very well.
15. Self-Assessment Checklist
I can define rate of reaction and explain how it is measured.
I can interpret a simple rate graph.
I can explain concentration, temperature and surface area using collision theory.
I can define activation energy.
I can explain how a catalyst works.
I can describe catalytic converters and catalyst poisons.