60-minute lesson
Year 11 Science: Rates of reaction
Lesson objective
- Explain collision theory.
- Predict and explain how temperature, surface area, concentration and catalysts affect reaction rate.
- Design and carry out a controlled investigation of reaction rate.
Success criteria:
- I can explain that particles must collide with enough energy to react.
- I can use collision theory to explain why a crushed tablet reacts faster.
- I can identify variables and present results in a table and graph.
Materials
- Effervescent vitamin C or antacid tablets (same brand and size)
- Clear glasses or cups
- Cold water from the fridge, room-temperature water and warm tap water
- A kitchen thermometer (optional)
- A measuring jug
- A stopwatch or phone timer
- A sealed zip-lock bag and a rolling pin for crushing a tablet
Introduction
6 minIntroduce today's words:
- reaction rate: How quickly reactants are used up or products are formed.
- collision theory: The idea that particles react only when they collide with enough energy and the right orientation.
- activation energy: The minimum energy colliding particles need for a reaction to occur.
- surface area: The total area of the surface of a solid that is exposed to other reactants.
- concentration: How much of a substance is dissolved in a given volume of solution.
- catalyst: A substance that speeds up a reaction without being used up, by lowering the activation energy.
Ask your child what they already know about rates of reaction.
Explanation
12 minEffervescent tablets contain an acid (such as citric acid) and sodium bicarbonate. When water dissolves them, they react to produce carbon dioxide gas, which makes the fizz. When the fizzing stops, the reaction has finished.
Collision theory says particles react only when they collide with enough energy (the activation energy) and in the right orientation. Anything that increases the number of successful collisions per second increases the reaction rate.
- Temperature: hotter particles move faster, collide more often and more of the collisions have enough energy. As a rough rule of thumb, many reactions go about twice as fast for every 10 °C rise.
- Surface area: crushing a solid exposes more particles to the other reactant, so collisions happen more often.
- Concentration: more particles in the same volume means more frequent collisions.
- Catalysts provide an alternative pathway with a lower activation energy. Enzymes are biological catalysts, and catalytic converters in car exhausts use metals such as platinum.
In a controlled investigation, change only the independent variable (for example, water temperature) and keep the tablet type, water volume and glass the same. Measure the time for the fizzing to stop and repeat each condition at least three times.
Worked examples
9 minTemperature investigation
How does water temperature affect the time for an effervescent tablet to finish reacting?
Step 1: Independent variable: water temperature (cold, room temperature, warm).
Step 2: Dependent variable: time until fizzing stops (s).
Step 3: Controlled variables: 200 mL of water, same tablet brand and size, same glass, whole tablet.
Step 4: Drop one tablet in, start the timer, stop it when bubbling ends. Repeat three times per temperature.
Step 5: Typical result: warm water gives the shortest time, cold water the longest.
Answer: Higher temperature gives a faster reaction (shorter time).
Calculating average time
Trials in cold water took 92 s, 88 s and 96 s. What is the average time?
Step 1: Add: 92 + 88 + 96 = 276 s.
Step 2: Divide by 3: 276 ÷ 3 = 92 s.
Answer: 92 s.
Explaining surface area
Why does a crushed tablet react faster than a whole one in the same water?
Step 1: Crushing breaks the tablet into many small pieces.
Step 2: More particles are exposed at the surface to water.
Step 3: More collisions occur per second, so the rate increases.
Answer: Greater surface area means more frequent collisions.
Guided practice (do together)
12 min1. According to collision theory, what must happen for particles to react?
(a) They must be the same colour (b) They must collide with enough energy and the right orientation (c) They must be in a solid (d) They must be heated above 100 °C
2. Which change would slow down the reaction of a tablet in water?
(a) Crushing the tablet (b) Using warmer water (c) Using colder water (d) Stirring the water
3. What does a catalyst do?
(a) Lowers the activation energy and is not used up (b) Raises the activation energy (c) Is used up as a reactant (d) Makes the reaction produce more product
4. Why are fine sawdust and flour dangerous in factories?
(a) They are heavy (b) They are always wet (c) They are acidic (d) Their huge surface area can make them burn explosively
Independent practice
15 min5. In the temperature investigation, which is a controlled variable?
(a) Water temperature (b) Time for fizzing to stop (c) Volume of water (d) Reaction rate
6. Trials took 40 s, 44 s and 42 s. What is the average time in seconds? (number)
7. Which gas causes the fizz when an effervescent tablet dissolves?
8. What name is given to biological catalysts?
Questions to check understanding
- Can you explain that particles must collide with enough energy to react?
- Can you use collision theory to explain why a crushed tablet reacts faster?
- Can you identify variables and present results in a table and graph?
- What was the trickiest part today?
Answer guide
1. They must collide with enough energy and the right orientation — Successful collisions need at least the activation energy and the right orientation.
2. Using colder water — Lower temperature means slower particles and fewer successful collisions.
3. Lowers the activation energy and is not used up — Catalysts provide a lower-energy pathway and remain at the end.
4. Their huge surface area can make them burn explosively — Fine powders expose enormous surface area to oxygen, so they can ignite very rapidly.
5. Volume of water — Volume of water must be kept the same; temperature is the independent variable.
6. 42 — (40 + 44 + 42) ÷ 3 = 126 ÷ 3 = 42 s.
7. carbon dioxide — The acid reacts with sodium bicarbonate to release carbon dioxide.
8. enzymes — Enzymes are proteins that catalyse reactions in living things.
Review
6 minRecap the success criteria together. Watch for these common misconceptions:
- Students often think a catalyst is used up or changes the amount of product. It only changes the rate.
- Some think a faster reaction makes more product. It makes the same amount, just sooner.
- Some confuse a shorter time with a slower rate. Rate is inversely related to time: shorter time means faster rate.
Extension activities
- Compare a whole tablet, a tablet broken into quarters and a crushed tablet in water at the same temperature.
- Make two small bowls of jelly: stir fresh pineapple pieces into one and tinned pineapple into the other. Fresh pineapple contains an enzyme (bromelain) that breaks down gelatine so the jelly may not set; canning heats the fruit and destroys the enzyme.
- Research why food is refrigerated, using the idea of reaction rates.
Suggested follow-up
- Revisit rates of reaction tomorrow with two or three quick questions from memory.
- Try the online practice check for this topic and look at any questions that need another go.
Tips for parents
- Measure water volume and check temperature carefully; small differences change the results.
- Help your child plot average time against temperature as a line graph.
- Safety: use warm tap water rather than boiling water, wear eye protection, do not drink the solutions or take extra tablets, and store medicines out of reach of young children.