Rates of Reaction and Collision Theory — Answers
Answer sheet with model answers and success criteria for the 10th Grade mixtures & chemical reactions worksheet.
Success TutoringWorksheetsAnswer sheet10th Grade · Science · Mixtures & Chemical ReactionsRates of Reaction and Collision Theory — Answers
Part A: Collision theory
Collision theory: particles can react only when they collide with enough energy (the activation energy) and in the right orientation. Anything that makes successful collisions happen more often increases the rate of reaction.
The rate increases with higher temperature, higher concentration (or pressure for gases), greater surface area (smaller pieces) and the use of a catalyst.
Answer: The particles must collide with enough energy (at least the activation energy) and in the correct orientation.
- AParticles become larger
- BParticles move faster, collide more often and more collisions have enough energy to react
- CThe activation energy increases
- DMore product is made in total
Answer: Particles move faster, collide more often and more collisions have enough energy to react
Both the frequency of collisions and the proportion of successful collisions increase.
Answer: The powder has a much larger surface area, so more particles are exposed to the acid. This gives more frequent collisions between acid particles and marble particles, so the rate is higher.
Answer: A more concentrated acid has more acid particles in the same volume, so they collide with the magnesium surface more often, giving more successful collisions per second.
Answer: A substance that speeds up a reaction without being used up. It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful.
Answer: False
A catalyst only makes the reaction reach the end faster. The amount of product depends on the amount of reactants.
Part B: Measuring rate
Marble chips (calcium carbonate, in excess) react with hydrochloric acid, producing carbon dioxide gas, which is collected in a gas syringe.
Volume of gas (cm³) — 0 s: 0 · 10 s: 24 · 20 s: 40 · 30 s: 50 · 40 s: 55 · 50 s: 56 · 60 s: 56
Average rate = amount of product formed ÷ time taken.
Answer: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- Left: Ca 1, C 1, O 3, H 2, Cl 2
- Right: Ca 1, Cl 2, H 2, O 1 + 2 = 3, C 1 (balanced)
Answer: 2.0 cm³/s
- rate = volume of gas ÷ time
- 40 ÷ 20 = 2.0 cm³/s
Answer: 0.75 cm³/s
- Gas produced = 55 − 40 = 15 cm³
- 15 ÷ 20 = 0.75 cm³/s
Answer: 1.12 cm³/s
- 56 ÷ 50 = 1.12 cm³/s
Answer: As the reaction goes on, the reactants are used up, so their particles become less concentrated and collide less often. The rate falls, and the reaction stops when one reactant (here the acid, since chips were in excess) is used up.
Answer: The gas would be produced faster at the start (a steeper graph), and the reaction would finish sooner. Because there is twice as much acid, about twice as much gas (about 112 cm³) would be produced in total.
Part C: Temperature and catalysts
Answer: About 20 s
- 20 °C → 30 °C: rate doubles, time halves to 40 s
- 30 °C → 40 °C: time halves again to 20 s
Answer: 0.025 s⁻¹
- 1 ÷ 40 = 0.025 s⁻¹
Answer: 2H₂O₂ → 2H₂O + O₂. The manganese dioxide is a catalyst: it speeds up the reaction without being used up, which is why the same mass can be recovered at the end.
Part D: Design an investigation
Model answer: Independent variable: temperature of the acid (for example 20, 30, 40, 50 °C). Dependent variable: time for the magnesium to disappear completely (or volume of hydrogen collected in 30 s). Controlled variables: the volume and concentration of acid, the length (mass) of magnesium ribbon, the type of container and whether it is stirred. Method: warm 25 mL of acid in a water bath to each temperature, checking with a thermometer; add a 2 cm piece of magnesium and start the stopwatch; stop it when the magnesium has disappeared. Repeat three times at each temperature and calculate the mean, ignoring outliers. Safety: wear safety glasses, use dilute acid, avoid naked flames because hydrogen is flammable, and use a water bath to heat the acid. Results: record in a table and draw a line graph of temperature (x-axis) against mean time (or rate = 1 ÷ time) on the y-axis.
Success criteria:
- Correct independent, dependent and controlled variables
- Clear, repeatable method including repeats and means
- At least two relevant safety precautions
- Appropriate table and graph described
Marking tips
- Give credit for correct method even when the final answer slips — the working shows where the thinking went right.
- For open-ended tasks, use the success criteria as a checklist rather than looking for one "right" answer.
- Celebrate what went well first, then pick one thing to work on next.
Great work. Here's what to try next: