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Rate of Reaction Paper 4: Error Clinic
Rate of Reaction Paper 4: Error Clinic
Ten recurring errors: a flat curve read as no reaction, average and instantaneous rates confused, unit and mole conversions wrong, and a catalyst mistaken for a yield booster.
Direct answers Answers and explanations for all ten questions
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: DSE Chemistry Rate of Reaction |
| Level | Paper 4 (error clinic) |
02 Questions (10 items)
- FoundationError 1: after 120 seconds the curve is almost flat, and a student answers “the reaction stopped, so the rate is zero”. What is the correct reading?
- FoundationError 2: 60 cm³ of gas in the first 20 seconds, then another 60 cm³ over the next 80 seconds. What is the average rate of each stage?
- FoundationError 3: 2.0 g of marble reacts in 24 seconds as powder and 96 seconds as one lump. How many times faster is the powder?
- IntermediateItem 4: 0.24 g of magnesium (relative atomic mass 24) dissolves completely in dilute acid. What volume of hydrogen, in cm³, is collected at room temperature?
- IntermediateItem 5: 0.06 mol of reactant is used up in 2 minutes. What is the rate in mol/s?
- IntermediateItem 6: a reaction takes 160 seconds at 20 °C. If the rate doubles for every 10 °C rise, roughly how long does it take at 40 °C?
- ChallengeItem 7: a student treats “adding a catalyst raises the yield” as a fact. Using rate and equilibrium, explain the error.
- ChallengeItem 8: 0.5 M hydrochloric acid takes 96 seconds with marble. Roughly how long with 1.0 M, and why is it not simply halved?
- ChallengeItem 9: 90 cm³ of gas is collected in 60 seconds, but 40 cm³ of it appears in the first 10 seconds. What are the average rates for the whole period and for the first stage?
- Error redoError 10: confusing rate with yield, and assuming that adding more reactant always raises the initial rate. How should they be separated?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | The reactants are largely used up, so the change in concentration is too small to see; the flat curve means the reaction has already happened. | A flat line only means a very small change over the sampling period, not that nothing ever reacted. |
| 2 | The first stage is 4.0 cm³/s and the second is 0.75 cm³/s, so the first is 5.333333333333333 times faster. | Equal volumes over unequal times must each be divided by their own time. |
| 3 | Four times: cutting the time to a quarter raises the rate to four times. | Rate is inversely proportional to the time needed, so compare reciprocals rather than time differences. |
| 4 | 0.01 mol of magnesium gives 0.01 mol of hydrogen, times 24 000 cm³/mol, which is 240.0 cm³. | Get moles from mass, convert with molar volume, and only then substitute into the rate expression. |
| 5 | 0.03 mol/min, which is 0.0005 mol/s. | Per minute and per second differ by a factor of sixty, so the unit asked for decides the divisor. |
| 6 | Two steps of doubling make it four times faster, so about 40.0 seconds. | Two doublings multiply by four, not two, so count the steps before dividing. |
| 7 | A catalyst lowers the activation energy so equilibrium is reached sooner without shifting its position, so the yield is unchanged. | What changes is the time to reach equilibrium; raising the yield needs different conditions or more reactant. |
| 8 | About 24.0 seconds: concentration and rate are roughly proportional, but surface area, temperature and gas diffusion all matter, so it is only an estimate. | Higher concentration is only one variable; hold the others fixed before using a simple ratio. |
| 9 | 1.5 cm³/s overall and 4.0 cm³/s in the first stage; the first stage is faster because reactant concentration is highest then. | An average rate describes the whole period and does not represent any single instant. |
| 10 | Adding reactant raises the final yield; the initial rate depends on starting concentration, temperature, surface area and catalysts. | Different factors control the two, so first ask whether the question is about how fast or how much. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Most errors come from mixing fast with much, or a time span with a single moment, rather than from missing the principle.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.