Rates of Reactions

11 Grade 10 General Science questions on Rates of Reactions, with worked marking schemes. 11 questions are available on this topic in full.

1. Question 1

   1 mark

   Achieng and Otieno each carry out a different reaction, X and Y, that gives off a gas. They collect the gas and record their results. Which observation shows that reaction X is faster than reaction Y?

   A. X gives a larger total volume of gas at the end

   B. X uses a larger mass of the solid reactant

   C. X gives the same volume of gas in less time

   D. X is carried out in a much larger flask

   (1 mark)

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2. Question 2

   3 marks

   Mueni is making tea for her family and finds that the sugar she adds takes a long time to dissolve. Outline three ways she could make the sugar dissolve faster. (3 marks)

   (3 marks)

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3. Question 3

   4 marks

   A fertiliser factory makes ammonia by reacting nitrogen gas with hydrogen gas. The gases are passed over iron at a high pressure.

   (a) Explain how using a high pressure increases the rate at which ammonia is formed. (2 marks)

   (b) Explain why iron is used in the process, giving one economic advantage to the factory. (2 marks)

   (4 marks)

   Marking scheme

   (a) 2 marks: - High pressure pushes the gas molecules closer together, increasing their concentration (number of molecules per unit volume) (1 mark) - The molecules therefore collide more frequently, so there are more successful collisions per second and ammonia forms faster (1 mark) (b) 2 marks: - Iron acts as a catalyst; it speeds up the reaction (by providing an alternative pathway with lower activation energy) without being used up (1 mark) - Any one economic advantage (1 mark): ammonia is produced faster so more product is made in a given time; the reaction can be run at a lower temperature, saving fuel/energy costs; the catalyst is not used up so it can be reused for a long time, reducing costs

4. Question 4

   1 mark

   A learner adds a small amount of manganese(IV) oxide to hydrogen peroxide solution and oxygen is given off rapidly. Which statement about the manganese(IV) oxide is correct?

   A. It reacts and is used up as oxygen is given off

   B. Its mass at the end is the same as at the start

   C. It increases the total volume of oxygen produced

   D. It provides the oxygen that is given off

   (1 mark)

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5. Question 5

   3 marks

   During a lesson, a learner places a conical flask containing calcium carbonate and dilute hydrochloric acid on an electronic balance. The mouth of the flask is loosely plugged with cotton wool. She records the reading on the balance every minute.

   (a) Describe what is meant by the rate of a chemical reaction. (2 marks)

   (b) State why the reading on the balance decreases as the reaction proceeds. (1 mark)

   (3 marks)

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6. Question 6

   1 mark

   A family in Eldoret keeps fresh milk in a refrigerator so that it stays fresh for several days instead of souring overnight. Which statement best explains this?

   A. Cold air removes the oxygen needed by bacteria

   B. Low temperature slows the enzymes of bacteria

   C. Low temperature kills all bacteria in the milk

   D. Cooling makes the milk sugar too thick to sour

   (1 mark)

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7. Question 7

   6 marks

   Sodium thiosulfate solution reacts with dilute hydrochloric acid to form a fine yellow precipitate of sulfur, which makes the mixture cloudy. Baraka wants to investigate how the concentration of sodium thiosulfate affects the rate of this reaction. He has a cross drawn in ink on a white paper, a stopwatch, measuring cylinders, a conical flask, distilled water, 2 M hydrochloric acid and 0.15 M sodium thiosulfate solution.

   (a) Describe how he would carry out the investigation. (4 marks)

   (b) State how he would use his results to compare the rates of reaction at the different concentrations. (1 mark)

   (c) State one safety precaution, specific to this reaction, that he should observe. (1 mark)

   (6 marks)

   Marking scheme

   (a) Any four of the following, 1 mark each (4 marks): - Place the conical flask on the paper marked with the cross - Measure a fixed volume of sodium thiosulfate solution (e.g. 50 cm³) into the flask - Add a fixed volume of the hydrochloric acid (e.g. 5 cm³) and start the stopwatch at the same time; swirl to mix - Look down through the mixture from above and stop the stopwatch when the cross can no longer be seen; record the time - Repeat using different concentrations of sodium thiosulfate made by diluting it with distilled water (e.g. 40 cm³ + 10 cm³ water, 30 cm³ + 20 cm³ water), keeping the total volume the same - Keep the volume of acid, the temperature and the same cross/observer constant in every run - Wash the flask thoroughly between runs (b) The shorter the time taken for the cross to disappear, the faster the reaction / rate is proportional to 1/time, so calculate 1/t for each concentration and compare (1 mark) (c) Any one of the following, 1 mark: - Carry out the experiment in a well-ventilated room or fume cupboard because sulfur dioxide gas, which is irritating/poisonous, is produced - Avoid inhaling the fumes from the flask - Pour the used mixture into a designated waste container promptly, not leaving it to give off fumes in the room

8. Question 8

   5 marks

   In a class experiment, excess marble chips were added to 50 cm³ of dilute hydrochloric acid at room temperature. The volume of gas produced was recorded every 30 seconds, as shown below. Time (s): 0, 30, 60, 90, 120, 150, 180 Volume of gas (cm³): 0, 24, 40, 50, 56, 58, 58

   (a) Calculate the average rate of the reaction during the first 60 seconds. Show your working. (2 marks)

   (b) Explain why the volume of gas produced in each 30-second interval decreases as the reaction proceeds. (2 marks)

   (c) State the time interval within which the reaction stopped. (1 mark)

   (5 marks)

   Marking scheme

   (a) Average rate = volume of gas produced / time taken (1 mark for formula with correct substitution) = 40 cm³ / 60 s = 0.67 cm³/s (accept 0.667 cm³ per second or 40 cm³ per minute) (1 mark for answer with unit) (b) 2 marks: - The acid is used up as the reaction proceeds, so its concentration decreases (1 mark) - There are fewer acid particles per unit volume, so fewer collisions occur per second between acid particles and the marble chips (1 mark) (c) Between 120 s and 150 s (accept: by 150 s, since the volume stays at 58 cm³ after this) (1 mark)

9. Question 9

   1 mark

   During a class activity, Wanjiru adds 2 g of calcium carbonate to 50 cm³ of the same dilute hydrochloric acid in each of four flasks, all at room temperature. In each flask the calcium carbonate is in a different form. Which form would give the fastest initial rate of reaction?

   A. 2 g of fine marble powder

   B. one 2 g lump of marble

   C. 2 g of small marble chips

   D. 2 g of large-sized marble chips

   (1 mark)

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10. Question 10

    1 mark

    A laboratory technician at a school in Kisii always stores silver nitrate solution in a brown glass bottle kept in a cupboard. What is the main reason for this?

    A. to block light that speeds up its decomposition

    B. to prevent water evaporating from the solution

    C. to keep the solution at a steady low temperature

    D. to stop it reacting with the glass of the bottle

    (1 mark)

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11. Question 11

    4 marks

    A baker in Nakuru mixes flour, sugar, yeast and water to make bread dough. She notices that dough left in a warm place at about 35 °C rises within an hour, while dough left on a cold night rises very slowly.

    (a) Explain why the dough rises faster in the warm place than on the cold night. (2 marks)

    (b) One day her assistant mixes the dough using boiling water and the dough does not rise at all. Explain this observation. (2 marks)

    (4 marks)

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