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GCSE · Chemistry

C6 Rate and Extent of Chemical Change

AQA 8462 (Chemistry topic 4.6); Combined Science Trilogy 8464 equivalent topic; higher-only equilibrium content also maps to 8461/8463 co-taught classes · Calculator allowed · about 85 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Chemists can measure how quickly a reaction happens.

(a)State what is meant by the term 'rate of reaction'.(1)

(b)A reaction between a solid and an acid produces a gas. Give two different ways a student could measure the rate of this reaction.(2)

(Total for Question 1 is 3 marks)

A student reacts magnesium ribbon with dilute hydrochloric acid.

A) Using powdered magnesium instead of a ribbon
B) Heating the acid before adding the magnesium
C) Using a more concentrated acid
D) Using a lower concentration of acid

(Total for Question 2 is 1 mark)

Collision theory explains how chemical reactions happen.

(a)State the two conditions that must be met for two particles to react when they collide.(1)

(b)Explain, using collision theory, why increasing the temperature of the reaction mixture increases the rate of reaction.(3)

(Total for Question 3 is 4 marks)

A student increases the concentration of hydrochloric acid used to react with magnesium ribbon.

(Total for Question 4 is 3 marks)

A student reacts marble chips (calcium carbonate) with dilute hydrochloric acid.

(a)The student crushes some marble chips into a powder and reacts this powder with the acid, instead of using whole chips of the same total mass. Explain why this increases the rate of reaction.(2)

(b)State one safety precaution the student should take when carrying out this experiment.(1)

(Total for Question 5 is 3 marks)

Catalysts are used in many industrial processes.

(a)State what is meant by the term 'catalyst'.(1)

(b)Explain, in terms of activation energy, how a catalyst increases the rate of a reaction.(2)

(c)State one economic advantage to a company of using a catalyst in an industrial process.(1)

(Total for Question 6 is 4 marks)

Enzymes and catalysts are used to speed up reactions in industry and in the body.

(a)Name the type of biological catalyst that speeds up reactions in the human body.(1)

(b)State one industrial process that uses a catalyst.(1)

(Total for Question 7 is 2 marks)

Some chemical reactions are reversible.

(a)State what is meant by a 'reversible reaction'.(1)

(b)Blue hydrated copper sulfate crystals can be heated to form white anhydrous copper sulfate powder and water. Write a word equation, using the reversible reaction symbol (≤>), to show this reaction, and state which direction is favoured by adding water to the white powder.(2)

(Total for Question 8 is 3 marks)

Ammonium chloride is a white solid that decomposes reversibly on heating: NH4Cl(s) ≤> NH3(g) + HCl(g)

(a)A sample of solid ammonium chloride is heated strongly in a test tube. State which direction of the reaction, forward or backward, is favoured by this heating.(1)

(b)The ammonia gas and hydrogen chloride gas produced are collected together in a sealed, cool container. Explain what would happen to these gases over time.(2)

(Total for Question 9 is 3 marks)

A student investigates the effect of temperature on the rate of reaction between magnesium ribbon and excess dilute hydrochloric acid, timing how long it takes for the ribbon to fully react.

(a)Identify the independent variable in this investigation.(1)

(b)Identify the dependent variable in this investigation.(1)

(c)State two variables that should be controlled (kept the same) in this investigation.(2)

(Total for Question 10 is 4 marks)

Figure 1 shows data from a student's investigation into the effect of hydrochloric acid concentration on the rate of reaction with excess marble chips (calcium carbonate). The volume of carbon dioxide gas produced was measured using a gas syringe at 10-second intervals.

(a)Identify the independent variable and the dependent variable in this investigation.(2)

(b)Calculate the mean rate of reaction between 0 and 30 seconds, in cm3/s.(2)

(c)Explain, in terms of collision theory, why the rate of reaction decreases as the reaction proceeds.(3)

(d)State how the graph in Figure 1 shows that the reaction has finished.(1)

(Total for Question 11 is 8 marks)

A student uses the disappearing cross method to investigate the effect of concentration on the rate of the reaction between sodium thiosulfate solution and dilute hydrochloric acid, which produces a precipitate of sulfur:
sodium thiosulfate + hydrochloric acid -> sodium chloride + sulfur dioxide + sulfur + water

(a)Describe how the student could carry out this investigation and use it to compare the rate of reaction at different concentrations of hydrochloric acid.(3)

(b)State the equation linking the rate of reaction and the time taken for the cross to disappear.(1)

(c)At a hydrochloric acid concentration of 40 g/dm3, the cross disappeared after 25 seconds. Calculate the rate of this reaction, giving the correct unit.(2)

(d)The student repeats the investigation using a higher concentration of hydrochloric acid, and the cross disappears more quickly. Explain this observation in terms of collision theory.(3)

(Total for Question 12 is 9 marks)

A student investigates the rate of a reaction between calcium carbonate chips and excess dilute hydrochloric acid by measuring the mass of the reaction flask (which is left unstoppered so gas can escape) over time.

(a)Calculate the mean rate of reaction, in g/min, over the first 3 minutes.(2)

(b)State, with a reason, at approximately what time the reaction stopped.(1)

(Total for Question 13 is 3 marks)

Figure 3 shows a graph of the volume of gas produced against time for a reaction between marble chips and hydrochloric acid. The curve rises steeply at first, then its gradient decreases until the line becomes flat at 60 seconds. A tangent has been drawn to the curve at t = 20 s; this tangent passes through the points (10 s, 10 cm3) and (30 s, 42 cm3).

Figure (to be drawn): A graph of volume of gas (cm3, y-axis, 0 to 60) against time (s, x-axis, 0 to 60). The curve starts at the origin, rises steeply, and gradually levels off to a flat line at a volume of 50 cm3 from t = 50 s onwards. A straight tangent line touches the curve at the point (20, 32) and is drawn extending through the points (10, 10) and (30, 42).

(a)Explain why a tangent must be drawn to calculate the rate of reaction at a specific point in time on this curved graph, rather than simply using two points on the curve.(2)

(b)Use the tangent shown in Figure 3 to calculate the rate of reaction at t = 20 seconds.(2)

(Total for Question 14 is 4 marks)

A different student reacts excess calcium carbonate chips with dilute hydrochloric acid and measures the total volume of gas produced over the whole reaction. The reaction produced 60 cm3 of gas in the first 20 seconds, and a total of 96 cm3 of gas by the time it finished at 80 seconds.

Figure (to be drawn): A graph of total volume of gas (cm3, y-axis) against time (s, x-axis) for the whole reaction. The curve rises steeply for the first 20 seconds (reaching 60 cm3), then rises much more gradually, levelling off completely at 96 cm3 by 80 seconds.

(a)Calculate the mean rate of reaction over the whole reaction (from 0 to 80 seconds), giving your answer to 2 significant figures.(2)

(b)Calculate the mean rate of reaction over the first 20 seconds, and use both mean rates to describe how the rate of reaction changes as the reaction proceeds.(2)

(Total for Question 15 is 4 marks)

A student is asked to investigate how the concentration of hydrochloric acid affects the rate of its reaction with sodium thiosulfate solution. They use the disappearing cross method: they place a flask containing sodium thiosulfate solution over a paper cross, add hydrochloric acid, and time how long it takes until the cross can no longer be seen from above. They test acid concentrations of 10, 20, 30 and 40 g/dm3, keeping the volume of thiosulfate solution, the volume of acid and the room temperature the same, and take one reading at each concentration.

(Total for Question 16 is 6 marks)

Higher tier only. A reversible reaction can reach a state called dynamic equilibrium.

(a)State what is meant by 'dynamic equilibrium'.(2)

(b)State one condition that a reversible reaction must be in for it to reach dynamic equilibrium.(1)

(Total for Question 17 is 3 marks)

Higher tier only. Nitrogen and hydrogen react reversibly to form ammonia in the Haber process: N2(g) + 3H2(g) ≤> 2NH3(g)

(Total for Question 18 is 2 marks)

Higher tier only. The forward reaction in the Haber process, N2(g) + 3H2(g) ≤> 2NH3(g), is exothermic.

(Total for Question 19 is 3 marks)

Higher tier only. In the Haber process, N2(g) + 3H2(g) ≤> 2NH3(g), there are 4 moles of gas on the reactants side of the equation and 2 moles of gas on the products side.

(Total for Question 20 is 3 marks)

Mark scheme · C6 Rate and Extent of Chemical Change

Question 1

(a) B1 how quickly a reactant is used up or how quickly a product is formed oe
(a) Answer: The rate of reaction is how quickly a reactant is used up or a product is formed.
(b) B1 measure the volume of gas produced (using a gas syringe) at regular time intervals oe
(b) B1 measure the loss in mass of the reaction flask (using a balance) at regular time intervals oe
(b) Answer: Measure the volume of gas produced over time using a gas syringe; measure the loss in mass of the flask and its contents over time using a balance.

Question 2

B1 D
Answer: D

Question 3

(a) B1 particles must collide with sufficient energy (equal to or greater than the activation energy) and in the correct orientation oe (both needed for the mark)
(a) Answer: They must collide with energy greater than or equal to the activation energy, and with the correct orientation.
(b) B1 at a higher temperature the particles have more kinetic energy and move faster oe
(b) B1 so the particles collide more frequently oe
(b) B1 a greater proportion of collisions have energy greater than or equal to the activation energy, so a greater proportion of collisions are successful oe
(b) Answer: Particles have more kinetic energy and move faster, so they collide more often; a greater proportion of these collisions have energy greater than or equal to the activation energy, so more collisions are successful, increasing the rate.

Question 4

B1 increasing the concentration means there are more acid particles in the same volume oe
B1 so collisions between reacting particles happen more frequently oe
B1 so there are more successful collisions per second/unit time, increasing the rate of reaction
Answer: Higher concentration means more acid particles in the same volume, so particles collide more often; this gives more successful collisions per second, increasing the rate of reaction.

Question 5

(a) B1 crushing the marble into a powder increases its surface area to volume ratio oe
(a) B1 this means more particles of acid can collide with (particles on the surface of) the marble per second, increasing the frequency of collisions and so the rate oe
(a) Answer: Powdering the marble increases its surface area to volume ratio, so more acid particles can collide with the marble's surface per second, increasing the rate of reaction.
(b) B1 wear eye protection oe (any sensible precaution, e.g. work in a well-ventilated area, handle acid with care)
(b) Answer: Wear eye protection.

Question 6

(a) B1 a substance that increases the rate of a reaction without being used up (chemically unchanged) at the end of the reaction oe
(a) Answer: A catalyst is a substance that speeds up a reaction without being used up itself.
(b) B1 a catalyst provides an alternative reaction pathway (mechanism) that has a lower activation energy oe
(b) B1 so a greater proportion of collisions have energy greater than or equal to this (lower) activation energy, so more collisions are successful oe
(b) Answer: A catalyst provides an alternative pathway with a lower activation energy, so a greater proportion of particle collisions have enough energy to react, increasing the rate.
(c) B1 lower temperatures/pressures can be used, reducing energy costs oe
(c) Answer: Using a catalyst allows lower temperatures to be used, reducing energy costs.

Question 7

(a) B1 enzyme
(a) Answer: Enzyme
(b) B1 the Haber process (iron catalyst) oe (accept catalytic converters, or any correctly named industrial catalytic process)
(b) Answer: The Haber process, which uses an iron catalyst.

Question 8

(a) B1 a reaction in which the products of the reaction can react together to re-form the original reactants oe
(a) Answer: A reversible reaction is one in which the products can react to reform the original reactants.
(b) B1 hydrated copper sulfate ≤> anhydrous copper sulfate + water
(b) B1 adding water favours the backward/reverse reaction (forms hydrated copper sulfate) oe
(b) Answer: Hydrated copper sulfate ≤> anhydrous copper sulfate + water. Adding water favours the reverse reaction, re-forming blue hydrated copper sulfate.

Question 9

(a) B1 forward (the solid decomposes into the two gases)
(a) Answer: Forward.
(b) M1 as the mixture cools, the backward/reverse reaction is favoured oe
(b) A1 the ammonia and hydrogen chloride gases react together to re-form solid ammonium chloride oe
(b) Answer: As the gases cool, the reverse reaction is favoured, so the ammonia and hydrogen chloride recombine to re-form solid ammonium chloride.

Question 10

(a) B1 temperature of the acid
(a) Answer: Temperature of the acid.
(b) B1 time taken for the magnesium ribbon to fully react/disappear
(b) Answer: Time taken for the magnesium ribbon to fully react.
(c) B1 concentration/volume of hydrochloric acid
(c) B1 length/mass of magnesium ribbon (any two suitable control variables credited, e.g. same person judging when reaction is complete)
(c) Answer: The concentration and volume of hydrochloric acid, and the length (mass) of magnesium ribbon used, should be kept the same each time.

Question 11

(a) B1 independent variable: concentration of hydrochloric acid
(a) B1 dependent variable: volume of carbon dioxide gas produced (in a given time)
(a) Answer: Independent variable: concentration of hydrochloric acid. Dependent variable: volume of CO2 gas produced.
(b) M1 42 / 30
(b) A1 1.4 cm3/s cao
(b) Answer: 1.4 cm3/s
(c) M1 as the reaction proceeds, hydrochloric acid (a reactant) is used up, so its concentration decreases oe
(c) M1 this means there are fewer acid particles in a given volume, so collisions between reacting particles are less frequent oe
(c) A1 fewer successful collisions occur per second, so the rate of reaction decreases
(c) Answer: As the reaction proceeds, the acid is used up so its concentration falls, meaning there are fewer acid particles per unit volume; collisions between reacting particles happen less often, so fewer are successful each second and the rate decreases.
(d) B1 the volume of gas produced stops changing/becomes constant (the line becomes flat/horizontal) between 50 s and 60 s
(d) Answer: The volume of gas stays constant at 50 cm3 between 50 s and 60 s, showing the reaction has stopped.

Question 12

(a) B1 place the flask containing sodium thiosulfate solution and acid on top of a paper marked with a cross, and view it from above oe
(a) B1 start a timer as soon as the acid is added, and stop it when the cross can no longer be seen (due to the sulfur precipitate) oe
(a) B1 repeat using different concentrations of hydrochloric acid, keeping all other variables (e.g. volume of thiosulfate, temperature) the same oe
(a) Answer: Add the acid to the thiosulfate solution in a flask standing on a paper marked with a cross; time from mixing until the cross disappears from view; repeat at different acid concentrations, keeping the volume of thiosulfate and the temperature constant each time.
(b) B1 rate of reaction = 1 / time
(b) Answer: Rate of reaction = 1 / time
(c) M1 1 / 25
(c) A1 0.04 s-1 (or /s) cao
(c) Answer: 0.04 s-1
(d) B1 a higher concentration means there are more acid particles in the same volume oe
(d) B1 so reacting particles collide more frequently oe
(d) B1 more successful collisions happen per second, so the rate of reaction increases and the cross disappears faster
(d) Answer: A higher acid concentration means more acid particles per unit volume, so collisions with thiosulfate particles happen more often; more successful collisions occur per second, so the reaction is faster and the cross disappears sooner.

Question 13

(a) M1 (100.00 - 99.64) / 3
(a) A1 0.12 g/min cao
(a) Answer: 0.12 g/min
(b) B1 at (approximately) 4 minutes, because the mass no longer changes after this time (stays at 99.61 g) oe
(b) Answer: At about 4 minutes, since the mass stays constant at 99.61 g from then on.

Question 14

(a) B1 the gradient of the curve is constantly changing, so a line joining two points on the curve would not give the rate at a single instant oe
(a) B1 a tangent touches the curve at only one point, so its gradient gives the instantaneous rate of reaction at that point in time oe
(a) Answer: The curve's gradient keeps changing, so joining two points on the curve gives an average rate over that interval, not the rate at one instant; a tangent touches the curve at a single point, so its gradient gives the true instantaneous rate there.
(b) M1 (42 - 10) / (30 - 10)
(b) A1 1.6 cm3/s cao
(b) Answer: 1.6 cm3/s

Question 15

(a) M1 96 / 80
(a) A1 1.2 cm3/s (2 sf) cao
(a) Answer: 1.2 cm3/s
(b) M1 60 / 20 = 3.0 cm3/s (ft their part a value if used correctly for comparison)
(b) A1 the rate over the first 20 s (3.0 cm3/s) is greater than the rate over the whole reaction (1.2 cm3/s), showing the reaction is fastest at the start and slows down as it proceeds oe
(b) Answer: 3.0 cm3/s over the first 20 s, compared with 1.2 cm3/s over the whole reaction; this shows the reaction is fastest at the start and gets progressively slower as the reactants are used up.

Question 16

Level 1 (1-2): Simple, isolated statements are made about the method and/or possible improvements, with little or no linkage between them. No clear conclusion is drawn about how the results would show the relationship between concentration and rate.
Level 2 (3-4): Some relevant strengths and/or weaknesses of the method are identified, with some suggested improvements. There is some linkage between the improvements and how they would affect the reliability or validity of the results.
Level 3 (5-6): A detailed and logically structured evaluation is given, identifying weaknesses in the method and linking clearly justified improvements to how they would improve the reliability and/or validity of the results. The answer explains how the improved results could be used to establish the relationship between concentration and rate.
Indicative content:
- Judging by eye exactly when the cross 'disappears' is subjective; different observers (or the same observer on different attempts) may judge the end point differently, reducing the reliability of the timings.
- Only one reading is taken at each concentration, so anomalous results cannot be identified; repeating each concentration and calculating a mean time would improve reliability.
- Only four concentrations are tested, over a limited range; using more concentrations, spread across a wider range (e.g. 5, 10, 15, ... g/dm3), would give a more precise picture of the trend.
- Using the same person to judge the end point each time, the same size of cross, and viewing from the same distance/lighting would reduce this source of error.
- The volume of thiosulfate, volume of acid and temperature have correctly been controlled, which is a strength of the method, since changing any of these would also affect the rate.
- Calculating the rate (1/time) for each concentration and plotting a graph of rate against concentration would show whether rate is directly proportional to concentration (a straight line through the origin) or whether the relationship is more complex.

Question 17

(a) B1 the forward and backward (reverse) reactions are happening at the same rate oe
(a) B1 so the concentrations of reactants and products remain constant (do not change) over time oe
(a) Answer: Dynamic equilibrium is when the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products stay constant.
(b) B1 it must be in a closed system (no reactants or products can enter or leave)
(b) Answer: The reaction must take place in a closed system.

Question 18

M1 increasing the concentration of nitrogen shifts the position of equilibrium to the right (towards the products) oe
A1 because the system acts to reduce the concentration of the added nitrogen (by using it up), so more ammonia is formed oe
Answer: The position of equilibrium shifts to the right (towards the products), so more ammonia is formed, because the system acts to use up the extra nitrogen that was added.

Question 19

M1 increasing the temperature shifts the position of equilibrium to the left (towards the reactants) oe
M1 this is the endothermic direction, so the system acts to oppose/reduce the increase in temperature by absorbing the extra energy oe
A1 less ammonia (and more nitrogen and hydrogen) is present in the equilibrium mixture, so the yield of ammonia decreases
Answer: The position of equilibrium shifts to the left (the endothermic, reverse direction), absorbing the extra energy to oppose the temperature rise; the equilibrium yield of ammonia decreases.

Question 20

M1 increasing the pressure shifts the position of equilibrium to the right (towards the products) oe
M1 because this is the side with fewer moles of gas, so the system acts to reduce the pressure (reduce the total number of gas particles) oe
A1 more ammonia is present in the equilibrium mixture, so the yield of ammonia increases
Answer: The position of equilibrium shifts to the right, towards the side with fewer gas moles (the products), because the system acts to reduce the increased pressure; the equilibrium yield of ammonia increases.

Rate and Extent of Chemical Change - Worksheets, Questions and Revision