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GCSE · Chemistry

C5a Exothermic and Endothermic Reactions

AQA 8464 · Calculator allowed · about 80 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Warming Up and Cooling Down: Energy Changes in Reactions

Original text written for Revision Library.

Every chemical reaction transfers energy somewhere. In an exothermic reaction, energy is transferred to the surroundings, so the temperature of the surroundings rises; in an endothermic reaction, energy is taken in from the surroundings, so the temperature falls. Chemists picture these changes using a reaction profile: a simple graph that shows the energy of the reactants, the energy of the products, and the hump in between, called the activation energy, that a reaction must climb over before it can happen at all. These ideas are not confined to the laboratory. A hand warmer tucked into a glove releases a slow, steady trickle of heat for hours by letting iron powder react gradually with oxygen, while a sports injury cold pack chills an ankle within seconds by dissolving a solid salt in water, an endothermic process. This worksheet explores how exothermic and endothermic reactions are represented on reaction profiles, how a required practical investigation can measure the temperature changes they produce, and how these ideas explain the everyday chemistry of staying warm or cooling down.

Define the term 'exothermic reaction'.

(Total for Question 1 is 1 mark)

Endothermic reactions are the opposite of exothermic reactions.

(a)Define the term 'endothermic reaction'.(1)

(b)Give one example of an everyday process that is endothermic.(1)

(Total for Question 2 is 2 marks)

A reaction profile diagram is used to show the energy changes during a reaction.

(a)State what is meant by a 'reactant'.(1)

(b)State what is meant by a 'product'.(1)

(c)State what a reaction profile diagram is used to show.(1)

(Total for Question 3 is 3 marks)

For each of the following changes, state whether it is exothermic or endothermic.

(a)Methane burning in a Bunsen burner: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)(1)

(b)Calcium carbonate breaking down when heated strongly in a lime kiln: CaCO3(s) -> CaO(s) + CO2(g)(1)

(c)Dilute hydrochloric acid neutralising sodium hydroxide solution: HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)(1)

(d)Solid ammonium nitrate dissolving in water.(1)

(Total for Question 4 is 4 marks)

Reactions need a minimum amount of energy before they can happen.

(a)State what is meant by 'activation energy'.(1)

(b)State the effect that a catalyst has on the activation energy needed for a reaction.(1)

(Total for Question 5 is 2 marks)

The diagram shows a reaction profile for a general reaction.

Figure (to be drawn): Figure 1: reaction profile axes labelled 'Energy' (y-axis) and 'Progress of reaction' (x-axis); a curve starts at the reactants' energy level, rises to a peak, then falls to the products' energy level, which is drawn lower than the reactants' level

(a)State whether this reaction profile represents an exothermic or an endothermic reaction.(1)

(b)State what is represented by the peak on the diagram.(1)

(c)Give one everyday device that makes use of an exothermic reaction like this one to produce warmth.(1)

(Total for Question 6 is 3 marks)

State two everyday uses of exothermic reactions, other than a hand warmer.

(Total for Question 7 is 2 marks)

A student is about to carry out an investigation into temperature changes using dilute hydrochloric acid and calcium carbonate chips.

State three safety precautions the student should take during this investigation.

(Total for Question 8 is 3 marks)

When investigating temperature changes in reacting solutions, other than the mass of a solid reactant, state two other independent variables that could be changed.

(Total for Question 9 is 2 marks)

Calcium carbonate breaks down when heated: CaCO3(s) -> CaO(s) + CO2(g). This is an endothermic reaction. The diagram shows the reaction profile, but the position of the products line has not yet been drawn in.

Figure (to be drawn): Figure 2: reaction profile axes labelled 'Energy' (y-axis) and 'Progress of reaction' (x-axis); the reactants line is drawn partway up the axis, a curve rises to a peak and falls again, but the height of the products line relative to the reactants line is left for students to determine

(a)State whether the products line should be drawn above or below the reactants line on the profile.(1)

(b)Explain your answer to part (a).(2)

(c)Give one observation a student could make in the school laboratory that would show this reaction is endothermic.(1)

(Total for Question 10 is 4 marks)

Hydrogen peroxide decomposes in the presence of a manganese dioxide catalyst: 2H2O2(aq) -> 2H2O(l) + O2(g). The reaction profile for this reaction has these energy values:
Energy of reactants = 40 kJ
Energy of the peak (transition state) = 90 kJ
Energy of products = 25 kJ

Figure (to be drawn): Figure 3: reaction profile axes labelled 'Energy (kJ)' (y-axis) and 'Progress of reaction' (x-axis); the curve starts at the reactants' energy (40 kJ), rises to a peak at the transition state (90 kJ), then falls to the products' energy (25 kJ), which is below the reactants line

(a)Calculate the activation energy of the reaction.(1)

(b)Calculate the overall energy change for the reaction, and state whether energy is released or absorbed overall.(2)

(c)Use the values given to explain why this reaction is exothermic.(2)

(d)A different catalyst is used that lowers the activation energy of this reaction. Describe the effect this would have on the shape of the reaction profile, and explain why a catalyst has this effect.(2)

(Total for Question 11 is 7 marks)

Copper carbonate decomposes when heated: CuCO3(s) -> CuO(s) + CO2(g). This is an endothermic reaction. The reaction profile for this reaction has these energy values:
Energy of reactants = 55 kJ
Energy of the peak (transition state) = 110 kJ
Energy of products = 80 kJ

Figure (to be drawn): Figure 4: reaction profile axes labelled 'Energy (kJ)' (y-axis) and 'Progress of reaction' (x-axis); the curve starts at the reactants' energy (55 kJ), rises to a peak at the transition state (110 kJ), then falls to the products' energy (80 kJ), which is above the reactants line

(a)Calculate the activation energy of the reaction.(1)

(b)Calculate the overall energy change for the reaction.(1)

(c)State how the reaction profile diagram shows that this reaction is endothermic.(1)

(d)A different decomposition reaction has a lower activation energy than this one, but the same overall energy change. State how this would be shown differently on a reaction profile diagram.(1)

(Total for Question 12 is 4 marks)

A student added a small piece of magnesium ribbon to excess dilute hydrochloric acid in a polystyrene cup, and measured the temperature every 10 seconds. The temperature rose from 19.0 degreesC to 34.0 degreesC over the first 40 seconds, then stayed constant.

(a)Calculate the overall temperature change during the first 40 seconds.(1)

(b)Calculate the mean rate of temperature increase, in degreesC per second, during the first 40 seconds.(2)

(Total for Question 13 is 3 marks)

A student investigated how the mass of ammonium nitrate added to 25 cm³ of water affects the temperature change of the water. Dissolving ammonium nitrate in water is an endothermic process. The student used an insulated polystyrene cup with a lid, added a measured mass of ammonium nitrate, stirred, and recorded the lowest temperature reached. Each mass was tested three times. The results are shown below.
Mass of ammonium nitrate added (g): 1.0, 2.0, 3.0, 4.0
Trial 1 temperature change (C): 2.4, 4.9, 7.4, 9.9
Trial 2 temperature change (C): 2.6, 5.1, 7.6, 10.1
Trial 3 temperature change (C): 2.5, 5.0, 11.0, 10.0
Mean temperature change (C): 2.5, 5.0, ?, 10.0

(a)Give one variable that should be controlled in this investigation, other than the mass of ammonium nitrate.(1)

(b)State the independent variable and the dependent variable in this investigation.(2)

(c)Identify the anomalous result in the table.(1)

(d)Calculate the mean temperature change for 3.0 g of ammonium nitrate, ignoring the anomalous result.(1)

(e)Use the pattern in the table to predict the mean temperature change that would occur if 5.0 g of ammonium nitrate were dissolved under the same conditions.(2)

(f)Explain why the student used a polystyrene cup with a lid, rather than an open glass beaker, in this investigation.(2)

(g)The thermometer used had a resolution of 1 degreesC. Suggest one improvement to the apparatus that would increase the accuracy of the results, and explain why.(2)

(h)This investigation models the chemistry used in a sports injury cold pack. Explain, in terms of energy transfer, how a cold pack works when its two compartments (ammonium nitrate and water) are mixed.(2)

(Total for Question 14 is 13 marks)

The student in Question 14 wants to display how the mean temperature change varies with the mass of ammonium nitrate added.

(a)State the most appropriate type of graph to display this data, and explain your choice.(2)

(b)State which variable should be plotted on the x-axis.(1)

(Total for Question 15 is 3 marks)

In a different investigation, a student adds increasing masses of magnesium ribbon to a fixed volume of dilute hydrochloric acid, which is always in excess: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g).

(a)Predict what would happen to the temperature change as the mass of magnesium added is increased (while the acid remains in excess).(1)

(b)Explain your answer to part (a).(2)

(Total for Question 16 is 3 marks)

A hand warmer contains 75 g of iron powder. The iron reacts slowly with oxygen in the air: 4Fe(s) + 3O2(g) -> 2Fe2O3(s) (exothermic). Each gram of iron that fully reacts releases approximately 1.6 kJ of energy as heat.

(a)Calculate the total energy released when all 75 g of iron in the hand warmer has fully reacted.(2)

(b)The hand warmer transfers this energy over 6 hours of use. Calculate the mean rate of energy transfer, in kJ per hour.(1)

(Total for Question 17 is 3 marks)

A student wants to compare how exothermic two different reactions are by measuring the maximum temperature change each one produces.

State three factors, other than the identity of the reactants, that should be kept the same for this to be a fair comparison.

(Total for Question 18 is 3 marks)

A company is developing a new type of hand warmer for use during winter sports and is comparing two possible exothermic reactions.
Reaction 1: iron powder reacting with oxygen in the air (as used in current hand warmers): 4Fe(s) + 3O2(g) -> 2Fe2O3(s)
Reaction 2: magnesium ribbon reacting with dilute hydrochloric acid: Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

Compare the two reactions and evaluate which would be more suitable for use in a hand warmer that needs to stay warm for several hours. You should refer to energy transfer and to practical/safety considerations in your answer.

(Total for Question 19 is 6 marks)

Higher tier only. A scientist is designing a reusable hand warmer that contains a supersaturated solution of sodium ethanoate (sodium acetate). When a small metal disc inside the pack is clicked, the solution rapidly turns into solid crystals, and the pack becomes warm. The pack can be made ready to use again by placing it in boiling water, which turns the crystals back into a solution, ready to be clicked again.

(a)State whether the crystallisation (solidifying) of the sodium ethanoate solution is exothermic or endothermic. Use information from the question to justify your answer.(2)

(b)Explain, in terms of energy transfer, why reheating the pack in boiling water is needed to make it reusable.(2)

(Total for Question 20 is 4 marks)

Mark scheme · C5a Exothermic and Endothermic Reactions

Question 1

B1 a reaction that transfers energy to the surroundings, usually shown by a rise in the temperature of the surroundings (oe)
Answer: A reaction that transfers energy to the surroundings, usually shown by an increase in the temperature of the surroundings.

Question 2

(a) B1 a reaction that takes in energy from the surroundings, usually shown by a fall in the temperature of the surroundings (oe)
(a) Answer: A reaction that takes in energy from the surroundings, usually shown by a decrease in the temperature of the surroundings.
(b) B1 any one valid example, e.g. dissolving ammonium nitrate in water, thermal decomposition (e.g. of calcium carbonate), a sports injury cold pack in use (oe, any one)
(b) Answer: Dissolving ammonium nitrate in water, as used in a sports injury cold pack.

Question 3

(a) B1 a starting substance in a chemical reaction, before it has reacted (oe)
(a) Answer: A starting substance in a chemical reaction, before it has reacted.
(b) B1 a substance formed as a result of a chemical reaction (oe)
(b) Answer: A substance that is formed as a result of a chemical reaction.
(c) B1 how the energy of the reactants and products (and the energy needed to start the reaction) changes as a reaction takes place (oe)
(c) Answer: How the energy of the substances involved changes as the reaction proceeds, from reactants to products.

Question 4

(a) B1 exothermic (cao)
(a) Answer: Exothermic.
(b) B1 endothermic (cao)
(b) Answer: Endothermic.
(c) B1 exothermic (cao)
(c) Answer: Exothermic.
(d) B1 endothermic (cao)
(d) Answer: Endothermic.

Question 5

(a) B1 the minimum amount of energy that particles must have when they collide for a reaction to occur (oe)
(a) Answer: The minimum amount of energy that particles need when they collide for a reaction to happen.
(b) B1 a catalyst lowers the activation energy needed, by providing an alternative reaction pathway (oe)
(b) Answer: A catalyst lowers the activation energy needed for the reaction, by providing an alternative reaction pathway.

Question 6

(a) B1 exothermic, because the products line is drawn lower than the reactants line (cao)
(a) Answer: Exothermic (the products line is lower than the reactants line).
(b) B1 the activation energy: the minimum energy needed for the reaction to happen (oe)
(b) Answer: The activation energy, the minimum energy needed for the reaction to occur.
(c) B1 any valid example, e.g. a hand warmer, a self-heating can (oe, any one)
(c) Answer: A hand warmer (or a self-heating drinks can).

Question 7

B1 any one from: self-heating cans (e.g. for coffee or food); the setting/curing of some adhesives or cements; combustion reactions used for heating homes or cooking; fireworks or flares producing heat and light (oe)
B1 any second, different example from the same list (oe)
Answer: Any two from: self-heating food or drink cans; combustion used for heating or cooking; the setting of some adhesives or cements; fireworks or flares.

Question 8

B1 any one from: wear eye protection/safety goggles
B1 any one from: take care handling the dilute acid, avoiding skin or eye contact and washing off any spills
B1 any one from: stand up during the practical rather than sitting; do not put the thermometer down on the desk/use it to stir; work in a well-ventilated area, as carbon dioxide gas is produced (any one, oe; any three different precautions from this list)
Answer: Any three from: wearing eye protection; handling the dilute acid carefully and washing off spills; standing up during the practical; not using the thermometer to stir; working in a well-ventilated area.

Question 9

B1 any one from: the concentration of a solution used; the volume of solution used; the identity/type of reactant used (e.g. which metal or acid); the starting temperature of the solution (oe)
B1 any second, different variable from the same list (oe)
Answer: Any two from: the concentration of a solution used; the volume of solution used; which reactant (e.g. metal or acid) is used; the starting temperature of the solution.

Question 10

(a) B1 above (cao)
(a) Answer: Above the reactants line.
(b) B1 energy is absorbed (taken in) from the surroundings during the reaction (oe)
(b) B1 so the products end up with more energy than the reactants (oe)
(b) Answer: Energy is absorbed from the surroundings, so the products end up with more energy than the reactants.
(c) B1 the temperature of the reaction mixture/surroundings decreases (oe)
(c) Answer: The temperature of the reaction mixture (and its surroundings) would decrease.

Question 11

(a) B1 50 kJ (cao) [90 - 40]
(a) Answer: 50 kJ
(b) M1 correctly subtracts to find 40 - 25 (= 15)
(b) A1 15 kJ released overall (exothermic) (allow -15 kJ) (cao)
(b) Answer: 15 kJ released overall (the reaction is exothermic).
(c) M1 the products (25 kJ) have less energy than the reactants (40 kJ) (oe)
(c) A1 so overall energy is transferred (released) to the surroundings (oe)
(c) Answer: The products (25 kJ) have less energy than the reactants (40 kJ), so overall energy (15 kJ) is released to the surroundings.
(d) M1 the peak (transition state) would be drawn lower, closer to the reactants line; the reactants and products lines stay in the same place (oe)
(d) A1 because a catalyst provides an alternative reaction pathway that needs less energy for the reaction to happen (oe)
(d) Answer: The peak of the profile would be lower (a smaller hump), because the catalyst provides an alternative reaction pathway with a lower activation energy; the reactants and products energy levels stay the same.

Question 12

(a) B1 55 kJ (cao) [110 - 55]
(a) Answer: 55 kJ
(b) B1 25 kJ absorbed (allow +25 kJ) (cao) [80 - 55]
(b) Answer: 25 kJ absorbed (endothermic).
(c) B1 the products line is drawn above (higher than) the reactants line (oe)
(c) Answer: The products line is drawn above (higher than) the reactants line.
(d) B1 the peak (transition state) would be drawn lower, closer to the reactants line, with the reactants and products lines the same distance apart (oe)
(d) Answer: The peak (transition state) would be lower, nearer the reactants line, while the reactants and products lines stay the same distance apart.

Question 13

(a) B1 15.0 degreesC increase (cao) [34.0 - 19.0]
(a) Answer: 15.0 degreesC increase.
(b) M1 correctly divides the temperature change by the time taken: 15.0 / 40 (ecf from candidate's answer to (a))
(b) A1 0.375 degreesC per second (allow awrt 0.38 degreesC/s, or 0.4 degreesC/s to 1 sf) (cao, ecf)
(b) Answer: 0.375 degreesC per second (accept awrt 0.38 degreesC/s).

Question 14

(a) B1 any one valid controlled variable, e.g. the volume of water used; the starting temperature of the water; the type of container/level of insulation used; the stirring method or time (oe)
(a) Answer: e.g. the volume of water used (25 cm³ each time).
(b) B1 independent variable: the mass of ammonium nitrate added
(b) B1 dependent variable: the temperature change of the water
(b) Answer: Independent variable: mass of ammonium nitrate added. Dependent variable: temperature change of the water.
(c) B1 the trial 3 result for 3.0 g of ammonium nitrate (11.0 degreesC) (cao)
(c) Answer: The trial 3 reading for 3.0 g of ammonium nitrate (11.0 degreesC).
(d) B1 7.5 degreesC (allow ft from candidate's answer to (c)); (7.4 + 7.6) / 2
(d) Answer: 7.5 degreesC
(e) M1 identifies that the mean temperature change increases by about 2.5 degreesC for every extra 1 g of ammonium nitrate added (oe), or recognises the direct proportional relationship
(e) A1 12.5 degreesC (allow ft from candidate's identified pattern; accept a reasonable range, e.g. 12.0-13.0 degreesC, with clear supporting working)
(e) Answer: About 12.5 degreesC.
(f) M1 polystyrene (and the lid) reduce the transfer of energy between the water and the surroundings, by conduction, convection and/or evaporation (oe)
(f) A1 so the temperature change measured is closer to the true (actual) energy change of the reaction, making the results more accurate/valid (oe)
(f) Answer: A polystyrene cup with a lid reduces energy transfer to or from the surroundings, so the measured temperature change is a more accurate reflection of the true energy change of the reaction.
(g) M1 use a thermometer, temperature probe or data logger with a finer resolution, e.g. 0.1 degreesC (oe)
(g) A1 this allows smaller temperature changes to be measured more precisely, reducing the uncertainty in each reading (oe)
(g) Answer: Use a thermometer (or temperature probe/data logger) with a finer resolution, such as 0.1 degreesC, as this reduces the uncertainty in each temperature reading.
(h) M1 dissolving ammonium nitrate in water is an endothermic process, so energy is taken in from the surroundings (oe)
(h) A1 this causes the temperature of the pack (and the injured area it is placed against) to decrease (oe)
(h) Answer: Dissolving ammonium nitrate in water is endothermic, so it takes in energy from the surroundings; this makes the temperature of the pack fall, which can help reduce swelling or pain at an injury.

Question 15

(a) B1 a line graph (allow scatter graph with a line of best fit)
(a) B1 because both variables are continuous numerical data (oe)
(a) Answer: A line graph, because both the mass added and the temperature change are continuous numerical variables.
(b) B1 the mass of ammonium nitrate added, the independent variable (cao)
(b) Answer: The mass of ammonium nitrate added (the independent variable).

Question 16

(a) B1 the temperature change would increase (oe)
(a) Answer: The temperature change would increase.
(b) M1 a greater mass of magnesium means more reactant particles take part in the exothermic reaction (oe)
(b) A1 so more energy in total is transferred to the surroundings (the fixed volume of acid), giving a bigger temperature rise (oe)
(b) Answer: A greater mass of magnesium means more reactant reacts, releasing more energy in total to the fixed volume of acid, so a bigger temperature rise is measured (until the acid is no longer in excess).

Question 17

(a) M1 75 x 1.6
(a) A1 120 kJ (cao)
(a) Answer: 120 kJ.
(b) B1 20 kJ per hour (allow ft from candidate's answer to (a) divided by 6)
(b) Answer: 20 kJ per hour.

Question 18

B1 any one from: the mass/amount of reactant used
B1 any one from: the volume (and starting temperature) of the water or solution used
B1 any one from: the concentration of any solutions used; the type of container/level of insulation used (any one, oe; any three different factors from this list)
Answer: Any three from: the mass/amount of reactant used; the volume and starting temperature of the water or solution used; the concentration of any solutions used; the type of container/level of insulation used.

Question 19

Level 3 (5-6): A detailed, logical comparison of both reactions is given, referring to the rate/duration of energy release and to safety/practicality, leading to a fully justified conclusion about which reaction is more suitable for a long-lasting hand warmer.
Level 2 (3-4): Some relevant comparison is made between the two reactions, using ideas about the rate of energy release and/or safety or practicality, but the judgement on suitability is only partly justified.
Level 1 (1-2): Simple statements are made about the reactions, e.g. identifying that both are exothermic, with little or no comparison or reasoned judgement.
Level 0 (0): No relevant content.
Indicative content:
- Both reactions are exothermic and transfer energy to the surroundings, raising the temperature.
- The iron and oxygen reaction is relatively slow, releasing energy gradually over a long period, giving sustained warmth over several hours.
- The magnesium and hydrochloric acid reaction is fast and vigorous, releasing energy quickly and then stopping, giving only a short burst of warmth rather than several hours of warmth.
- The iron/oxygen reaction produces only a solid product (iron oxide), so it is safe to seal in a packet with no gas build-up.
- The magnesium/acid reaction produces hydrogen gas, which is flammable and would need to escape from the packaging, and also involves handling a corrosive liquid acid, both of which are safety and practicality concerns.
- Iron-based hand warmers can be designed to start reacting only when air reaches the iron powder, for example when the packet is opened, which is a practical, controllable trigger.
- A reasoned conclusion is required: the iron and oxygen reaction is more suitable for a hand warmer that must stay warm for several hours, because it releases energy slowly and safely over a long period, whereas the magnesium and acid reaction is too fast, less safe, and impractical to carry as a liquid.

Question 20

(a) M1 exothermic
(a) A1 because the pack becomes warm as the solution crystallises, showing that energy is released to the surroundings (oe)
(a) Answer: Exothermic; the pack becomes warm as it crystallises, showing that energy is released to the surroundings.
(b) M1 heating the solid crystals in boiling water melts/redissolves them, an endothermic change that takes in (absorbs) energy from the water (oe)
(b) A1 this reverses the crystallisation, resetting the pack so the same amount of energy can be released again the next time it is clicked (oe)
(b) Answer: Heating redissolves the crystals in an endothermic process that absorbs energy from the boiling water; this reverses the crystallisation and resets the pack so it can release that same energy again next time it is used.

Exothermic and Endothermic Reactions - Worksheets, Questions and Revision