5.5.1.1Energy transfer during exothermic and endothermic reactions
AQA GCSE Combined Science (8464), Higher tier · Chemistry › Energy changes › Exothermic and endothermic reactions
Practise Energy transfer during exothermic and endothermic reactions. 19 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Every chemical reaction transfers energy, either to the surroundings (exothermic) or from the surroundings (endothermic). You need to classify reactions from temperature changes, give examples and uses of each type, and describe the required practical on temperature changes in reacting solutions. Expect data questions, short recall questions and 4 to 6 mark method questions.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define exothermic and endothermic reactionsExothermic reactions transfer energy to the surroundings so their temperature rises; endothermic reactions take in energy so their temperature falls.
3
Give examples of exothermic reactionsCombustion, many oxidation reactions and neutralisation are all exothermic.
4
Give examples of endothermic reactionsThermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are endothermic.
4
Classify a reaction from temperature dataA rise in temperature means the reaction is exothermic; a fall means it is endothermic.
5
Explain energy conservation in a reactionEnergy is not created or destroyed, so in an exothermic reaction the products have less energy than the reactants by the amount transferred.
6
Describe the temperature change required practicalMix measured amounts in a polystyrene cup, stir, and record the starting and the highest (or lowest) temperature.
6
Evaluate uses of exothermic and endothermic reactionsJudge hand warmers, self-heating cans and cold packs using given data on temperature change, cost, safety and reuse.
7
Suggest and explain improvements to the practicalFor example, a lid and extra insulation reduce energy transfer to the surroundings, and repeats let you calculate a mean.
Notes
Energy is conserved
Energy is never created or destroyed in a chemical reaction. There is the same amount of energy in the universe after a reaction as before it.
If a reaction transfers energy to the surroundings, the products must have less energy than the reactants, by exactly the amount transferred.
If a reaction takes in energy from the surroundings, the products have more energy than the reactants.
The surroundings are everything around the reacting particles: the water the chemicals are dissolved in, the container, the thermometer and the air.
Exothermic reactions
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
Examples: combustion (burning fuels), many oxidation reactions, neutralisation (acid + alkali), metals reacting with acids, and displacement reactions.
Everyday uses: self-heating cans of food or drink, and hand warmers.
Endothermic reactions
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
Examples: thermal decomposition (e.g. calcium carbonate → calcium oxide + carbon dioxide) and the reaction of citric acid with sodium hydrogencarbonate.
Everyday use: some instant cold packs for sports injuries.
Required practical: temperature changes
You investigate a variable that affects the temperature change of reacting solutions, e.g. acid + metal, acid + carbonate, neutralisation or metal displacement.
Measure a set volume of one solution into a polystyrene cup. Stand the cup in a beaker so it cannot fall over.
Record the starting temperature. Add the other reactant, stir, and record the highest (or lowest) temperature reached.
Temperature change = highest (or lowest) temperature − starting temperature.
The independent variable could be the type or mass of metal, or the volume or concentration of a solution. Keep everything else the same.
Polystyrene is a good thermal insulator and a lid reduces energy transfer to (or from) the air, so the measured temperature change is closer to the true value. grade 6+
If you add an alkali to an acid in small portions, the temperature is highest when the acid has just been neutralised. After that no more energy is released, and the extra solution added cools the mixture. grade 7+
Cheatsheet
Exothermic: energy transferred to the surroundings, so their temperature rises
Endothermic: energy taken in from the surroundings, so their temperature falls
Energy is conserved: the total energy is the same before and after a reaction
Temperature change = highest (or lowest) temperature − starting temperature
Practical: polystyrene cup and lid reduce energy transfer with the surroundings
How to answer each type of question
Identify exothermic or endothermic from data
1 to 2 marks4
Work out the temperature change for each reaction (final − starting temperature).
A rise in temperature means exothermic; a fall means endothermic.
Back up your choice with the data, e.g. 'the temperature fell by 4.5 °C'.
Example. A student carried out three reactions in polystyrene cups and recorded the temperatures. P: iron filings + copper sulfate solution: start 19.0 °C, end 27.5 °C Q: citric acid + sodium hydrogencarbonate solution: start 20.5 °C, end 16.0 °C R: nitric acid + potassium hydroxide solution: start 19.5 °C, end 31.0 °C (a) Which reaction is endothermic? Give a reason for your answer. (2) (b) Calculate the temperature change in reaction R. (1)
Show the model answer
(a) Q (1) because the temperature decreased / fell by 4.5 °C (1) (b) 31.0 − 19.5 = 11.5 °C (1)
Explain how the energy of the products compares with the reactants
2 marks5
Say whether energy is transferred to or from the surroundings.
Compare the energy of the products with the energy of the reactants, using 'more' or 'less'.
Link to conservation of energy: the difference is the energy transferred.
Example. When methane burns, the temperature of the surroundings increases. Explain how the energy of the products compares with the energy of the reactants. (2)
Show the model answer
Energy is transferred to the surroundings, so the reaction is exothermic (1). The products have less energy than the reactants, by the amount of energy transferred to the surroundings (1).
Describe a method (required practical)
4 to 6 marks6
Name the apparatus: measuring cylinder, polystyrene cup in a beaker, lid, thermometer, balance.
Give the steps in order with amounts: measure, record the starting temperature, add, stir, record the highest (or lowest) temperature.
Say how you change the independent variable and what you keep the same.
Finish with repeats and how you use the results (temperature change, mean).
Example. A student wants to find out how the temperature change depends on the metal used when magnesium, zinc and iron powders react with dilute hydrochloric acid. Describe a method the student could use. (6)
Show the model answer
Measure 25 cm³ of dilute hydrochloric acid with a measuring cylinder and pour it into a polystyrene cup standing in a beaker (1). Measure and record the starting temperature of the acid with a thermometer (1). Weigh 0.5 g of magnesium powder on a balance, add it to the acid, put on a lid and stir (1). Record the highest temperature reached and work out the temperature change (1). Repeat with 0.5 g of zinc powder and then 0.5 g of iron powder, using fresh acid of the same volume and concentration each time (1). Repeat each experiment and calculate a mean temperature change for each metal (1). This type of question is marked in levels: a complete, logically ordered method like this is Level 3 (5 to 6 marks).
Evaluate the use of a reaction in a product
3 to 4 marks6
Decide whether the product needs an exothermic reaction (to heat) or an endothermic reaction (to cool).
Use the data: temperature reached, cost, safety, and whether it can be reused.
Give points for and against, each linked to the data.
End with a conclusion that says which is best and why.
Example. A company is designing a self-heating can of soup. The reaction must heat the soup from 20 °C to at least 60 °C. The company tests three reactions. X: highest temperature 75 °C, cost per can 12p, product is a harmless solid Y: highest temperature 52 °C, cost per can 4p, product is a harmless solid Z: highest temperature 90 °C, cost per can 8p, gives off a toxic gas Evaluate which reaction the company should use. (4)
Show the model answer
Y is the cheapest, but it only reaches 52 °C, so it would not heat the soup to 60 °C (1). Z reaches the highest temperature and is cheaper than X, but it gives off a toxic gas, so it is not safe to use with food (1). X reaches 75 °C, which is above 60 °C, and its product is harmless (1). The company should use X, even though it costs the most, because it is the only reaction that is both hot enough and safe (1).
Shortcuts and memory tricks
EXothermic: energy EXits to the surroundings. ENdothermic: energy ENters from the surroundings.
Burning, neutralising and oxidising are exothermic. Splitting a compound up by heating it (thermal decomposition) is endothermic.
Products that warm you (hand warmers, self-heating cans) use exothermic reactions; products that cool you (sports injury packs) use endothermic reactions.
Where marks are lost
Saying an endothermic reaction 'gives out cold'. It takes in energy from the surroundings, which makes them colder.
Saying energy is 'made', 'used up' or 'destroyed'. Energy is transferred, never created or destroyed.
Mixing up the direction: in an endothermic reaction the thermometer reading falls, because energy is taken in from the solution around the reacting particles.
Giving a classification with no reason. When the question says 'give a reason', 'exothermic' alone scores only 1 of the 2 marks; add 'because the temperature increased'.
Leaving out control variables in a method: the volume and concentration of the solution, the mass of solid and the type of cup should all stay the same.
Exam technique
When you classify a reaction, always quote the data, e.g. 'exothermic, because the temperature increased by 8.5 °C'.
In method questions, give amounts and equipment (e.g. '25 cm³ of acid measured with a measuring cylinder') and keep the steps in a logical order.
Give temperature changes to the same precision as the data, with the unit °C, e.g. 27.5 − 19.0 = 8.5 °C.
In 'evaluate' questions, use numbers from the data for both sides and always finish with a justified conclusion.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write the ionic equation for the reaction. Include state symbols.
H+(aq) + OH−(aq) → H2O(l)
When a metal carbonate is heated strongly, it breaks down into a metal oxide and carbon dioxide. This type of reaction is always endothermic. Name this type of reaction.
Thermal decomposition
A student is investigating how the mass of magnesium ribbon added to dilute hydrochloric acid affects the temperature change of the reaction mixture. The student uses a polystyrene cup with a lid, and a thermometer. Write a hypothesis for this investigation.
The greater the mass of magnesium used, the bigger the temperature rise.
Two students each investigated the temperature change when the same mass of zinc powder was added to 25 cm3 of copper sulfate solution.
Student A repeated the experiment three times using the same apparatus and got temperature rises of 14.8 °C, 15.0 °C and 14.9 °C.
Student B, working in a different room with different apparatus, got a mean temperature rise of 14.7 °C.
What is meant by a result being reproducible?
A different person, or different equipment, gets very similar results.
A student investigates how the mass of sodium hydrogencarbonate added to citric acid solution affects the temperature change. The student:
measures 50 cm3 of citric acid solution into a polystyrene cup and records its temperature
adds a weighed mass of sodium hydrogencarbonate powder and stirs
records the lowest temperature reached.
The student repeats this with different masses of sodium hydrogencarbonate. What is the dependent variable in this investigation?
The temperature change (starting temperature − lowest temperature).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Some products use chemical reactions to heat things up or to cool things down.
(a) Which product uses an endothermic reaction? Tick (✓) one box.[1]
A hand warmer
A self-heating can of soup
A cold pack for a sports injury
A camping stove
(b) A self-heating can has a sealed compartment containing two chemicals. When the chemicals are mixed they react exothermically. The compartment is surrounded by the soup. Explain how the reaction heats the soup.[2]
(c) The chemicals in a cold pack are kept in two separate compartments until the pack is needed. Suggest why.[1]
(d) Energy is conserved in chemical reactions. What does this mean?[1]
Show the answer and mark scheme
(a)Answer: A cold pack for a sports injury
(b)Answer: Energy is transferred from the reacting chemicals to the soup, so the temperature of the soup increases.
energy is transferred from the reaction (mixture) to the soup / to the surroundings
so the temperature of the soup increases
(c)Answer: So that the reaction only starts when the pack is needed.
the reaction only starts when the chemicals are mixed / so the pack does not get cold before it is needed
(d)Answer: The total amount of energy is the same before and after the reaction.
the total amount of energy is the same before and after the reaction / energy cannot be created or destroyed
Question 2Medium7 marks
A student is investigating how the mass of magnesium ribbon added to dilute hydrochloric acid affects the temperature change of the reaction mixture. The student uses a polystyrene cup with a lid, and a thermometer.
(a) Write a hypothesis for this investigation.[1]
(b) Identify the independent variable and the dependent variable in this investigation.[2]
(c) Give two variables the student should control.[2]
(d) Explain why the cup used is made of polystyrene and has a lid.[2]
Show the answer and mark scheme
(a)Answer: The greater the mass of magnesium used, the bigger the temperature rise.
the greater the mass of magnesium (used), the bigger the temperature rise / increase
(b)Answer: Independent: mass of magnesium; dependent: temperature rise.
independent variable: mass of magnesium (ribbon)
dependent variable: the temperature rise (highest temperature reached minus starting temperature)
(c)Answer: Volume of acid; concentration of acid.
volume of hydrochloric acid
concentration of hydrochloric acid
starting temperature of the acid
form of the magnesium (e.g. ribbon rather than powder) / its surface area
same insulated cup and lid each time
(d)Answer: To reduce energy transfer to the surroundings, so the measured temperature change is closer to the true value.
to reduce energy transfer to the surroundings (from the reaction mixture)
so the measured temperature change is closer to the true (maximum) value
Question 3Hard6 marks
A student measured the temperature rise when a fixed mass of magnesium reacted with excess dilute hydrochloric acid in a polystyrene cup with no lid.
The student read the thermometer from a different position each time, sometimes looking from slightly above the liquid level and sometimes from slightly below it.
Energy was transferred from the reaction mixture to the surrounding air throughout the experiment.
(a) State whether reading the thermometer from different positions causes a random error or a systematic error. Explain your answer.[2]
(b) State whether energy transfer to the surrounding air causes a random error or a systematic error. Explain your answer.[2]
(c) Suggest one change to the apparatus that would reduce the systematic error.[1]
(d) Suggest one change to the method that would reduce the random error caused by reading the thermometer.[1]
Show the answer and mark scheme
(a)Answer: Random error: it makes readings vary unpredictably above and below the true value.
random error
because the readings vary unpredictably, sometimes above and sometimes below the true value (not consistently in one direction)
(b)Answer: Systematic error: it makes every temperature rise recorded lower than the true value, in the same direction each time.
systematic error
because it makes every temperature rise recorded lower than the true value (the error is always in the same direction)
(c)Answer: Put a lid on the cup (or use better insulation, e.g. stand the cup in a beaker packed with cotton wool).
put a lid on the cup / add more insulation (e.g. a second cup, or cotton wool around the cup)
(d)Answer: Always read the thermometer with the eye level with the top of the liquid.
always read the thermometer with the eye level with the liquid (straight on) / use a digital thermometer or temperature probe