5.5.1.1Energy transfer during exothermic and endothermic reactions
AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Energy changes › Exothermic and endothermic reactions
Practise Energy transfer during exothermic and endothermic reactions. 12 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.
Key facts
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
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
diagram
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.
The cup insulates, the lid cuts energy transfer to the air and the beaker stops the cup falling over.
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.
How to answer each type of question
Identify exothermic or endothermic from data
1 to 2 marksGrade 4
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.
Reaction
Reactants
Starting temperature in °C
Final temperature in °C
P
iron filings + copper sulfate solution
19.0
27.5
Q
citric acid + sodium hydrogencarbonate solution
20.5
16.0
R
nitric acid + potassium hydroxide solution
19.5
31.0
(a) Which reaction is endothermic? Give a reason for your answer. (2) (b) Calculate the temperature change in reaction R. (1)Show the model answerHide 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 marksGrade 5
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 answerHide 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).
Don’t lose marks
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.
More tips
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.
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.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
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.
Grade 3
Give examples of exothermic reactionsCombustion, many oxidation reactions and neutralisation are all exothermic.
Grade 4
Give examples of endothermic reactionsThermal decomposition and the reaction of citric acid with sodium hydrogencarbonate are endothermic.
Grade 4
Classify a reaction from temperature dataA rise in temperature means the reaction is exothermic; a fall means it is endothermic.
Grade 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.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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