5.1.1Energy transfer in exothermic and endothermic reactions
AQA GCSE Chemistry (8462), Higher tier · Energy changes › Exothermic and endothermic reactions
Practise Energy transfer in 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.
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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