AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Endothermic reactions include thermal decompositions and the reaction of…
Full specification wording
Energy transfer during exothermic and endothermic reactions: Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate.
Thermal decomposition means breaking a compound down by heating, for example heating calcium carbonate to form calcium oxide and carbon dioxide; energy must be taken in continuously, so it is…
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 8 of 12
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Thermal decomposition means breaking a compound down by heating, for example heating calcium carbonate to form calcium oxide and carbon dioxide; energy must be taken in continuously, so it is endothermic. The reaction of citric acid with sodium hydrogencarbonate is also endothermic: when the two solids or solutions mix, they fizz to produce carbon dioxide and the mixture cools. In both cases energy is taken in from the surroundings, so the temperature of the surroundings falls. These examples show that endothermic reactions are not limited to one type of chemistry; they include decomposition driven by heating and a reaction between an acid and a hydrogencarbonate.
What earns the marks
- Thermal decomposition is an endothermic process in which a compound is broken down by heating.
- A named thermal decomposition, such as calcium carbonate forming calcium oxide and carbon dioxide, releases carbon dioxide and requires energy input.
- The reaction of citric acid with sodium hydrogencarbonate is endothermic and produces carbon dioxide gas.
- In both examples, energy is taken in from the surroundings, so the temperature of the surroundings decreases.
- The citric acid and sodium hydrogencarbonate reaction can be observed as fizzing and a temperature fall.
Common mistakes and how to put them right
Calling thermal decomposition exothermic because heat is applied.
Put it right: Energy is taken in overall, so it is endothermic.
Writing that the citric acid and sodium hydrogencarbonate reaction warms the mixture.
Put it right: The mixture cools as energy is taken in.
Confusing the gas produced with the energy change.
Put it right: Carbon dioxide production is a chemical observation, while the temperature fall shows the endothermic energy change.
Exam tips
- Name the products in a thermal decomposition, for example calcium oxide and carbon dioxide, to show the reaction clearly.
- For citric acid and sodium hydrogencarbonate, state both observations: fizzing and a decrease in temperature.
- Use the phrase energy taken in from the surroundings to explain why these reactions are endothermic.
Try the lesson
The start of the Energy transfer during exothermic and endothermic reactions: Endothermic reactions include thermal decompositions and the reaction of citric acid and sodium hydrogencarbonate. lesson, made from checked teaching notes for this specification point.
Have a go first
Which of these is an example of an endothermic reaction?
Listen
Thermal decomposition means breaking a compound down by heating, for example heating calcium carbonate to form calcium oxide and carbon dioxide; energy must be taken in continuously, so it is endothermic. The reaction of citric acid with sodium hydrogencarbonate is also endothermic: when the two solids or solutions mix, they fizz to produce carbon dioxide and the mixture cools. In both cases energy is taken in from the surroundings, so the temperature of the surroundings falls. These examples show that endothermic reactions are not limited to one type of chemistry; they include decomposition driven by heating and a reaction between an acid and a hydrogencarbonate. A strong answer earns marks for these points. Thermal decomposition is an endothermic process in which a compound is broken down by heating. A named thermal decomposition, such as calcium carbonate forming calcium oxide and carbon dioxide, releases carbon dioxide and requires energy input. The reaction of citric acid with sodium hydrogencarbonate is endothermic and produces carbon dioxide gas. In both examples, energy is taken in from the surroundings, so the temperature of the surroundings decreases. A common slip: Calling thermal decomposition exothermic because heat is applied. Instead, energy is taken in overall, so it is endothermic. Exam tip: Name the products in a thermal decomposition, for example calcium oxide and carbon dioxide, to show the reaction clearly.
See it
Key words
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Thermal decomposition
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Temperature decrease as evidence
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Energy input required for decomposition
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See every definition
- Thermal decomposition
- Breaking a compound down by heating, for example heating calcium carbonate to form calcium oxide and carbon dioxide.
- Temperature decrease as evidence
- Energy is taken in from the surroundings, so the temperature of the surroundings falls.
- Energy input required for decomposition
- Energy must be taken in continuously, so thermal decomposition is endothermic.
Label the diagram
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1Box 1
2Box 2
3Box 3
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