Where does the energy in fuels come from, and how do we measure it?
Energy from fuels: fossil fuels as finite resources, complete and incomplete combustion, exothermic reactions, and calculating the energy released using Eh equals cmDeltaT.
An SQA National 5 Chemistry answer on energy from fuels, covering fossil fuels as finite resources, complete and incomplete combustion, exothermic reactions, and calculating the energy released by a burning fuel using Eh equals cmDeltaT.
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What this key area is asking
The SQA wants you to describe fossil fuels as finite resources, explain complete and incomplete combustion, recognise combustion as an exothermic reaction, and calculate the energy released by a burning fuel using . This key area combines knowledge of fuels with a calculation that uses the data booklet.
Fossil fuels are finite
Combustion is exothermic
Complete and incomplete combustion
Calculating the energy released
The energy a burning fuel transfers to water is found by measuring the temperature rise of the water and using the energy relationship:
The mass of water must be in kilograms because the specific heat capacity is per kilogram.
Worked example: energy from a burning fuel
Examples in context
This calculation is exactly what happens in a spirit-burner experiment, where students compare fuels by measuring how much each warms a fixed mass of water. In real life, the same chemistry powers central heating boilers and car engines, where complete combustion is wanted for maximum energy and minimum carbon monoxide. The finite nature of fossil fuels, and the carbon dioxide their combustion releases, is the driving force behind renewable fuels such as bioethanol made by fermenting plant sugars.
Try this
Q1. Name the two products of the complete combustion of a hydrocarbon. [1 mark]
- Cue. Carbon dioxide and water.
Q2. A fuel heats of water by . Calculate the energy released (). [2 marks]
- Cue. .
Q3. State one reason fossil fuels are described as finite. [1 mark]
- Cue. They form over millions of years but are used far faster, so they will run out.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2018 style3 marksA burning fuel raised the temperature of 200 g of water by 25 degrees C. Using the specific heat capacity of water as 4.18 kJ per kg per degree C, calculate the energy released, in kJ. (Take the mass of water as 0.2 kg.)Show worked answer →
Markers reward the correct relationship, the substitution, and the answer with units.
Use the energy relationship for heating water:
Substitute the specific heat capacity, the mass in kg, and the temperature change:
The energy released by the fuel is 20.9 kJ. A common error is leaving the mass of water in grams; the specific heat capacity here is per kilogram, so the mass must be in kg.
SQA N5 2020 style3 marksExplain why the combustion of a fuel is described as an exothermic reaction, name the two products of the complete combustion of a hydrocarbon, and state one problem caused by incomplete combustion.Show worked answer →
A 3 mark answer needs the exothermic point, the two products, and one problem.
Combustion is exothermic because it releases energy to the surroundings, usually as heat (and light). The temperature of the surroundings rises.
The complete combustion of a hydrocarbon, in plenty of oxygen, produces carbon dioxide and water.
Incomplete combustion, where there is too little oxygen, produces the poisonous gas carbon monoxide and sometimes soot (carbon). Carbon monoxide is dangerous because it is toxic, which is a valid problem to state.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)