Measuring energy changes Edexcel International GCSE Chemistry revision
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In plain words
You can measure the heat from a reaction by letting it warm some water and watching a thermometer. The more water you heated and the hotter it got, the more energy was released. Dividing by the amount of chemical used gives the energy per mole.
Three things to know
- Heat energy change: Q = m × c × ΔT. m is the mass of water (or solution) in grams, c is its specific heat capacity (4.18 or 4.2 J/g °C for water) and ΔT is the temperature change.
- Molar enthalpy change: ΔH = Q ÷ moles. Give it in kJ/mol, with a minus sign if the temperature rose.
- Use a polystyrene cup and a lid to cut heat losses, which are the main source of error.
Worked example
Burning 0.02 mol of ethanol heats 100 g of water by 25 °C. Find the molar enthalpy change. (c = 4.2 J/g °C)
- Q = m × c × ΔT = 100 × 4.2 × 25 = 10 500 J = 10.5 kJ.
- ΔH = Q ÷ moles = 10.5 ÷ 0.02 = 525 kJ/mol.
- The water got hotter, so the reaction is exothermic: ΔH = −525 kJ/mol.
Tips and tricks
- m is the mass of the water being heated, not the mass of the fuel or solid.
- Change J into kJ (divide by 1000) before dividing by the moles.
It lands in your notebook with its questions as flashcards.
Measuring energy changes: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
In Q = mcΔT, what is m?
It is whatever is being warmed up.
100 g of water is heated by 5 °C. How much heat energy is that? (c = 4.2 J/g °C)
100 × 4.2 × 5.
Why is a polystyrene cup used rather than a glass beaker?
Losing heat to the surroundings makes the temperature change too small.
8.4 kJ of heat is released by 0.1 mol of a substance. What is the molar enthalpy change?
8.4 ÷ 0.1, with a minus sign because heat is given out.
What are the units of molar enthalpy change?
It is an amount of energy for each mole.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
3 questions, 7 marks. Write your answers on paper, then check them.
Measuring energy changes
Edexcel International GCSE Chemistry 4CH1 · 7 marks · papermunch.org
Name ______________________________ Date ______________
50 g of water is heated by 10 °C. Calculate the heat energy taken in by the water. (c = 4.2 J/g °C)[2]
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2100 J. 50 × 4.2 × 10.
A reaction of 0.05 mol of a substance releases 4.2 kJ of heat. Calculate the molar enthalpy change, with its sign.[3]
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−84 kJ/mol. 4.2 ÷ 0.05, and negative because heat is released.
In an experiment to measure the energy released by a burning fuel, the value found is much lower than the true value. Suggest two reasons.[2]
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Any two: heat is lost to the surrounding air; heat is used to warm the container; the fuel does not burn completely; some fuel evaporates.
Answers: Measuring energy changes
- 1. 2100 J. 50 × 4.2 × 10.
- 2. −84 kJ/mol. 4.2 ÷ 0.05, and negative because heat is released.
- 3. Any two: heat is lost to the surrounding air; heat is used to warm the container; the fuel does not burn completely; some fuel evaporates.