AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The heat change, q, in a reaction is given by the equation q = mc∆T where…
Full specification wording
Calorimetry: The heat change, q, in a reaction is given by the equation q = mc∆T where m is the mass of the substance that has a temperature change ∆T and a specific heat capacity c.
In a calorimetry experiment the reaction does not cool or warm itself: it warms or cools the solution around it, and q = mc∆T measures the heat gained or lost by that solution.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a calorimetry experiment the reaction does not cool or warm itself: it warms or cools the solution around it, and q = mc∆T measures the heat gained or lost by that solution. The m is the mass in grams of the liquid whose temperature you measured, usually taken as the volume in cm³ because the solution is assumed to have the density of water, 1.00 g cm⁻³. The c is the specific heat capacity, the energy needed to raise one gram by one kelvin, 4.18 J g⁻¹ K⁻¹ for water. Because a kelvin and a degree Celsius are the same size, ∆T can be read straight off the thermometer. The answer comes out in joules. A temperature rise means the surroundings gained heat, so the reaction released it and ∆H is negative.
What earns the marks
- Using the mass of the solution, not the mass of the solid added, as m in q = mc∆T.
- Correct substitution into q = mc∆T with matching units, giving q in joules.
- Converting joules to kilojoules before dividing by the number of moles.
- Assigning the correct sign, negative ∆H for a temperature rise and positive ∆H for a temperature fall.
Common mistakes and how to put them right
Using the mass of the dissolved solid or of the metal rather than the mass of the water that changed temperature.
Treating 25.0 cm³ of solution as 25.0 kg, or leaving the answer in joules and calling it kJ mol⁻¹.
Converting ∆T from °C to K by adding 273, when only the difference is needed and the size of the unit is identical.
Forgetting the minus sign when the temperature rose, so an exothermic reaction is reported as endothermic.
Using the specific heat capacity of the metal or solid when the temperature measured was that of the water.
Exam tips
- Write q = mc∆T, then label m, c and ∆T with the number and unit you are putting in, before you press a key.
- Divide by 1000 once and say so in your working, so an examiner can follow the joule to kilojoule step.
- If a question asks how to reduce the percentage uncertainty in ∆T, increase the temperature change, for example by using a larger mass of solid in the same volume of water.
Try the lesson
The start of the Calorimetry: The heat change, q, in a reaction is given by the equation q = mc∆T where m is the mass of the substance that has a temperature change ∆T and a specific heat capacity c. lesson, made from checked teaching notes for this specification point.
Have a go first
In the equation q = mc∆T, which quantity is represented by the symbol c?
Listen
In a calorimetry experiment the reaction does not cool or warm itself: it warms or cools the solution around it, and q = mc∆T measures the heat gained or lost by that solution. The m is the mass in grams of the liquid whose temperature you measured, usually taken as the volume in cm³ because the solution is assumed to have the density of water, 1.00 g cm⁻³. The c is the specific heat capacity, the energy needed to raise one gram by one kelvin, 4.18 J g⁻¹ K⁻¹ for water. Because a kelvin and a degree Celsius are the same size, ∆T can be read straight off the thermometer. The answer comes out in joules. A temperature rise means the surroundings gained heat, so the reaction released it and ∆H is negative. A strong answer earns marks for these points. using the mass of the solution, not the mass of the solid added, as m in q = mc∆T. correct substitution into q = mc∆T with matching units, giving q in joules. converting joules to kilojoules before dividing by the number of moles. assigning the correct sign, negative ∆H for a temperature rise and positive ∆H for a temperature fall. Exam tip: Write q = mc∆T, then label m, c and ∆T with the number and unit you are putting in, before you press a key.
Key words
Say what each word means before you turn the card.
heat change q
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mass of solution
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specific heat capacity
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temperature change
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See every definition
- heat change q
- Given by the equation q = mc∆T, where m is the mass of the substance that has a temperature change ∆T and c is its specific heat capacity.
- mass of solution
- The m in q = mc∆T is the mass in grams of the liquid whose temperature you measured, not the mass of the solid added.
- specific heat capacity
- The energy needed to raise one gram by one kelvin, 4.18 J g⁻¹ K⁻¹ for water.
- temperature change
- ∆T, which can be read straight off the thermometer because a kelvin and a degree Celsius are the same size.
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