Edexcel IGCSE Chemistry · Spec 3.2-3.4, 3.8
Calorimetry and Molar Enthalpy Calculations
Covers collect reliable calorimetry data, Calculate Q using mc delta T and Calculate molar enthalpy change with the correct sign.
Chemistry revision video
Calorimetry and Molar Enthalpy Calculations
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Explained
From a temperature rise to a value per mole
Calorimetry turns a temperature change into an energy value. The whole calculation runs in four steps, and doing them in order is what keeps it straightforward.
Collecting the data
For a reaction in solution, use an insulated cup with a lid, record the starting temperature, mix, stir, and record the highest or lowest temperature reached.
Recording at regular intervals is better than taking one final reading, because you can plot the results and extrapolate back to the moment of mixing. That gives the temperature change that would have occurred with no energy lost to the surroundings.
For a burning fuel, weigh the burner before and after, so the difference is the mass of fuel used, and heat a known mass of water in a metal can.
The four steps
- Find the temperature change, the final temperature minus the initial one.
- Find the energy transferred: Q equals mass times specific heat capacity times temperature change.
- Find the number of moles of the substance that reacted or burned.
- Divide the energy by the moles, then divide by a thousand to convert joules to kilojoules per mole.
Finally, give the sign. Negative for exothermic, positive for endothermic. The sign is part of the answer, not decoration, and leaving it off on an exothermic value is a common way to lose the last mark.
The two masses
Step two uses the mass of the water or solution being heated. Step three uses the amount of the reactant or fuel. Mixing them up is the single most common error in this calculation.
For solutions, take 1 cm cubed as 1 g and use the specific heat capacity of water, 4.2 joules per gram per degree, because the solution is mostly water.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme works through exactly this calculation for burning ethanol, and awards five marks for five steps: the mass of ethanol used from the difference in the burner masses, the amount in moles by dividing by the relative formula mass, the energy divided by the moles to give joules per mole, the conversion to kilojoules per mole, and finally the negative sign.
Its notes then say that a correct answer without working scores all five, and that an error carried forward is allowed throughout. So a mistake at step one does not have to cost the other four marks, provided every step is visible.
On the earlier part it shows the same generosity, awarding both marks for a correct energy value without working, and one mark where an incorrect temperature change was used correctly in the equation.
The lesson is the same one these papers repeat everywhere: write the whole chain down. The examiners are crediting the method separately from the arithmetic, and an unwritten method cannot be credited.
Why the answer is usually too small
Energy escapes to the surroundings, warms the container and the thermometer rather than the water, and in a fuel experiment some fuel burns incompletely and some of the flame's energy misses the can entirely.
All of these mean less energy reached the water than the fuel actually released, so the calculated value is smaller in magnitude than the true one. If asked to compare your value with a data book figure, that is the explanation, and naming two specific losses is worth more than saying energy was lost.
Improvements follow from the losses: a lid, insulation around the can, a shield against draughts, and moving the flame closer to the can. Each reduces one identified loss, and mark schemes want the change and the reason together.
Spec 3.2-3.4, 3.8
What you need to know
- Collect reliable calorimetry data
- Calculate Q using mc delta T
- Calculate molar enthalpy change with the correct sign
Active recall
Quick check
Answer each question before opening the answer.
Write the calorimetry equation
Q = mc delta T
Why is an insulated cup used?
To reduce heat transfer to the surroundings
What sign does an exothermic delta H have?
Negative
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