Edexcel IGCSE Chemistry · Spec 3.8
Energy Changes (Required Practical)
Required practical: investigating the temperature changes of reactions.
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Energy Changes (Required Practical)
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Explained
Measuring an energy change in a cup
The idea behind this practical is simple. If a reaction gives out energy, the mixture gets warmer. If it takes energy in, the mixture gets colder. Measure the temperature before and after and you know which it was and roughly how much.
The method
Measure a known volume of one solution into a polystyrene cup and record its starting temperature. Add a known volume of the second solution, stir, and record the highest or lowest temperature reached.
The temperature change is the final reading minus the initial one. A rise means the reaction was exothermic and a fall means it was endothermic.
The polystyrene cup is not a detail. Polystyrene is a poor conductor, so it reduces the energy transferred between the mixture and the room, which would otherwise distort the reading. A lid does the same job for the energy that would escape from the surface, and also stops any splashing.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks how a method could be improved. It awards the first mark for using a polystyrene cup instead of a beaker, or for putting a lid on the beaker, and the second for saying that less heat is lost. Then it adds one word of instruction: reject prevent heat loss.
That distinction is worth more than it looks. Insulation reduces the energy escaping; it never stops it. Writing prevents overstates what the apparatus does, and the mark is withheld for it. Write reduces or less is lost.
The same mark scheme, on a burning fuel version of the experiment, credits keeping the spirit burner the same distance from the copper can, followed by the reason that the same amount of heat loss occurs. The reason is a separate mark from the change, which is the usual pattern in improvement questions.
On the calculation it shows its working in stages: the temperature change, then energy equals mass times specific heat capacity times temperature change. A correct answer without working still scores, and an error carried forward from a wrong temperature change is allowed to earn the later marks.
The calculation
Energy transferred equals mass multiplied by specific heat capacity multiplied by temperature change.
Two conventions catch people out. The mass is the mass of the solution being heated, not the mass of the reactant, and since these solutions are mostly water you take 1 cm cubed as 1 g. The specific heat capacity used is that of water, 4.2 joules per gram per degree Celsius, for the same reason.
Divide the energy by the number of moles of the reactant that was used up to get a value per mole, and convert joules to kilojoules by dividing by a thousand. Then give the sign: negative for exothermic, positive for endothermic.
Sources of error
Energy is lost to the surroundings and to the cup and thermometer themselves, so a measured value is usually smaller than the true one. The reaction may not be complete when you take the reading. And a thermometer reading to the nearest degree limits how precisely a small change can be known.
Improvements follow directly from those: insulate better, add a lid, take readings continuously and extrapolate back, and use a thermometer with finer divisions or a temperature probe.
Spec 3.8
What you need to know
- Describe an experiment to measure a temperature change
- Carry out the steps
- Decide whether a reaction is exothermic or endothermic
Active recall
Quick check
Answer each question before opening the answer.
How do you measure the temperature change of a reaction in solution?
Carry it out in an insulated cup with a lid, and record the starting and highest/lowest temperature.
What is the main source of error in this practical?
Heat lost to (or gained from) the surroundings.
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