Edexcel IGCSE Chemistry · Spec 3.1-3.4
Exothermic & Endothermic Reactions; Calorimetry
Exothermic and endothermic reactions, and measuring temperature change by calorimetry.
Chemistry revision video
Exothermic & Endothermic Reactions; Calorimetry
Prefer to watch on YouTube? Open this video on YouTube.
Explained
Which way the energy goes, and how to measure it
Every reaction transfers energy to or from its surroundings, and there are only two possibilities.
An exothermic reaction gives energy out to the surroundings, so the temperature of the surroundings rises. An endothermic reaction takes energy in from the surroundings, so the temperature falls.
The thermometer in the mixture is measuring the surroundings, not the chemicals. That is why an exothermic reaction shows a temperature rise: the energy has left the chemicals and gone into the solution around them.
Which is which
Exothermic: combustion, neutralisation, oxidation, respiration, and most displacement reactions. Anything that burns is exothermic.
Endothermic: thermal decomposition, photosynthesis, and the reaction between citric acid and sodium hydrogencarbonate, which is the standard demonstration because the beaker becomes cold enough to feel.
Bond breaking takes energy in and bond making gives energy out, so the direction overall depends on which total is larger. If more energy is released making the new bonds than was needed to break the old ones, the reaction is exothermic.
What the mark scheme accepts and rejects
An Edexcel International GCSE Chemistry mark scheme asks what exothermic means and credits that heat energy or thermal energy is given out, released or lost to the surroundings. Its note then adds a restriction: not just energy, it needs to be heat released or thermal energy released.
So the word energy on its own is not enough. Name the store, and say where it went.
The same mark scheme allows an alternative phrasing, that it gives out energy and raises the temperature of the surroundings, which earns the mark because the consequence identifies what kind of energy is meant.
On the calorimetry side, another mark scheme sets out a fair test as three pairs of points, each with a change and a reason: the same volume of water, so the same amount of heat is required; stir the water, so the temperature is uniform throughout; and keep the burner the same distance from the can, so the same amount of heat loss occurs. It ignores references to time.
Every pair is a change plus a reason. Listing three changes with no reasons earns half of what is available.
Calorimetry
To measure the energy released by a fuel, burn it under a metal can containing a known mass of water and record the temperature rise. To measure a reaction in solution, carry it out in a polystyrene cup and record the temperature change.
The energy transferred is mass times specific heat capacity times temperature change. Two conventions apply. The mass is the mass of the water or solution being heated, not the mass of the fuel or reactant, and 1 cm cubed of a dilute solution is taken as 1 g.
To get a value per mole, divide the energy by the number of moles of the substance used up, then divide by a thousand to convert joules to kilojoules. Finally give the sign: negative for exothermic, positive for endothermic.
Why measured values come out low
Energy escapes to the air, warms the apparatus itself rather than the water, and in a fuel experiment some of the fuel may burn incompletely.
All of these make the measured energy release smaller than the true value. Insulating the container, adding a lid and shielding the flame from draughts reduce the loss, though none of them removes it, and the mark schemes are strict about that distinction.
Spec 3.1-3.4
What you need to know
- Tell exothermic and endothermic reactions apart
- Give examples of each
- Describe how calorimetry measures an energy change
Active recall
Quick check
Answer each question before opening the answer.
What is the difference between exothermic and endothermic reactions?
Exothermic reactions release heat so the temperature rises (e.g. combustion, neutralisation); endothermic reactions take in heat so the temperature falls (e.g. thermal decomposition).
What does calorimetry measure?
The temperature change of a reaction, used to work out the energy released or absorbed.
Chemistry revision app
Take this topic further in the app
275 guided topics with questions marked as you answer them, section checkpoints and timed practice papers. Fifteen topics are free to try, with no card needed.