Eduqas GCSE Chemistry · Topic 8 — Energy changes in chemistry
Mini-Lesson
Energy changes in chemistry
This mini-lesson walks you through the whole of Eduqas Topic 8 — Energy changes in chemistry: exothermic & endothermic reactions, how we measure temperature changes, reaction profiles with activation energy, and the Higher-tierbond-energy ΔH calculation.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Exothermic & endothermic
Energy out, or energy in?
Every reaction either releases energy to its surroundings or takes energy in from them. Eduqas asks you to describe this in terms of temperature change and energy transferred to or from the surroundings.
Exothermic — energy is transferred TO the surroundings, so the surroundings warm up (temperature rises). Examples: combustion, neutralisation, and displacement reactions of a metal with a salt solution.
Endothermic — energy is taken IN from the surroundings, so the surroundings cool down (temperature falls). Examples: thermal decomposition (e.g. heating calcium carbonate), and some dissolving (e.g. dissolving ammonium nitrate in water).
Watch out: "exo" means exit — energy exits to the surroundings, which therefore feel hotter. A common slip is to say an exothermic reaction "gets cold". It doesn't — the surroundings warm up because the chemicals push energy out into them.
Sort it
Exothermic or endothermic?
Tap a reaction, then tap the box it belongs in.
🔥 Exothermic (surroundings warm)
❄️ Endothermic (surroundings cool)
Measuring temperature changes
The calorimetry experiment
To find out whether a reaction is exo- or endothermic — and how much — we measure the temperature change of the mixture. This is the core required practical for this topic.
Record the start temperature, mix the reactants, then read the highest (or lowest) temperature. The change, Δθ, tells you the direction and size of the energy transfer.
Use an insulated container (a polystyrene cup) with a lid to reduce energy lost to the air.
A temperature rise ⇒ exothermic; a temperature fall ⇒ endothermic.
Typical investigations: neutralisation (acid + alkali), displacement (e.g. zinc added to copper(II) sulfate), and dissolving salts.
Quick check
Reading the thermometer
?A student adds zinc powder to copper(II) sulfate solution in a polystyrene cup. The temperature rises from 21 °C to 34 °C. What does this tell you?
Quick check
How big was the change?
?In the experiment above, the temperature went from 21 °C up to 34 °C. What is the temperature change, Δθ, in °C?
°C
Hint: Δθ = highest temperature − starting temperature.
Reaction profiles
Drawing the energy story
A reaction profile plots energy against the progress of the reaction. You must be able to show the relative energies of reactants and products, the activation energy, and the overall energy change.
Exothermic profile: products are lower than reactants, so energy is released. The hump is the activation energy Eₐ.Endothermic profile: products are higher than reactants, so energy is taken in. The activation energy is the climb to the peak.
The activation energy is the hump: Eₐ is the minimum energy the colliding particles need to start the reaction — it is the height from the reactant level up to the top of the curve, in BOTH exo- and endothermic reactions. It is not the overall energy change.
Quick check
Read the profile
?In this profile the products are higher in energy than the reactants. What type of reaction is shown?
Higher tier only
Energy stored in bonds
Where does the energy come from? It is all about breaking and making bonds:
Breaking bonds takes energy IN — it is endothermic.
Making bonds releases energy OUT — it is exothermic.
If making the new bonds releases more energy than breaking the old ones took in, the reaction is overall exothermic. If less, it is overall endothermic.
ΔH = (bonds broken) − (bonds made)
Add up every bond energy in the reactants (broken) and subtract every bond energy in the products (made). A negative ΔH = exothermic; a positive ΔH = endothermic.
Watch out: two classic traps. (1) Bond breaking is endothermic (energy in), bond making is exothermic (energy out) — students often swap these. (2) The order is broken minus made — do it the other way round and your sign flips.
Higher tier only
A worked ΔH calculation
For the reaction H–H + Cl–Cl → 2 H–Cl, the bond energies (kJ/mol) are:
Working
Bonds broken: one H–H + one Cl–Cl = 436 + 242 = 678 kJ/mol
Bonds made: two H–Cl = 2 × 432 = 864 kJ/mol
ΔH = broken − made = 678 − 864 = −186 kJ/mol
ΔH is negative, so the reaction is exothermic — more energy was released making bonds than was used breaking them.
Higher · calculation
Your turn: ΔH from bonds
?For H–H + Cl–Cl → 2 H–Cl, using H–H = 436, Cl–Cl = 242 and H–Cl = 432 kJ/mol, calculate ΔH in kJ/mol. (Include the minus sign.)
kJ/mol
Hint: broken = 436 + 242; made = 2 × 432; then ΔH = broken − made.
Higher · calculation
One more: is it exo or endo?
?A reaction breaks bonds totalling 2750 kJ/mol and makes bonds totalling 2600 kJ/mol. What is ΔH in kJ/mol? (Include the sign.)
kJ/mol
Hint: ΔH = bonds broken − bonds made = 2750 − 2600. A positive answer means endothermic.
Quick check
Breaking vs making
?Which statement about bonds is correct?
Quick check
What is activation energy?
?On a reaction profile, the activation energy is best described as…
Recap
Topic 8 in a nutshell
Exothermic: energy out → surroundings warm (combustion, neutralisation, displacement).
Endothermic: energy in → surroundings cool (thermal decomposition, some dissolving).
Measuring: use an insulated cup + thermometer; rise = exo, fall = endo.
Profiles: show reactant & product levels, activation energy (the hump) and the overall change.