Cambridge IGCSE Chemistry 0620 · Topic 5 · Chemical energetics
Every reaction breaks bonds and makes bonds. Breaking takes energy in. Making gives energy out. Whichever wins decides if things get hotter or colder. Go through the tabs in order.
Every reaction, and every change like melting or boiling, either gives out heat or takes in heat. That's it.
EXO: the orange heat dots fly out of the beaker. The thermometer goes up. Your hand feels warm.
Heat exits, like people leaving through an exit door. Heat energy is released, so the surroundings get hotter.
Heat enters (andar). The reaction absorbs the heat, so the surroundings get colder.
Feels hot → EXO. Feels cold → ENDO. The thermometer says the same: up means exo, down means endo.
Read each one out. The class shouts EXO or ENDO, then tap the answer.
Once the class is sure about hot and cold, climb one step at a time.
In every chemical reaction, reactants → products. The bonds of the reactants are broken and new bonds between the products are formed. Press play and watch it happen.
Bond breaking of reactants: energy is ABSORBED → it is an ENDOTHERMIC process.
Bond formation of products: energy is RELEASED → it is an EXOTHERMIC process.
Two molecules: H–H and Cl–Cl. Each line is a bond.
You must pay energy to pull two atoms apart. Nothing breaks for free.
When atoms join, energy is given out. You get paid.
It is the energy needed to break 1 mole of bonds. Units: kJ/mol (kilojoule per mole).
The bigger the bond energy, the stronger the bond, and the more energy you must absorb to break it.
Pick a reaction. The class guesses first. Then add the reactants and watch the thermometer. Going up means exo. Going down means endo. Core 5.1.1–5.1.2
Pick a reaction
Reaction
Plants make their own food in the leaves, using carbon dioxide from the air and water from the soil, in the presence of sunlight and chlorophyll.
Why is it endothermic? Because sunlight (energy) is absorbed by the leaves of the plant.
The thermometer sits in the water around the reaction, so it measures the surroundings. Heat released makes it rise. Heat absorbed makes it fall.
Pick a process. Watch the particles. Do they move apart or come together?
Melting, boiling / vaporisation, sublimation
Freezing, condensation
| Process | Change | Particles | Heat energy | |
|---|---|---|---|---|
| Melting | solid → liquid | move apart | ABSORBED | ENDO |
| Boiling / vaporisation | liquid → gas (all through the liquid) | move far apart | ABSORBED | ENDO |
| Evaporation | liquid → gas (surface only) | move far apart | ABSORBED | ENDO |
| Sublimation | solid → gas (skips liquid) | move far apart | ABSORBED | ENDO |
| Freezing | liquid → solid | come together | RELEASED | EXO |
| Condensation | gas → liquid | come together | RELEASED | EXO |
In melting, boiling and sublimation the heat energy is absorbed during the process, which is why all three are endothermic.
The reactants come in carrying energy. The products leave carrying energy. The difference is what goes out to, or comes in from, the surroundings. Press play.
Press play to run the reaction.
ΔH is the overall heat of a reaction. Delta (Δ) means difference. Supplement 5.1.4–5.1.8
ΔH = [ energy absorbed by reactants in bond breaking ] + [ energy released by products in bond formation ]
The first bracket is always a +ve value (energy goes in). The second is always a −ve value (energy comes out). Add them, and the sign of the answer tells you the type.
Minus. More energy came out than went in, so the reaction overall is EXOTHERMIC.
Plus. More energy went in than came out, so the reaction overall is ENDOTHERMIC.
M3 is the mark most students miss. Saying only "the products have less energy" says nothing about bonds and scores nothing.
Same idea with real numbers. Add up what is absorbed breaking every bond, add up what is released forming every bond, then add the two with their signs.
Step 1 · Energy absorbed breaking the bonds of the reactants
Step 2 · Energy released forming the bonds of the products
Step 3 · Compare the two
Step 4 · ΔH with a sign and units
Typical values in kJ/mol. In the exam, always use the values printed in the question.
The same reaction, drawn as a hill. Press play to send the reaction over it. Core 5.1.3 Supplement 5.1.4–5.1.6
Ea is the energy absorbed by the reactants to convert into products — the push every reaction needs to start, like a match to light a fire. It is the height of the hill.
Exo: the reaction ends lower than it started, so ΔH is −ve. Endo: it ends higher, so ΔH is +ve.
In a catalysed reaction the activation energy is lowered, so the reaction is faster. But catalysts do not affect ΔH — the start and finish levels do not move. Use the "Show catalyst" button above to see the dashed lower hill.
The other graph they ask for. Press play to draw it.
The simple meaning is for understanding. In the exam, write the exact wording. These follow the 0620 syllabus.
A reaction that transfers thermal energy to the surroundings, leading to an increase in the temperature of the surroundings.
Simple meaning: heat energy is released, things around get hotter.
Examiner-safe · syllabus wordingA reaction that takes in thermal energy from the surroundings, leading to a decrease in the temperature of the surroundings.
Simple meaning: heat energy is absorbed, things around get colder.
Examiner-safe · syllabus wordingThe energy needed to break one mole of bonds. Units: kJ/mol.
Simple meaning: how much you must pay to break that bond.
Examiner-safeThe overall heat (thermal energy) change of a reaction. ΔH is negative for exothermic reactions and positive for endothermic reactions. Units: kJ/mol.
Simple meaning: delta means difference — how much heat went out (minus) or came in (plus).
Examiner-safe · syllabus wordingThe minimum energy that colliding particles must have to react. (Said the way it was taught: the energy absorbed by the reactants to convert into products.)
Simple meaning: the height of the hill, the starting push.
Examiner-safe · syllabus wordingBond breaking is an endothermic process. Bond making is an exothermic process.
Simple meaning: breaking absorbs energy, forming releases energy.
Examiner-safe · syllabus wordingQ. The combustion of methane is exothermic. Explain why, in terms of bond breaking and bond making. [3]
Show every line. If one total is wrong, later marks can still be earned by error carried forward — but only if the working is visible.
Start with the easy quiz. When the class gets most right, move to the exam quiz. Every answer shows the reason.