Matter and energy — exo/endo and energy profile diagrams
Every chemical reaction is also an energy story. Today you meet the two energy shapes a reaction can take — exothermic (energy released, surroundings warm) and endothermic (energy absorbed, surroundings cool) — and you turn that energy story into a picture: the energy profile diagram. By the end of the lesson you'll classify a reaction from a temperature change, sketch its profile, and label the activation energy and overall energy change.
Learning Intentions + Success Criteria
LITo classify reactions as exothermic or endothermic, and to draw and read an energy profile diagram.
SC: I can:
01I can explain the difference between exothermic (releases heat → surroundings warm) and endothermic (absorbs heat → surroundings cool).
02I can use the bond-breaking + bond-forming model to explain whether a reaction is exothermic or endothermic.
03I can label reactants, products, activation energy (Eₐ), and overall energy change on a profile diagram.
04I can sketch a profile that matches a given reaction and use the shape (products lower or higher) to classify it.
05I can give an everyday example of each (e.g. instant hot pack, instant cold pack; combustion, photosynthesis).
01
Engage
5 min
↺Quick recap · from last class
L17 · §4.6 Factors that affect rate of reactions — concentration and catalysts
Try these 3questions before today's new content. Click an answer for instant feedback — your teacher will walk through them with you.
▸Predict · your turn
Write before you watch
An instant hot pack gets WARMER when squeezed; an instant cold pack gets COLDER when squeezed. Both started at room temperature. Where does the heat come from for the hot pack, and where does it go for the cold pack?
02
Explicit
15 min
Today's procedure — classify a reaction as exo or endo
START — a reaction is happening
↓
1. Check the bond-energy balance
breaking reactant bonds → ABSORBS energy forming product bonds → RELEASES energy
↓
2. Or measure: did the surroundings warm or cool?
↓
3. Which way does energy flow OVERALL?
↓
↓ Energy OUT
EXOTHERMIC Released > Absorbed. Surroundings warm. ΔE negative. e.g. combustion, neutralisation, hot pack
Memory hook: exo = OUT (energy leaves), endo = IN (energy enters).
Chemical reactions involve energy changes
Every chemical reaction transfers energy between the reacting particles and their surroundings. The reason: making new substances always means breaking some bonds and forming others, and each of those steps moves energy in or out.
That gives every reaction one of two shapes:
Exothermic — energy is released (often as heat or light). The surroundings warm. Examples: combustion, neutralisation, many displacement reactions.
Endothermic — energy is absorbed from the surroundings. The surroundings cool. Examples: photosynthesis, some decomposition reactions.
Total energy is conserved either way; only how it is stored or transferred changes.
Figure 4.34 — Endothermic reactions absorb heat from their surroundings (which cool); exothermic reactions release heat to their surroundings (which warm).
Bond-breaking vs bond-forming — at a glance
Break bondsEnergy IN ↓
Always absorbs energy from somewhere. You have to pull bonded atoms apart, and that costs energy.
Picture: pulling two magnets apart — you have to put work in.
Make bondsEnergy OUT ↑
Always releases energy. New bonds are more stable than the loose atoms, and the leftover energy escapes (often as heat).
Picture: two magnets snapping together — they release a click of energy.
Instant cold and hot packs
First-aid packs use the same idea, run in opposite directions. The cold-pack example absorbs heat from the surroundings; the hot-pack example releases heat to the surroundings.
Step
Cold pack (endo)
Hot pack (exo)
Textbook example
ammonium nitrate dissolves in water
iron powder reacts with oxygen in air
Main change
NH₄NO₃ separates into NH₄⁺ and NO₃⁻ ions
iron and oxygen form iron(III) oxide
Net energy
energy is absorbed from surroundings
energy is released to surroundings
You feel
pack goes cold
pack goes warm
NH4NO3(s)H2O▶NH4+(aq)+NO3−(aq)
4Fe(s)+3O2(g)⟶2Fe2O3(s)
Energy profile diagrams — turning the energy story into a picture
So far we've classified reactions from their temperature change. The same story can be drawn as a picture: the energy profile diagram. It plots energy (vertical axis) against reaction progress (horizontal axis) and tells you three things at a glance — where the reactants sit, how big the activation-energy hump is, and where the products end up.
Exothermic. Products sit below reactants. The drop = energy released to surroundings. Overall energy change is negative.
Endothermic. Products sit above reactants. The rise = energy absorbed from surroundings. Overall energy change is positive.
In both cases the curve climbs first to the Eₐ peak (the "hump") before settling at the product level. The bigger the hump, the more energy is needed to start the reaction.
Figure 4.35 — Exothermic vs endothermic energy profiles
Exothermic energy profile (textbook Figure 4.35). The curve climbs from reactants up to a peak (the activation energy hump), then drops past the reactant level to a lower products level. The overall change in energy is negative — energy is released.
Endothermic energy profile (textbook Figure 4.35). The curve climbs from reactants to the peak then drops only partway, leaving the products at a higher energy level than the reactants. The overall change in energy is positive — energy is absorbed.
Figure 4.37 — Cold and hot pack profiles
Cold pack · ENDOTHERMIC
NH₄NO₃ (ammonium nitrate) dissolves: products higher than reactant — energy absorbed from surroundings (which cool).
Hot pack · EXOTHERMIC
Iron rusts: products lower than reactants — energy released to surroundings (which warm).
Core vocabulary for today
Keywords
exothermic
A reaction that releases energy (often as heat or light) to its surroundings; the surroundings warm.
endothermic
A reaction that absorbs energy from its surroundings; the surroundings cool.
surroundings
Everything outside the reaction itself — the solvent, beaker, air and your hand.
bond energy
The energy needed to break a chemical bond, or the energy released when one forms.
energy profile diagram
A diagram that shows how the energy of a reaction changes from reactants to products, including the activation-energy hump and the overall energy change.
⚠Watch out · common traps
1
Trap 1
“Bond-breaking releases energy.”
Wrong — bond-breaking always ABSORBS energy. You have to put energy in to pull bonded atoms apart.
It's the opposite step — bond-MAKING — that releases energy as new, more stable bonds form.
Why students get this wrong: in everyday language, "breaking" often sounds like releasing energy. In chemistry, the textbook rule is simpler: bonds need energy to break, and new bonds release energy when they form.
Rule: breaking bonds = energy IN; making bonds = energy OUT. The overall reaction is exo or endo depending on which side wins.
2
Trap 2
“An instant cold pack feels cold because the reaction releases cold.”
Wrong — there is no such thing as "cold". Cold isn't a substance that flows out; it's the absence of heat.
A cold pack feels cold because the endothermic reaction (NH₄NO₃ dissolving) absorbs heat from the surroundings — which includes the water in the pack and your hand. As heat flows OUT of your hand and INTO the reaction, your hand loses heat → it feels cold.
Rule: heat always flows from warmer to cooler. Cold packs cool by taking heat away, not by giving cold out.
3
Trap 3
“All chemical reactions release heat.”
Wrong — only exothermic reactions release heat. Endothermic reactions absorb heat, cooling their surroundings:
The textbook rule: every reaction's energy direction depends on the balance between energy absorbed to break bonds and energy released when new bonds form.