19P2
Fri 22 May
L19
CAT revision
Week 5 · Lesson 16 of 17

Chapter 4 CAT revision (with energy profile reference)

Today is the revision period before the Chapter 3-4 CAT next period. Your teacher will walk through the printed cheat sheet — the §4.4 acid reactions panel (heaviest), collision theory, energy profiles, and the rate graph. This page keeps the full energy profile diagrams content as a quick reference.
Learning Intentions + Success Criteria

LITo revise the §4.2-§4.7 high-yield CAT patterns, and to draw and interpret energy profile diagrams.

SC: I can:

  1. 01I can read the cheat sheet's §4.4 panel: pH scale, indicator colours, and the four acid reactions (acid + base, metal, carbonate, oxide).
  2. 02I can label reactants, products, activation energy (Ea), and overall energy change on a profile diagram.
  3. 03I can sketch a profile that matches a given reaction (exothermic or endothermic).
  4. 04I can use the shape (products lower or higher than reactants) to classify a reaction.
01

Engage

5 min
Quick recap · from last class
L18 · §4.7 Matter and energy — exo/endo and energy profile diagrams

Try these 2questions 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

If you had to draw a picture showing how the energy of a reaction changes from start to finish, what shape would it have? A flat line? A hill? A slide? Sketch a quick guess on paper before reading on.

Once you've sketched your guess, watch this short FuseSchool clip — it connects the earlier collision-theory work to activation energy, the "bump" you'll see in every energy profile diagram today.

YouTube · Collision theory & reactions — Part 2 · open in new tab
02

Explicit

17 min
Today's procedure — drawing an energy profile diagram
START — you are given a reaction; draw its energy profile
1. Plot REACTANTS on the left (flat horizontal line)
y-axis = energy; x-axis = reaction progress
2. Add the activation-energy (Eₐ) HUMP — curve rises from reactants up to a peak
tall hump = slow reaction; short hump = fast
3. Is the reaction exothermic or endothermic?
↓ Exothermic
Products LOWER than reactants.
Curve drops past the reactant line down to a lower products line.
ΔE negative (energy released).
e.g. hot pack: 4 Fe + 3 O₂ → 2 Fe₂O₃ (iron + oxygen → iron(III) oxide)
↓ Endothermic
Products HIGHER than reactants.
Curve drops only partway from the peak; products line sits above the reactants line.
ΔE positive (energy absorbed).
e.g. cold pack: NH₄NO₃ (ammonium nitrate) dissolving
Catalyst rule (recap from §4.6): the hump gets LOWER but reactants/products lines stay where they were — faster, not more exothermic.

Energy profile diagrams — exo vs endo

An energy profile diagram plots energy (vertical axis) against reaction progress (horizontal axis). It 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 below reactants. The drop = energy released to surroundings. Overall energy change is negative.
  • Endothermic. Products 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

Reaction progress →Energy (E) →reactantsproductsEₐOverall change negative (energy released)

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.

Reaction progress →Energy (E) →reactantsproductsEₐOverall change positive (energy absorbed)

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 — Energy profiles for cold and hot packs (textbook examples)

Cold pack · ENDOTHERMIC

Energy →NH₄NO₃(s)NH₄⁺ + NO₃⁻ (aq)ΔE +REACTION PROGRESS →

NH₄NO₃ (ammonium nitrate) dissolves: products higher than reactant — energy absorbed from surroundings (which cool).

Hot pack · EXOTHERMIC

Energy →4Fe + 3O₂2Fe₂O₃ΔE −REACTION PROGRESS →

Iron rusts: products lower than reactants — energy released to surroundings (which warm).

Two real first-aid examples, side by side:

Cold pack (endo)Hot pack (exo, Fe version)
Reactant(s)NH₄NO₃(s)4 Fe(s) + 3 O₂(g)
Product(s)NH₄⁺(aq) + NO₃⁻(aq)2 Fe₂O₃(s)
Profile shapeproducts above reactantproduct below reactants
Energy flowabsorbed from surroundingsreleased to surroundings

Key terms

Keywords

energy profile diagram
A diagram that shows energy changes as a reaction progresses from the reactants to products, including activation energy and whether energy is released or absorbed.
Watch out · common traps
Trap 1
“If the products are higher than the reactants, the reaction can't happen.”

Wrong — endothermic reactions still happen; they just need energy supplied from the surroundings.

Textbook examples include photosynthesis and instant cold packs. They can happen because energy is supplied from the surroundings — sunlight for photosynthesis, or heat from the surroundings for a cold pack.

Rule: endothermic = absorbs energy (light, heat, electricity). The reaction proceeds whenever that energy supply is available.

Trap 2
“Activation energy is the overall energy of the reaction.”

Wrong — Eₐ and ΔE are two different distances on the same diagram:

  • Eₐ = the height of the hump (reactant level → peak). The minimum energy needed to start the reaction.
  • ΔE = the overall change (reactant level → product level). The energy released (exo, ΔE < 0) or absorbed (endo, ΔE > 0).

Two reactions can share Eₐ but have very different ΔE values. Don't confuse the climb with the final drop.

Rule: Eₐ = climb the hill; ΔE = where you end up.

Trap 3
“A catalyst makes the reaction more exothermic.”

Wrong — catalysts only lower the hump (Eₐ). They do not change where the reactants or products sit on the energy axis, so ΔE is unchanged.

The reaction releases (or absorbs) the same total energy as before — it just gets there sooner because more collisions clear the smaller barrier.

Rule: catalyst → faster, same ΔE. It changes the rate, not the energetics.

03

Apply

25 min
Question 1Identify the energy profile
Question 2Label the diagram

Fill in the blanks to label the parts of an energy profile diagram.

Question 3Sketch the cold-pack energy profile
Your turnShort answer · Have a go first
On paper, sketch and FULLY LABEL an energy profile diagram for the dissolution of ammonium nitrate in water (the instant cold-pack reaction). Label both axes, the reactant level (NH₄NO₃), the activation-energy hump (Eₐ), and the product level (NH₄⁺ + NO₃⁻). Mark whether the overall energy change is positive or negative, and justify why.
Question 4Match the feature to its meaning
04

Catch

5 min
05

Reflect

10 min
Your turnReflect · One thing you learned

One thing I now understand about energy profile diagrams that I didn't understand at the start of the lesson:

Success criteria — where are you right now?

Next class (Fri 22 May, P3): Chapters 3-4 CAT — 60 minutes, 31 marks.