Lesson 1- Endothermic and Exothermic Reactions
When I begin the Energy Changes topic, I like to start with something familiar: chemical reactions. Students already know that reactants turn into products, but before introducing any new terminology, I remind them that something important happens in between — bonds have to be broken and new ones formed..I’ll write a simple reaction on the board, something like:
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Then I ask:
“What has to happen to the magnesium atoms before they can become part of magnesium chloride?”
“What happens to the H–Cl bonds in the acid?”
With a bit of prompting, students remember that breaking bonds is part of the reaction process. That leads naturally into the first two key questions of the lesson:
Does bond breaking need energy or release energy?
Does bond making need energy or release energy?
Most students aren’t sure at first, so I like to get a few guesses. Then we discuss that breaking bonds always requires energy — you’re pulling particles apart that are attracted to each other. Making new bonds releases energy, because the atoms are moving to a more stable arrangement and energy is given out in the process.I’ll often act it out: two students holding hands tightly represent a chemical bond. To “break” the bond, someone has to use energy to pull them apart. When they form a new bond with someone else, energy is released. The visual helps the concept stick immediately. Following this we move onto a simple practical.
I set up four reactions for students to investigate — two exothermic and two endothermic:
Exothermic reactions:
Sodium hydroxide and hydrochloric acid (neutralisation)
Magnesium and hydrochloric acid (metal + acid reaction)
Endothermic reactions:
Ammonium nitrate and water (dissolving)
Citric acid and sodium bicarbonate (fizzing reaction)
Each group gets polystyrene cups, thermometers, and a results table. I remind them to measure the starting and final temperature.
Then we collect results, students see that some reactions cause a temperature rise, and others a fall. We discuss what this means in terms of bond breaking and making:
Exothermic: More energy released making new bonds than absorbed breaking old ones → surroundings get warmer.
Endothermic: More energy absorbed breaking bonds than released when new ones form → surroundings get cooler.
This is also the point where the big misconception comes up. Many students think that if a reaction releases energy, the temperature should drop, because “energy has left the chemicals.” It helps to clarify that we always measure the temperature of the surroundings, not the reaction itself. When energy is released, it goes into the surroundings, so the thermometer reading goes up. When energy is absorbed, it’s taken from the surroundings, so the thermometer reading goes down.
I like to sketch two quick energy diagrams to end the lesson — not with numbers, just a simple visual.
For exothermic, the products end up at a lower energy level than the reactants, with an arrow showing energy being released.
For endothermic, the products are higher, with an arrow showing energy being taken in.
We finish by returning to those first two questions:
“Which process needs energy?”
“Which process releases energy?”
By now, they can answer confidently — bond breaking needs energy, bond making releases energy — and they can explain how the balance between the two decides whether a reaction is endothermic or exothermic.
That’s where I end the first lesson: with students seeing energy change not as “hot” or “cold,” but as a story of bonds breaking, bonds forming, and energy moving in or out.
Lesson 2 Reaction Profiles
We start by reminding ourselves of what we saw: some groups’ cups warmed up, some cooled down. I point out that those temperature changes told us about energy moving into or out of the surroundings, and that now we’re going to look inside the reaction using a different kind of picture — the reaction profile.
I draw the axes slowly, narrating every choice as I go. “Horizontal axis: this shows the progress of the reaction — time is one way to think about it, but really it’s just ‘where we are between reactants and products’. Vertical axis: this represents the energy of the system — not the thermometer reading, but the internal energy of the chemicals involved.” I pause and ask them to repeat that last bit: the vertical axis is the chemicals’ energy, while the thermometer measures the surroundings.
Step 1 — the starting point: “Here are the reactants.” I draw a dot and label it ‘reactants’. “This is their energy level.” I ask: “Would you expect this point to be high or low? Think about the bonds we broke in Lesson 1.” A few students suggest reasons; we reinforce that the height is a way to compare stored chemical energy qualitatively.
Step 2 — the bump: I sketch a hill between reactants and products and label the top ‘activation energy’. I explain it as a barrier — “to get from reactants to products, bonds must be broken and rearranged; that needs an initial input of energy.” I use a classroom analogy: pushing a bike over a hump — you have to give it a push even if it will roll down the other side. We make the important point that activation energy is needed even for exothermic reactions.
Step 3 — the end point: I draw the products point and ask the class to compare its height to the reactants. “If the products end up lower than reactants, energy has been released overall; if they’re higher, energy has been absorbed overall.” I label the vertical difference ΔH (but I don’t dive into signs and numbers yet). We practice: “Which would be exothermic? Which endothermic?” Students point and we name them.
Step 4 — arrows and interpretation: I draw a big arrow from the top of the reactants down to the products for the exothermic case and say, “This arrow isn’t the thermometer — it shows that the chemicals have lost energy overall. Where did that energy go?” A chorus answers: “The surroundings.” I draw an arrow pointing from the reaction arrow to a little thermometer icon, showing the energy flowing to the surroundings. I repeat the picture for an endothermic profile: products higher than reactants, an arrow from surroundings into the reaction, and the thermometer cooling down. This is where I explicitly tie back to the common misconception: “Energy released by the chemicals makes the surroundings warmer — the chemicals don’t cool the surroundings by losing energy to become colder; they transfer energy into the room.”
Step 5 — activation energy revisited: We revisit the hill and I ask why some reactions never seem to start on their own in the lab. Students suggest ideas — small collisions, low temperature. I show how a catalyst would lower the hump and draw a smaller hill to illustrate faster reaction without changing ΔH. We link that to real examples (spark for combustion, enzyme in biology if they’re ready).
Step 6 — reading profiles as evidence: I give students three printed profiles (simple sketches) and three short experimental summaries from last lesson (e.g., “temperature rose by 5 °C”, “temperature dropped by 3 °C”, “no observable temperature change but bubbles released”). Working in pairs they match profile to experiment and annotate: mark activation energy, mark ΔH and write a sentence explaining what the surroundings would feel like. I circulate and listen for language: “energy absorbed to break bonds,” “energy released making bonds,” “surroundings warmed.” This activity cements reading the graph and linking it to what they observed.
Along the way I drop in quick checks: “If ΔH is negative, what will the surroundings do?” “Does activation energy tell you whether a reaction is endo or exo?” (no — it only controls how easily the reaction starts). These help correct slip-ups and the persistent backwards-thinking that I watch for.
To push higher students, I sketch a combined profile showing a reaction with a very large activation energy but exothermic ΔH and ask them why it’s slow at room temperature but fast with heat or a catalyst. For foundation groups, I keep to clear, bold sketches and make them annotate with words rather than symbols.
We finish by returning to the core story: they saw temperature changes, we drew profiles to “peer inside” the reactions, and now they can explain why some reactions heat the surroundings and others cool them — because bond breaking needs energy (the hill), bond making gives energy (the slope down or up), and the balance between those determines whether energy moves into or out of the room. I end with a short challenge: “Sketch a profile for a reaction you might observe at home that releases energy, and explain in one sentence why the room would warm.”