Exothermic & Endothermic Reactions
Where the energy in a reaction comes from, and where it goes
How this deck works. Every slide is colour-coded: Read for the explanation, Look for diagrams and models, Do for experiments and simulations, and Check for questions. Move with the buttons at the bottom, or the ← → keys on a keyboard.
Mass is always conserved. Temperature is not.
Where you left off · Topic 4
You proved that atoms are never created or destroyed — sealed bag, kitchen scale, no exceptions. Every reaction in this unit obeyed that rule exactly.
But something else was happening the whole time, and you have already felt it twice. Damp steel wool got warm with no flame anywhere near it. And if you have ever snapped an instant cold pack on a sprained ankle, you have felt a chemical process go the other way.
Reactions do not just rearrange atoms. They also move energy — sometimes out into the room, sometimes in from it. That is the last idea in this unit, and it takes you all the way back to Topic 1, where you learned what temperature actually measures.
Every chemical bond is stored energy
A chemical reaction has two halves. Old bonds have to break, and new bonds have to form. Those two halves do opposite things with energy:
- Breaking a bond always absorbs energy. Bonds hold atoms together; pulling them apart costs something, the way pulling two magnets apart costs effort.
- Forming a bond always releases energy. When atoms snap together into a new bond, the energy that was needed to hold them apart is let go.
So every reaction is a subtraction problem. Compare the energy it cost to break the old bonds with the energy released by forming the new ones, and whichever is bigger decides what you feel.
Released more than it absorbed? The extra energy leaves the chemicals and spreads into everything around them — the container, the air, your hand. Those things get warmer.
Absorbed more than it released? The reaction has to take that energy from somewhere, and it takes it from everything around it. Those things get colder.
The chemicals doing the reacting are called the system. Everything else — the beaker, the air, your skin, the thermometer — is the surroundings. A thermometer never measures the system directly. It always tells you what happened to the surroundings.
Two shapes, two outcomes
Height on these graphs means energy stored in the chemicals. Left to right is the reaction happening.
Test yourself
You mix two liquids in a beaker and the outside of the beaker becomes noticeably warm. What kind of reaction is it?
Exothermic: energy heads out
An exothermic reaction releases energy to its surroundings. Forming the new bonds gives back more than breaking the old ones cost, and the leftover leaves the system — usually as thermal energy, sometimes as light.
The surroundings get warmer. The thermometer goes up.
Ones you already know:
- Combustion — anything burning. Natural gas in a furnace, gasoline in an engine, wood in a fire pit.
- Rusting — your steel wool from Topic 3. Slow, flameless, and the thermometer still climbed.
- Respiration — your own cells reacting sugar with oxygen. It is the reason your body sits at about 98.6 °F on a 40 °F morning.
- Setting concrete — a fresh sidewalk pour is genuinely warm to the touch.
Teacher note · common misconception
"Where does the heat come from?" is a fair question and deserves a real answer, not "from the reaction." It comes from energy that was already stored in the chemical bonds of the reactants. Nothing is created — stored energy is converted to thermal energy, exactly as conservation demands. Students who accepted conservation of mass in Topic 4 will accept this readily if you name the parallel out loud.
Run the reaction and watch the thermometer
Choose a reaction type, then drag the slider from start to finish. The dot is the chemicals; the thermometer is the room.
Push the slider slowly and watch three things at once:
- The dot climbs the hump first, in both cases. Even a reaction that ends up releasing energy has to be given some energy to get started — that is why gasoline needs a spark.
- On the exothermic curve the dot finishes lower than it started. The chemicals ended with less stored energy, and the difference went into the room.
- On the endothermic curve the dot finishes higher. That extra energy had to come from somewhere, and the thermometer shows you exactly where.
Test yourself
In an exothermic reaction, where does the released energy come from?
The fizz that gets colder
You used this reaction in Topic 4 to test mass. Now put a thermometer in it.
Materials
- Baking soda
- White vinegar
- A thermometer
- A tall glass or jar
- A measuring spoon
- Paper for a data table
Steps
- Pour about half a cup of vinegar into the glass and stand the thermometer in it.
- Wait 3 full minutes so the thermometer settles, then record the starting temperature. Do not skip the wait — a thermometer that came out of a warm hand will ruin the result.
- Add 2 teaspoons of baking soda all at once and stir gently with the thermometer.
- Record the temperature every 30 seconds for 5 minutes.
- Graph your readings: time on the bottom, temperature up the side.
The temperature drops — usually by several degrees — and then slowly creeps back up toward room temperature.
This is an endothermic reaction. Breaking the bonds in the baking soda and the acetic acid costs more energy than forming the bonds in the products gives back. The reaction makes up the difference by pulling thermal energy out of the liquid around it, so the liquid cools.
The slow rise afterward is not the reaction restarting. Once the fizzing stops, ordinary heat transfer takes over: the room is warmer than the glass, so energy flows from the room into the glass until they match — exactly the rule you learned in Topic 1.
Notice how badly "looks energetic" predicts the answer. Vigorous bubbling tells you a gas is being produced quickly. It says nothing at all about which direction energy is moving.
Teacher note
This is the most valuable prediction in the unit, because nearly every student picks "warmer" and is wrong. Have them write the prediction down and initial it before adding the baking soda — the memory of being confidently wrong is what makes the definition stick.
Endothermic: energy is pulled in
An endothermic reaction absorbs energy from its surroundings. Breaking the old bonds costs more than forming the new ones gives back, so the reaction takes the shortfall from everything around it.
The surroundings get colder. The thermometer goes down.
Ones you already know:
- Photosynthesis — plants need a constant supply of sunlight, because building sugar out of carbon dioxide and water absorbs energy. Cut off the light and it stops.
- Cooking an egg — the pan has to keep supplying energy the whole time. Turn off the burner halfway and the change halts where it is.
- Baking soda and vinegar — your experiment. Loud, fast, and cold.
- An instant cold pack — squeeze it and the temperature plunges within seconds.
Teacher note · accuracy
Most instant cold packs work by dissolving a salt such as ammonium nitrate in water, which is an endothermic physical process rather than a chemical reaction. It is a perfectly good example of energy being absorbed — just be ready to say so if a sharp student asks whether a new substance was formed. Hand warmers, by contrast, really are chemical: iron oxidizing, the same reaction as the rusting steel wool.
Test yourself
A plant stops growing when you move it into a dark closet. What does that tell you about photosynthesis?
Foam you can feel
The same measurement, the opposite result.
Safety: use ordinary 3% drugstore hydrogen peroxide only, never a stronger salon or industrial grade. Do this with an adult, wear eye protection, work in the sink or on a tray, and use an open container — never a sealed bottle. The foam and the container get hot.
Materials
- 3% hydrogen peroxide
- Active dry yeast
- Warm water
- Dish soap
- A thermometer
- A tall plastic cup on a tray
Steps
- In a small cup, stir 1 tablespoon of yeast into 3 tablespoons of warm water. Let it sit 3 minutes.
- Pour about half a cup of hydrogen peroxide into the tall cup, add a squirt of dish soap, and swirl gently.
- Stand the thermometer in the peroxide, wait 3 minutes, and record the starting temperature.
- Pour in the yeast mixture and step back. Foam will rise quickly.
- Record the temperature every 30 seconds for 5 minutes, and carefully touch the outside of the cup near the end.
- Graph these readings on the same axes as your baking soda graph from the last experiment.
The temperature rises clearly — often 10 to 20 °F above where it started — and the cup feels warm.
Hydrogen peroxide (H₂O₂) breaks down into water and oxygen gas. It does this on its own very slowly, which is why the bottle has an expiration date. The yeast contains an enzyme that speeds the process up enormously without being used up itself — that is what a catalyst does. The soap is only there to trap the oxygen as foam so you can see it.
This reaction is exothermic. The bonds in the water and oxygen that form hold less energy than the bonds in the peroxide did, and the difference goes into the cup, the foam and the air.
Put the two graphs side by side. Same equipment, same kind of measurement, opposite slopes. That single pair of lines is the entire point of this topic.
Teacher note
The catalyst is worth naming even though it is beyond the workbook. Students often assume the yeast is a reactant being consumed; ask them where the yeast is at the end. Also check the peroxide is not years old — expired bottles have already decomposed and give a disappointing result.
Exothermic or endothermic?
One question each time: do the surroundings end up warmer or colder? Ignore how dramatic it looks.
Test yourself
On an energy diagram, the products sit higher than the reactants. What does that mean?
Same shape, opposite direction
A hand warmer and a cold pack are the same idea run backwards.
Eight words you now own
Tap any card to flip it. Tap again to flip it back.
Where you already use this
Hand warmers at a November football game
Open the pouch and air reaches iron powder inside. It oxidizes — the same reaction as your rusting steel wool — and releases energy for hours. Exothermic.
An instant cold pack on the sideline
Squeezing it breaks an inner pouch and lets a salt dissolve in water. That process absorbs energy fast, so the pack drops below freezing temperatures within seconds. Endothermic.
The furnace in your basement
Methane plus oxygen becomes carbon dioxide and water, releasing a large amount of energy. That is Topic 3's equation doing a job. Exothermic.
Fireworks over the Mall on July 4th
Energy stored in the chemical bonds leaves as light, sound and heat all at once. Loudly exothermic.
A cake in the oven
The oven has to keep supplying energy the entire baking time. Turn it off at minute ten and the cake stops changing. Endothermic.
Your own body, right now
Cells react sugar with the oxygen you just inhaled and release energy. That is why a room full of people warms up, and why you are 98.6 °F. Exothermic.
Test yourself
Two reactions both foam vigorously. One thermometer rises and the other falls. What is the best conclusion?
Topic 5 quiz — 8 questions
One try per question. You get the reasoning right after you answer.
End of the unit · the whole chain
Particles move, and their motion is thermal energy. Those particles are built from atoms. Atoms rearrange in chemical reactions. Through every rearrangement the atoms are conserved — and the energy stored in their bonds moves either out into the room or in from it. Five topics, one idea, followed all the way down.