Chemistry does not create heat or cold. It moves energy between the reacting substances and everything around them — and it keeps a strict ledger. How much a reaction releases says nothing at all about how fast it goes.
⚖️Reading the Ledger: Breaking Costs, Forming Pays
Breaking a bond always costs energy. Forming a bond always releases it. Every reaction does both — and which side of the ledger wins decides which direction the energy flows.
🔥EXOTHERMIC — energy OUT
More energy comes out of bond forming than went into bond breaking. The leftover escapes as heat.
ΔH is negative
Products hold less stored chemical energy than the reactants did.
A hand warmer is iron rusting fast: 4 Fe + 3 O2 → 2 Fe2O3. Burning methane: ΔH ≈ −890 kJ/mol.
🧊ENDOTHERMIC — energy IN
Bond breaking costs more than bond forming pays back, so the shortfall is borrowed from the surroundings.
ΔH is positive
Products hold more stored chemical energy, and everything nearby gets colder.
An instant cold pack: ammonium nitrate dissolving absorbs about +26 kJ/mol.
The temperature you feel is the surroundings talking, not the reaction. A cold pack does not make cold and does not send cold into your hand — it takes energy out of your hand, and “cold” is what the absence of that energy feels like. Get this sentence right and half of thermochemistry stops being confusing.
🧹An honest footnote about the cold pack. Ammonium nitrate dissolving is not strictly a chemical reaction — no new compounds form, the ions just separate and get surrounded by water. But the energy accounting is identical: attractions are broken, less energy is recovered, and the shortfall is taken from the surroundings. Noticing distinctions like this one is what chemistry rewards.
📈The Energy Diagram Gives You THREE Numbers
Reaction progress along the bottom, energy up the side. Most students read one number off it and lose marks. Read it carefully and it hands you the forward activation energy, the overall energy change, and the reverse activation energy — three different quantities.
How to read any energy diagram, in four moves
1
Forward activation energy = peak − reactants. Here 230 − 150 = 80 kJ. This is the climb, the toll every collision must pay.
2
Overall energy change ΔH = products − reactants. Here 90 − 150 = −60 kJ. Negative, so this reaction is exothermic.
3
Reverse activation energy = peak − products. Here 230 − 90 = 140 kJ — a much taller climb, which is why the reaction runs strongly forwards.
4
Never confuse the hill with the drop. The height of the hump (80) and the height of the step down across it (60) are different quantities answering different questions: how hard to start versus how much comes out.
If exothermic reactions release energy, why doesn’t petrol in a tank simply become CO2 and water on its own? Because every reaction — exothermic ones included — must first pay an activation energy toll to reach the transition state.
🚴The cyclist pictureA deeper valley waits on the far side, but the cyclist must still pedal over the ridge first. Downhill overall does not mean effortless to start.
⚡The transition stateThe top of the ridge: bonds half broken and half formed. It exists for less than a trillionth of a second and cannot be isolated or bottled.
🗑️Striking a matchFriction turns a trace of red phosphorus on the strip into white phosphorus, which ignites almost instantly — and that flash is the activation energy the match head needed all along.
⚠️The most common exam error: assuming exothermic reactions have no activation energy, or that they must be fast. Neither is true. A diamond is thermodynamically unstable relative to graphite — the conversion is downhill — yet the activation barrier is so tall that a diamond will sit unchanged for longer than the Earth has existed. Energetics and rate are separate questions.
🛠️A Catalyst Lowers the Hill — It Does Not Move the Ends
A catalyst offers the reaction a different route whose highest point is lower, and it emerges unchanged, ready to work again. It changes the rate. It does not change ΔH, and it cannot squeeze out extra product energy.
🧬Catalase, the enzyme in your bloodHydrogen peroxide decomposing on its own faces about 75 kJ/mol. Add catalase — which your blood and a raw potato both contain — and the barrier drops to roughly 8 kJ/mol. That is why peroxide foams instantly on a cut while the same bottle sits quietly on a shelf for years.
🦠Enzymes are catalysts life buildsAlmost always proteins, each with an active site shaped to grip one particular substrate in exactly the orientation that makes reacting easy. That specificity is why you need thousands of them — and why overheating a cell is lethal: enough thermal energy unravels the protein’s shape, a change called denaturing.
⏱️Four Levers on Reaction Rate
All four work through the same picture: reactions need collisions that are frequent enough and energetic enough. Each lever pulls on one of those two things — reason it out rather than memorising it.
🌡️1 · TemperatureHelps twice over: particles collide more often, and — far more importantly — a much larger fraction of collisions now carries enough energy to clear the barrier. Rough classroom rule: a 10 °C rise roughly doubles the rate of many reactions.
🧺2 · Concentration (or pressure)More particles packed into the same volume means collisions happen more frequently. It does not change how energetic each collision is — only how many there are.
🧺3 · Surface areaGrinding a solid into powder exposes far more of its particles to attack. The same mass of zinc that dissolves lazily as a lump reacts vigorously as a powder — identical atoms, far more of them reachable.
🛠️4 · A catalystOpens a route with a lower barrier, so a larger fraction of ordinary collisions is now energetic enough. Not consumed, and ΔH is untouched.
🧊Your fridge is a rate-control device. Spoilage is chemistry — enzymes and bacteria running reactions in your leftovers. Dropping from 20 °C to 4 °C does not stop those reactions; it starves collisions of energy so the rate falls by roughly three- to fivefold. Freezing slows it further still. Preservation is not magic; it is the temperature lever, pulled backwards.
🔑Key Terms
🔥ExothermicReleases energy to the surroundings, usually as heat. Products hold less stored chemical energy than the reactants, so the surroundings warm up.
🧊EndothermicAbsorbs energy from the surroundings. Products hold more stored chemical energy than the reactants, so the surroundings cool down.
⛰️Activation Energy (Ea)The minimum energy a collision must supply to reach the unstable arrangement where old bonds break and new ones form. Every reaction has one.
⚡Transition StateThe highest-energy, half-made arrangement at the very top of the barrier — bonds partly broken, partly formed. It lasts under a trillionth of a second.
📈Energy DiagramReaction progress on the x-axis, energy on the y-axis. Shows reactant energy, the peak, product energy, both activation energies, and ΔH all at once.
🏷️Enthalpy Change (ΔH)The overall heat released or absorbed at constant pressure, in kJ/mol. Negative = exothermic; positive = endothermic.
🛠️CatalystSpeeds up a reaction by providing an alternative pathway with lower activation energy. Not consumed, and it changes neither ΔH nor how much product forms at equilibrium.
🧬EnzymeA biological catalyst, almost always a protein, that lowers the activation energy of one specific reaction. Its active site holds the substrate in exactly the position needed to react.
⏱️Reaction RateHow fast reactants are used up or products formed — a change in amount per unit time. Rises with temperature, surface area, concentration, and a catalyst.
💥CombustionA rapid exothermic reaction between a fuel and oxygen, releasing heat and usually light. Complete combustion of a hydrocarbon gives CO2 and H2O.
🌐Where This Shows Up
🚗Catalytic converters. A honeycomb coated with platinum, palladium and rhodium gives toxic exhaust gases a lower-activation-energy route to safer products — carbon monoxide becomes carbon dioxide, and nitrogen oxides become nitrogen and oxygen. The metals are never consumed, which is why one converter can last the life of the car (and why thieves steal them for the metal).
🧤Disposable hand warmers are not doing anything exotic — they are rusting. Iron powder, salt, water and activated carbon let 4 Fe + 3 O2 → 2 Fe2O3 run in hours instead of years, releasing the heat all at once. The powder is the surface-area lever; the salt and water speed the electrochemistry.
🔥Why a campfire needs kindling. Thin splinters have enormous surface area and a small activation demand, so a match can get them over the barrier. A log has neither. Every fire-lighting technique ever invented is really a way of manipulating surface area and activation energy until the exothermic reaction can sustain itself.
📌Remember This
1Exothermic means energy leaves and the surroundings warm (ΔH negative); endothermic means energy enters and the surroundings cool (ΔH positive). The temperature you feel is the surroundings talking, not the reaction itself.
2Every reaction — exothermic ones included — must pay an activation-energy toll to reach the transition state. Whether a reaction releases energy overall says nothing about how fast it goes.
3A catalyst lowers the hill by opening a different pathway. It is not consumed, and it changes the rate without changing ΔH. Enzymes are catalysts built by living things.
🤔 Think about it
Petrol and oxygen sitting together in a fuel tank would release an enormous amount of energy if they reacted — yet the tank does not explode on its own. If exothermic reactions are energetically “downhill,” what is holding this one back, and what does that tell you about the difference between how much energy a reaction releases and how fast it happens?
A catalyst speeds up the forward reaction. Look at the diagram above and reason it out: what must the catalyst be doing to the reverse reaction at the same time? What does that imply about whether a catalyst can ever push a reaction to produce more product than it otherwise would?
⭐Remember: the hill and the drop are two different numbers. The drop tells you how much energy you get. The hill tells you how hard it is to start — and a catalyst only ever lowers the hill.
✏️ ClickClass Anchor Chart · Stretch Chemistry 6 — Energy in Reactions