Exothermic & Endothermic
Every reaction climbs a hill before it can fall. The hill's height sets the drama; the landing height sets the heat.
Builds on: 2.1 Chemical Reactions & Balancing
Bonds are energy accounts
Breaking a bond always costs energy; forming one always pays out. A reaction does both, so its net heat is a simple balance: energy of bonds formed minus energy of bonds broken. Pay out more than you spent and the difference leaves as heat — the reaction is exothermic (burning, rusting, your metabolism). Spend more than you recoup and the reaction drinks heat from its surroundings — endothermic (photosynthesis, cold packs, baking soda in vinegar).
The hill in the middle
If burning releases energy, why doesn’t the wood pile ignite itself? Because before new bonds can form, old ones must be loosened — and that upfront cost is the activation energy Ea. Picture the reaction as a ball that must be shoved over a hill before it can roll down into the valley of products. Wood + oxygen sit behind a high hill: at room temperature almost no collision carries enough energy to cross it. A match delivers the shove — and once some molecules cross, the heat they release shoves their neighbours. That chain hand-off is what a flame is.
Catalysts: a tunnel through the hill
A catalyst offers the reaction an alternative route with a lower Ea — and emerges unchanged, ready to serve again. It doesn’t alter ΔH: the start and end valleys stay where they were; only the pass between them drops. Your car’s catalytic converter, the enzymes running your cells and the industrial catalysts behind fertilizer all play the same trick: lower the hill, multiply the crossings.
Diamond is less stable than graphite — turning into pencil lead would release energy. It never visibly happens because the activation hill is astronomically high. Chemistry has two separate questions: “downhill or uphill?” (thermodynamics, ΔH) and “how high is the pass?” (kinetics, Ea). Diamond is thermodynamically doomed and kinetically immortal.
⚗️ Lab — The Energy Landscape
Shape the reaction profile yourself, then fire collisions at the hill.
- Set a collision weaker than Ea — the ball rolls back: no reaction.
- Add the catalyst and retry the same collision through the lowered pass.
- Make ΔH positive and see what an endothermic landscape looks like.