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Endothermic vs exothermic: what's the difference?

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University Course Reader · STEM

The reaction is strongly exothermic, so the flask is cooled in an ice bath as the acid is added.

The reader highlighted one word in a textbook passage. Clicked broke down the chemistry term “exothermic” into plain English:

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Overview

An exothermic reaction gives out heat, so its surroundings warm up. An endothermic reaction takes heat in from its surroundings, so they cool down. Burning wood is exothermic: the wood gives out heat and the room warms up. An instant cold pack is endothermic: the salt inside it dissolves and takes heat from whatever it touches, so a hand holding it goes cold. Which way the heat moves is the whole difference between the two.
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Overview

An exothermic reaction gives out heat, so whatever is around it warms up. An endothermic reaction takes heat in, so whatever is around it cools down. Mix baking soda and vinegar and the fizzing cup goes cold in your hand, because that reaction is taking heat in from everything it touches, cup and hand included. Light a candle, hold a hand above it, and the hand warms, because the burning wick is giving heat out. 😎

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Detail

An exothermic reaction gives out energy, mostly as heat, so its surroundings warm up. An endothermic reaction takes energy in, so its surroundings cool down. When wood burns it gives out heat and light, and the room warms up. In an instant cold pack a salt dissolves in water and takes heat from whatever it touches, so the pack goes cold. Why does one give energy out and the other take it in? Breaking a bond between two atoms costs energy, and making a bond releases energy. A reaction breaks some bonds and makes new ones, and only the difference between the energy it spends and the energy it gets back reaches the surroundings. When the new bonds release more energy than the old ones cost, the reaction gives out the extra, and that makes it exothermic. When the new bonds release less than the old ones cost, the reaction takes the shortfall from its surroundings, and that makes it endothermic. Most reactions need a push to start, which is why wood needs a match, and that first push is called the activation energy. An exothermic reaction pays the push back in heat, so once lit it keeps itself going. An endothermic reaction runs only while something outside supplies heat, and it stops as soon as that supply stops.
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Detail

An exothermic reaction gives heat out and an endothermic reaction takes heat in. Mix baking soda and vinegar and the cup goes cold, because that reaction is pulling heat out of the cup and the hand around it. Hold a hand over a candle and it warms, because burning is pushing heat out. Why the difference? Every reaction snaps some bonds between atoms and forms new ones, and snapping costs energy while forming gives energy back. If the new bonds give back more than the snapped ones took, the reaction has energy to spare and pours it out as heat, and that is exothermic. If they give back less, the reaction is short, and it pulls the missing heat in from whatever is around it, and that is endothermic. Nearly every reaction needs a shove to get started, and a spark or a flame is that shove; chemists call it the activation energy. A fire pays the shove back with interest and keeps itself burning, and your body runs the same way all day, which is why you sit at 37 degrees in a 20 degree room. An egg in a pan is the other kind: the pan has to keep giving it heat the whole time it cooks, and the moment you lift the pan off the hob the cooking stops. Exothermic keeps itself going; endothermic stops the moment the heat supply does. 😎

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Analogy

An exothermic reaction is a guest at a shared dinner who brings more food than they eat, and an endothermic reaction is a guest who eats more than they bring. The table is the surroundings and the food is the energy. Bringing a dish is making bonds, because making a bond gives energy out; eating is breaking bonds, because breaking one takes energy in. Every guest does both, so the table ends the evening fuller or emptier only by the difference between what that guest brought and what they ate. The first guest leaves the table fuller than they found it, and the second leaves it emptier. That is a reaction: bonds broken and bonds made, and the surroundings warmed or cooled by the difference.
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Analogy

An exothermic reaction is like a house with solar panels that sends more electricity to the grid than it draws from it, and an endothermic reaction is like a house that draws more than it sends. The grid plays the surroundings, and the electricity plays the energy. Making bonds releases energy, which is the house sending power out; breaking bonds takes energy, which is the house drawing power in. Every solar house does both across a day, and the meter records only the difference: at the end the first house's meter shows more sent than drawn, and the second's more drawn than sent. Every reaction is one of the two houses, and the room around it ends up warmer or colder by that difference. 😎
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Formal definition — The same term, explained the usual way

An exothermic reaction is one that releases energy to its surroundings, mainly as heat, so that the enthalpy of the products is lower than that of the reactants; the enthalpy change ΔH is negative. An endothermic reaction absorbs energy from its surroundings, so that the enthalpy of the products is higher than that of the reactants; ΔH is positive. The sign is determined by bond energies: energy is required to break the bonds in the reactants and released when the bonds in the products form, and the net enthalpy change is the difference. Both classes of reaction generally require an activation energy to proceed. The terms also apply to physical processes such as dissolving and changes of state.

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