Endothermic and Exothermic

Every chemical reaction involves energy. Some reactions release energy (usually as heat) into their surroundings. These are called exothermic. Other reactions absorb energy from their surroundings. These are called endothermic. The simple labels matter a lot in real life. Exothermic reactions warm hand warmers, run cars and power your body. Endothermic reactions cool ice packs, cook food and run the photosynthesis of every plant on Earth.

• ExothermicReleases energySurroundings get hotter
• EndothermicAbsorbs energySurroundings get cooler
• Exo prefixGreek for "out"Energy flows out
• Endo prefixGreek for "in"Energy flows in
• Famous exoBurning, rusting, fireworksHot, bright, fast
• Famous endoPhotosynthesis, cooking, ice packQuietly cooling or absorbing

Why energy changes

Atoms in molecules are held together by chemical bonds. Breaking a bond takes energy. Making a bond releases energy.

• If the new bonds released more energy than was needed to break the old ones, the reaction overall releases energy: exothermic.
• If the new bonds released less energy than was needed to break the old ones, the reaction overall absorbs energy: endothermic.

Either way, the law of conservation of energy still holds. Energy is not created or destroyed; it just flows in or out of the reaction.

Exothermic reactions

Most everyday reactions are exothermic. The energy released is usually felt as heat, and sometimes seen as light.

• Burning fuels: methane burning in your cooker, petrol burning in a car engine, wood burning on a bonfire. (See combustion.)
• Rusting: a very slow exothermic reaction, releasing tiny amounts of heat over months and years. (See oxidation and rusting.)
• Hand warmers: a packet of iron filings that rust slowly when you crack open an air seal, releasing heat for a few hours.
• Setting concrete: cement reacts with water in an exothermic reaction so warm it can crack concrete if not managed.
• Neutralising acid with a base: pour vinegar onto baking soda and feel the slight warmth.
• Fireworks: chemical mixtures that release energy quickly as light, heat and sound.
• Explosives: extreme exothermic reactions, releasing huge amounts of energy in fractions of a second.
• Respiration in your body: slow, controlled exothermic reactions that release energy from glucose; this is how you stay warm.

Fact Your body is a giant collection of slow, controlled exothermic reactions. The energy released as you "burn" the glucose from your food is used partly to power your muscles, brain and chemistry, and partly to keep you warm. The reason you feel cold on a winter day is that you are losing heat to the air faster than your body chemistry is making it.

Endothermic reactions

Endothermic reactions take energy in. They make the surroundings cool down. Sometimes they need a constant supply of energy to keep going (light, electricity or applied heat).

• Photosynthesis: plants take in light energy and use it to convert carbon dioxide and water into sugar. (See photosynthesis.)
• Cooking food: many cooking reactions only happen with continuous heat input. Boiling, baking, frying.
• Instant cold packs: when you squeeze a sports cold pack, you break a small inner bag of water that mixes with a chemical like ammonium nitrate. The salt dissolving is endothermic, so the pack cools down rapidly.
• Electrolysis: passing electricity through water to split it into hydrogen and oxygen. The electrical energy is absorbed.
• Melting ice: technically a physical change, but it absorbs energy. That is why ice cools your drink: it pulls heat from the liquid to melt.
• Evaporation of sweat: takes heat from your skin, cooling you down. Again, this is a physical change but follows the same rule.
• Mixing baking soda with citric acid in water: cools the solution noticeably.

How to tell which is which

If you have a thermometer, you can measure the temperature before and after a reaction:

• Temperature rises = exothermic. Energy was released.
• Temperature falls = endothermic. Energy was absorbed.

Even without a thermometer, your skin can usually tell. A reaction that warms or burns is exothermic. A reaction that feels cold is endothermic.

Did you know? Many reactions need a small amount of energy to get started, even if they release much more energy overall. This starter energy is called the activation energy. That is why a match needs the friction of striking before it bursts into flame, even though the combustion is highly exothermic. Catalysts work by lowering the activation energy needed to start a reaction.

Energy graphs

Chemists often draw simple graphs of energy versus reaction progress:

• In an exothermic graph, the products end up lower in energy than the reactants. The difference is released as heat.
• In an endothermic graph, the products end up higher in energy. The difference came from the surroundings.
• Both graphs have an activation energy hump in the middle that the reaction has to climb before tipping into the products.

Why this matters

Knowing whether a reaction is exo or endo is a key piece of chemistry. It affects:

• Safety: exothermic reactions can run away if they release heat faster than it can escape. Lithium battery fires are a famous example.
• Manufacturing: industrial chemists must add or remove heat to keep big reactors at the right temperature. Cooling jackets, heating coils and even ice baths are common.
• Cooking: many cooking processes are endothermic, requiring you to keep adding heat from the cooker.
• Energy: every fuel you use (wood, petrol, gas, food) releases energy through exothermic reactions. Understanding exactly how much is the basis of energy science.

Try this Mix a tablespoon of baking soda with two tablespoons of citric acid powder in a small cup. Stir in a few tablespoons of water. The cup feels noticeably colder. This is an endothermic reaction. Now try mixing a tablespoon of warm water into a teaspoon of calcium chloride pellets (sold as a drying agent or de-icer). This time the cup gets quite warm: an exothermic dissolving. Two safe kitchen reactions, opposite energy directions.

Deeper dive: why ice melting cools your drink

This is technically a physical change rather than a chemical reaction, but the energy rules are exactly the same.

To melt 1 gram of ice into 1 gram of water at 0 degrees Celsius takes about 334 joules of energy. This is called the latent heat of melting. The ice does not get warmer as it melts: it stays at 0 degrees. All the energy goes into breaking the bonds between water molecules in the solid crystal so they can flow as a liquid.

When you drop an ice cube into a warm drink, the ice pulls heat out of the drink to melt itself. The drink cools sharply, while the ice slowly turns to water. For every gram of ice you melt, the drink loses about 334 joules of energy. That is enough to cool 80 grams of water by 1 degree Celsius. A typical ice cube weighs about 30 grams and can therefore cool a 250 ml drink by 4 degrees, even before the cold water further mixes in.

The same effect explains why sweat cools you. As sweat evaporates from your skin, each gram of water takes about 2,260 joules of energy with it (much more than melting takes, because turning liquid into gas takes even more energy than turning solid into liquid). That is why you feel cooler in a breeze: the breeze speeds up evaporation, which pulls heat out of your skin faster. Mammals and many other animals rely on this clever bit of physics to avoid overheating.

For more, see combustion and photosynthesis.