What are exothermic reactions?

The exothermic reactions are chemical reactions that release energy as heat. The word exothermic is made up of  exo  which means “outwards”, and “thermos” which means heat, so “exothermic reaction” literally means, reaction in which the heat goes outwards.

These reactions are very common and can be found everywhere, from our homes to the cells that are part of our body.

At home we can find them in the kitchen when the gas is burned on the stove to cook our food. We also find them inside car engines when they burn fuel.

An example of an exothermic reaction

On the other hand, exothermic reactions are essential for life, since they provide cells with the energy they require to live, grow and reproduce.

They release energy in the form of heat

This is the main characteristic that makes them exothermic reactions.

The released energy comes from the same reactants

Every chemical reaction requires a certain activation energy to occur. In the case of exothermic reactions, this energy comes from the same reactants when they are transformed into products.

The products of exothermic reactions are always more stable than the reactants.

The reactants when they become products, lose part of their energy in the form of heat. For this reason, the products are always left with less energy than the reactants, which makes them more stable.

They have a negative enthalpy change (AH).

This is an easy way to tell if a reaction is exothermic or not. Any reaction that has a negative AH (read delta H) will be exothermic.

  • For example , the combustion reaction of butane (the gas we burn in the kitchen) has an enthalpy variation of -11,823 kcal / mol. Therefore, it is an exothermic reaction.

They only need one trigger to occur in most cases.

Many exothermic reactions just need something to give them enough energy to start. After this, the same heat that they release is enough to keep the reaction going.

  • For example , when burning gas, it only takes one spark, then the flame continues to burn until the gas runs out.

Differences between exothermic reactions and endothermic reactions

Exothermic reactions are the opposite of endothermic reactions. In the latter, the reactants absorb energy in the form of heat instead of releasing it, as occurs in exothermic reactions.

Exothermic reactions

Endothermic reactions

They release energy in the form of heat.

They absorb energy in the form of heat.

They have a negative enthalpy change, that is DH <0.

They have a positive enthalpy change, that is, DH> 0.

They tend to increase the temperature of the surroundings.

They tend to lower the temperature or cool the surroundings.

The energy that keeps the reaction going comes from the reactants themselves.

The energy that keeps the reaction going comes from the surroundings.

More stable products with less energy are always obtained.

Less stable products are always obtained with a higher energy level.

Examples of exothermic reactions

Cooking gas combustion

In most households in the world, the gas used to cook food is butane, which has the molecular formula C 4 H 10 . This hydrocarbon burns with oxygen in the air through the following combustion reaction, releasing large amounts of heat:

The explosion of dynamite

Dynamite contains an explosive called nitroglycerin (C 3 H 5 N 3 O 9 ). This compound decomposes very violently, releasing large amounts of heat, causing a large explosion. The chemical reaction is:

This reaction has a Δ H = -368 kcal/mol , so is an exothermic reaction.

Spontaneous combustion of white phosphorus

The element phosphorus (P) can be found as red phosphorus or white phosphorous. White phosphorus burns spontaneously when it comes into contact with air, which is why it is used as a wick for hand grenades. The reaction is:

Cellular respiration

The cellular respiration is an exothermic chemical reaction is the oxidation of the foods we eat. The heat released by this reaction is harnessed by cells to carry out a myriad of endothermic chemical reactions that require energy to occur.


Fireworks contain black powder as the main explosive component. Gunpowder is a mixture containing potassium nitrate, carbon graphite and sulfur that react with each other through an exothermic reaction. The heat that this reaction releases generates the wide range of colors that we see in the sky when fireworks explode.

The chemical heating pad

Heating pads are used as hot compresses to alleviate sports injuries. These consist of a sealed bag that contains water and a chemical in a small tube. As the tube breaks, the chemical mixes with the water producing an exothermic chemical reaction that heats the pad.

Reaction of metallic sodium with water

Sodium (Na) is a very reactive alkali metal that reacts violently when it comes into contact with water, releasing a lot of heat and hydrogen gas. The reaction is:

This reaction releases 1900 calories for every gram of Na that reacts with water. This means that 1 gram of sodium would be enough to heat 3 medium cups of coffee.

Detergent dissolution

This is a reaction that anyone can experience at home. By taking a handful of powdered laundry detergent by hand and wetting it with a little water, your hand will immediately feel warm. This is because the process of dissolving the detergent in water is exothermic.

The termite reaction

Termite is a mixture of aluminum in the form of a very fine powder and an oxide of another metal such as iron. After a spark, the reaction that occurs is:

This reaction releases so much heat that it can melt iron and is widely used to weld train rails.

Water condensation

For all substances, phase changes from gas to liquid (condensation), from liquid to solid (solidification) and from gas to solid (deposition), always release heat and are therefore exothermic processes. In the case of water, the condensation of just 18 grams of water releases approximately 10,000 calories, which is enough to heat 15 small cups of coffee.

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