Endothermic and exothermic reactions

Leon is perfecting his lemonade recipe. He found out he can make it fizzy by adding some baking soda! Baking soda reacts with a substance called citric acid, that’s in the lemon juice. One of the products of this chemical reaction is carbon dioxide. Carbon dioxide forms gas bubbles that make the drink fizzy.

But there is something else about this reaction that we can investigate… Touch the cup in which you mixed your lemon juice and baking soda, Leon. It suddenly feels cold! Why? To understand this, we first need to know something about energy in chemical reactions. Chemical reactions are all about changing some substances — reactants — into other substances — products.

For this to happen, chemical bonds between atoms in the reactants are broken, so that the atoms can rearrange and create new bonds, forming products. The chemical bonds store energy. When bonds are broken and new bonds form, energy moves around. Let's look at the reaction occurring in Leon's lemonade. Bonds in baking soda and citric acid store less energy than the bonds in the products of the reaction.

So, for the products to form, extra energy is needed. Energy cannot just appear from nowhere, it can only move from one place to another. Here, the extra energy comes from the surroundings, and it is absorbed in the form of heat. The absorbed heat is transformed into energy stored in the chemical bonds of the products of the reaction. That’s what made the drink suddenly feel colder!

Reactions that absorb energy from the environment are called endothermic reactions. Another example of an endothermic reaction is the process in which plants produce food — photosynthesis. During photosynthesis, plants turn dioxide and water into glucose, but to do so they need to absorb energy from sunlight. Without energy from the sun, the reaction doesn’t occur. In endothermic reactions the energy level of reactants is lower than the energy level of products, and energy is absorbed.

But there are also chemical reactions in which the energy level of reactants is higher than that of products. This means that after the reaction takes place, there is energy left over. This extra energy gets released into the surroundings. You can test it yourself! Put some laundry detergent powder onto your hand, and mix it with some cool water until the detergent dissolves.

Can you feel the warmth? One of the laundry detergent ingredients is sodium carbonate. When sodium carbonate dissolves in water, a chemical reaction occurs. The products of this reaction store less energy in their bonds than the reactants, and the extra energy is released into the surroundings. That’s why the mixture starts to feel warm.

Reactions that release energy into the environment are called exothermic reactions. Another example of an exothermic reaction is burning, or combustion. When fuel, for instance wood, natural gas, or ethanol, burns in the presence of oxygen, energy is released in the form of heat and light — which you can observe as a flame. Since in all chemical reactions some bonds are broken and some new bonds form, there is always some energy transferred. So all chemical reactions either absorb energy from outside — if they are endothermic, or release energy to their surroundings — if they are exothermic.