Bonds break, bonds form

Section 1 of 4

In a chemical reaction, molecules meet, some of their bonds break, and the atoms join up again with new partners. The result is new molecules.

The molecules you start with are the reactants, and the new ones are the products. Chemists write a reaction as an equation, with an arrow from the reactants to the products. Hydrogen and oxygen make water: 2 H₂ + O₂ → 2 H₂O. A number in front of a formula counts molecules, so 2 H₂ means two hydrogen molecules.

Hydrogen and oxygen make water: the same atoms with new partners Ball-and-stick models. On the left, two hydrogen molecules, each two white hydrogen atoms joined by a single bond, and one oxygen molecule, two red oxygen atoms joined by a double bond. An arrow points right to two bent water molecules, each a red oxygen atom joined to two white hydrogen atoms. Both sides have 4 hydrogen atoms and 2 oxygen atoms. 2 H₂ + O₂ 2 H₂O 4 H atoms, 2 O atoms 4 H atoms, 2 O atoms
Two hydrogen molecules and one oxygen molecule make two water molecules. The H–⁠H and O=O bonds break, and new O–⁠H bonds form.

Every atom in the reactants ends up in the products. A reaction rearranges atoms, and the number of each kind stays the same, so both sides of an equation have the same atoms. Here there are 4 hydrogen atoms and 2 oxygen atoms on each side.

Energy in, energy out

A bond holds two atoms together, so pulling them apart takes energy, a bit like pulling two magnets apart. When two atoms join to make a bond, energy is given out, often as heat. A bond gives out the same amount of energy when it forms as it takes to break it.

Every reaction does both. When the new bonds give out more energy than the old bonds took to break, the reaction gives out heat. Chemists call it exothermic. Making water from hydrogen and oxygen is strongly exothermic, because the new O–⁠H bonds are strong. When breaking the old bonds takes more energy, the reaction takes heat in, and it is endothermic. Photosynthesis is endothermic: plants use energy from sunlight to turn carbon dioxide and water into sugar.

The rest of this lesson looks at three kinds of reaction that organic molecules take part in: burning, addition and condensation.

Quick check

In a reaction, some bonds break and new ones form. Which of these gives out energy?