Published: 30 January, 2003

Explosions serve to liberate chemical bonding energy

When hydrogen explodes and combines with oxygen, where does the energy that is released in the explosion come from?

This answer is a two-for-one deal! The energy that is released in an explosion (as light, heat, and sound) and in a firefly's tail (as light) is the energy that is stored in "chemical bonds," the glue that hold atoms together into molecules.

There are three molecules involved in your explosion: hydrogen and oxygen, and the product, water. The hydrogen molecule is made up of two atoms of hydrogen (written as H₂), the oxygen molecule is made up of two atoms of oxygen (O₂), and water is made up of two atoms of hydrogen plus one atom of oxygen (H₂O). Atoms are made up of a tiny positively charged nucleus surrounded by a cloud of negatively charged electrons. Sometimes when atoms come together they can reach a lower energy state by sharing their electrons. These shared electrons that spend their time in between two nuclei act as a glue to hold to nuclei together, and that's how you get molecules. These shared electrons are called chemical bonds. In H₂ you have two electrons that are shared between the two hydrogen nuclei; this is counted as one bond. Some bonds are stronger than others. For example, in O₂ you also have a bond, but the bond is stronger than in H₂. In H₂O you have two bonds, one from each hydrogen to the oxygen atom.

If you add up the energy it takes to break the bonds in H₂ and O₂, and subtract the energy it takes to form the bonds in H₂O, you find that there is energy left over. (You have to do the math carefully. To conserve atoms, you must have 2 H₂'s combining with 1 O₂ to give 2 H₂O. So you are breaking two H-H bonds and one O-O bond, and making four H-0 bonds). The "left over" bonding energy in this chemical reaction gets turned into the light, heat, and sound that we call an explosion. In a car, there is bonding energy left over when the gasoline molecules combine with oxygen in the air to make carbon dioxide and hydrogen, and we use that energy to move the pistons and make the car move. Many chemical reactions give off heat, only to a lesser degree. Put your hand into a pile of decaying leaves next fall and you'll find that it's warm. Some reactions even take up energy and feel cold, although these are less common. Amazingly, in some chemical reactions, like the reactions going on inside firefly's tails that glow in the dark, the excess bonding energy is converted almost completely into light instead of heat.