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Chemical Equations

Chemical reactions happen all around us: when we light a match, start a car, eat dinner, or walk the dog. A chemical reaction is the process by which substances bond together (or break bonds) and, in doing so, either release or consume energy (see our Chemical Reactions module). 

A chemical equation is the shorthand that scientists use to describe a chemical reaction. Let's take the reaction of hydrogen with oxygen to form water as an example. If we had a container of hydrogen gas and burned this in the presence of oxygen, the two gases would react together, releasing energy, to form water. 

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To write the chemical equation for this reaction, we would place the substances reacting (the reactants) on the left side of an equation with an arrow pointing to the substances being formed on the right side of the equation (the products). Given this information, one might guess that the equation for this reaction is written:H + O arrow H2OThe plus sign on the left side of the equation means that hydrogen (H) and oxygen (O) are reacting. Unfortunately, there are two problems with this chemical equation. 

First, because atoms like to have full valence shells, single H or O atoms are rare. In nature, both hydrogen and oxygen are found as diatomic molecules, H2 and O2, respectively (in forming diatomic molecules the atoms share electrons and complete their valence shells). Hydrogen gas, therefore, consists of H2 molecules; oxygen gas consists of O2. Correcting our equation we get:H2 + O2 arrow H2OBut we still have one problem. 

As written, this equation tells us that one hydrogen molecule (with two H atoms) reacts with one oxygen molecule (two O atoms) to form one water molecule (with two H atoms and one O atom). In other words, we seem to have lost one O atom along the way! To write a chemical equation correctly, the number of atoms on the left side of a chemical equation has to be precisely balanced with the atoms on the right side of the equation. How does this happen? In actuality, the O atom that we "lost" reacts with a second molecule of hydrogen to form a second molecule of water. During the reaction, the H-H and O-O bonds break and H-O bonds form in the water molecules, as seen in the simulation below. 

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The balanced equation is therefore written:2H2 + O2 arrow 2H2OIn writing chemical equations, the number in front of the molecule's symbol (called a coefficient) indicates the number of molecules participating in the reaction. If no coefficient appears in front of a molecule, we interpret this as meaning one.