The Conservation of Mass in a Chemical Reaction: Chemical reactions may seem to only take place in books and labs, but they are in fact taking place all around us. Every time you metabolize a meal, light a match, start a car, or even think a thought, chemical reactions are responsible. Reactions, which are often initiated by catalysts, take place when new molecular bonds are created or broken to link or break apart different elemental atoms. One such reaction is the reaction between hydrogen and oxygen to create water. In chemistry you represent a reaction by writing the reactants (the substances before the reaction) to the left of an arrow, and the products (the product of the reaction) to the right of the arrow. If you know that water is H2O, then this might lead you to represent the reaction like this: H + O H2O But this is wrong. Because atoms like to have full valence shells, H and O atoms by themselves are very rare. In fact, both hydrogen and oxygen react with themselves to create the molecules H2 and O2, respectively. It is much more common to see O and H atoms paired up in molecules of two than it is to see them alone. With this knowledge, one might assume then that the reaction between hydrogen and oxygen would look like this: H2 + O2 H2O This actually closer but still not quite right. This equation tells us that one hydrogen molecule (composed of two H atoms) and one oxygen molecule (composed of two O atoms) will form a single water molecule with 2 H atoms and 1 O atom. In this equation we seem to have lost one of the O atoms. But the principle of the conservation of mass tells us that no atoms are lost in chemical reactions. In other words the number of atoms on the left side of a chemical equations must be balanced with the number of atoms on the right side. In reality the O atom that we lost, under most circumstances, would then bond with a second molecule of hydrogen in essence creating two different water molecules. So the reaction is written: 2H2 + O2 2H2O In this reaction equation, the number in front of the molecule (which is called a coefficient) designates the number of molecules that participate in the reaction. To conclude, we can read this as saying: two separate hydrogen molecules (with 2 H atoms each) combine with a single oxygen molecule (with 2 O atoms) to create 2 individual molecules of water (each with two H atoms and a single O atom). |
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