Balanced equations and chemical reactions

There are four basic types of chemical reactions. The first is the simplest and is known as synthesis, or a composition reaction. This type of reaction combines two or more substances to form at least one new compound. One example has already been given which is when sodium metal, a solid, combines with chlorine gas to form sodium chloride. The reaction can be symbolized in the following way:

Na (s) + Cl₂ (g) NaCl (s)

The reactants are on the left hand side of the arrow and the product(s) is (are) on the right. It can be read to mean that the reactants, sodium and chlorine, will react to form a single product, sodium chloride. In the chemical equation above, the (s) stands for solid, and the (g) stands for gas which indicates which state of matter the substance is in. Sometimes the arrow is read as “yields”. As you look at this equation, the natural question to ask is why are there three atoms on one side of the equation and only two atoms on the other? What happened to the extra chlorine atom? Good question. This equation is not a balanced equation, and matter doesn’t just disappear and appear. A more accurate way to correctly represent this reaction is as follows:

2Na (s) + Cl₂ (g) 2NaCl (s)

This can be understood by saying that two sodium atoms combine with one diatomic chlorine molecule to form two ionic units of sodium chloride.

Another aspect of chemical reactions is the physical properties of compounds, or how atoms and molecules fit together. If we look at the crystal structure of NaCl, we notice that it is a closely packed cube.

The sodium chloride crystal structure has each atom with six of its nearest neighbors in an octahedral geometric pattern. Na ions are in light blue and Cl ions are darker green for contrast.

All reactions are simply rearrangements of matter in more stable ways. New matter is never created, and no matter is ever destroyed. This is known as the law of conservation of matter; matter can neither be created nor destroyed, but matter simply changes form.

sodium + chlorine → sodium chloride

2 Na (s) + Cl₂ (g) 2 NaCl (s)

2 moles + 1 mole = 2 moles

2(23 amu) + 2(35 amu) = 2(58 amu)

46 amu + 70 amu = 116 amu

116 amu = 116 amu

Using the atomic weighs from the periodic table, you can see that two sodium atoms weigh 46 atomic mass units. One chlorine molecule weighs 70 atomic mass units. Two units of a sodium chloride crystal weigh 116 atomic mass units. The total mass of the reactants must equal the total mass of the product.