Reading Chemical Equations

Chemical equations are symbolic representations of how chemicals interact with each other—how they react. The basis of the chemical equation is the chemical notation for molecules of the elements and compounds.

The periodic table serves as the basis for how compounds. Each element is represented by a chemical symbol. For instance, hydrogen is represented by the symbol H, sodium by Na, chlorine by Cl, and aluminum by the symbol Al.

Every symbol for every element begins with a capital letter. If there is more than one letter in the symbol, the following letters will be lower-case. This is very, very important!  CO₃ is not the same as Co₃.

When a single molecule of a given compound contains more than one atom of a particular element, it is sub-scripted or followed by a number telling how many. When there are more than one molecules present, the molecule is preceded by the number of molecules (called the coefficient).

With that in mind, the following should be understandable: 3H₂O

In the above, there are 3 molecules of the compound H₂O which includes two hydrogen atoms and one water atom.

Working with invisible molecules might not make the simplicity of the system obvious. In order to build familiarity with the notation system, consider how it might be used by a restaurant that serves breakfasts.

Suppose, on the menu, there were a few set items:

• A "side of bacon" is 2 strips of bacon.
• A "lite breakfast" is 2 eggs and one piece of toast.
• A "good morning" is 2 strips of bacon and 2 pancakes.
• A "hungry jack" is 2 eggs, 4 strips of bacon, and one piece of toast.

If the manager decided to use symbols for the pieces of food, the following would be one way to do that:

• Ba = bacon
• E = egg
• T = toast
• Pc = pancakes

Using these symbols and modeling after chemical notation, the waiter would, for each item on the menu, write:

• Ba₂ = side of bacon
• E₂T = lite breakfast
• Ba₂Pc₂= good morning
• E₂Ba₄T= hungry jack
• 3T = 3 orders of toast

This symbolic notation is exactly like that used for chemical compounds. If you can understand how the coefficients and subscripts work, then you can look at a chemical equation and pick out the different numbers present for each atom!

To use the restaurant model a bit further, consider how compounds could be combined.

Suppose the cook had prepared some food and plated it in anticipation of a regular customer. Say he made a few sides of bacon and a couple of lite breakfasts. Now, suppose that the town doctor rushes in and asks for one hungry jack to go!

While the cook started cooking more food, the waitress could grab one of the lite breakfasts and two sides of bacon, plate them up as one hungry jack, and send the doctor on his way (presumably to do something urgent.) Using the notation for chemical equations, that would look like this:

E₂T + 2Ba₂ -> E₂Ba₄T

In a balanced equation, the number of atoms on each side must always be the same. So in the above, on the left side of the arrow (which is generally read as "yields" — and is sometimes represented with an equal sign) there are 2 eggs, 1 piece of toast and 2 sets of 2 strips of bacon (4 bacons). On the right side, there is also 2 eggs, one piece of toast, and 4 strips of bacon.

Consider this "re-plating" of food:

2Ba₂Pc₂ + E₂T = E₂Ba₄T + 2Pc₂

  +       

=

  +  

This time, the left side has 4 bacons, 4 pancakes, 2 eggs, and one piece of toast. The right side has to end up with exactly that combination, but since the bacon was plated with the eggs and toast, the pancakes all ended up left over on two plates. The components of the breakfasts are switched around and re-organized so that the new plates are composed of the same things, but in different order.

The principle is the same for chemical compounds. Using the principles above, the following notation of a reaction should make sense:

2Al + 6HCl = 2AlCl₃ + 3H₂

On the left are 2 aluminums, 6 hydrogens, and 6 chlorines. Thus, on the right those atoms are re-organized into 2 molecules of aluminum chloride and 3 molecules of hydrogen. (Since the aluminum chloride molecule has 3 chlorines, and there are 2 molecules, that accounts for the 6 chlorines on the left. Similarly, each hydrogen molecule has 2 hydrogens, so there are 3 molecules of hydrogen.)

Just as in the example of the breakfasts, chemical equations must end up with the same number of parts (atoms) as they begin with.

Examples

In NaHCO₃ there are 1 Na, 1 H, 1 C and 3 O.

In 2H₂O there are 4 H and 2 O

In 4NaCl there are 4 Na and 4 Cl

In 3H₂SO₄ there are 6 H, 3 S, and 12 O

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Summary Remarks

• Each element is represented by a chemical symbol that begins with a capital letter, and if there is more than one letter in the symbol, the following letters will be lower-case.
• When a single molecule of a given compound contains more than one atom of a particular element, it is sub-scripted or followed by a number telling how many.
• When there are more than one molecules present, the molecule is preceded by the number of molecules (called the coefficient).
• In a balanced equation, the number of atoms on each side must always be the same.
• The equations tell how many of each molecule combines and what the resulting compounds are.