Chemistry - Chemical Formulas

For words you are unfamiliar with, please reference the Chemistry Glossary related to this material but clicking HERE.

Let's begin by reviewing a chemical formula.

Remember, a chemical formula is like a recipe for a compound. These recipes include the elements that make up the compound and the specific ratios for each element in the compound. Those ratios are indicated by the coefficients (large numbers in front of the chemical symbols) and the subscripts (the little numbers at the "foot" or the lower left of the chemical symbols).

See these demonstrated in the image below:

 

On the left hand side, we see the reactants, as Hydrogen and Oxygen are used to make water. The "red" indicates coefficients - how many molecules we have, which pertains also to the entire compound of water as well.

On the right hand side, we see the product - water. The subscripts in "blue" represent the number of individual element molecules in the recipe. Consider what a recipe is: it is the instructions for making a desired product - change it, even a little, and we might get something else. So, the subscripts never change for any given formula.

Adding up the molecules is important when balancing equations to determine the needed coefficients. 

Notice, the recipe for water is H₂O. There are 2 hydrogen, and just one oxygen. But, why then, is there a coefficient of 2 in the product above? 

Except for in rare occasions, the noble gasses do not exist alone; they are instead paired. Notice the H₂  and the O₂ on the reactants side of the equation. Thus, this reaction must move forward in such a way that 2 molecules of oxygen are consumed. Well, we do not want to get H₂ O₂(hydrogen peroxide), because that is hydrogen peroxide.

Notice, too, that the hydrogen reactant has a coefficient of 2. In order to get the correct ratio for our recipe, we need to balance our equations.

A simple table makes this task much quicker.

2H₂ +  O₂ ---> 2H₂O

 

Reactants			Products
Element	Amount		Element	Amount
H	4		H	4
O	2		O	2

Because the coefficient of 2 in the product applies to the entire water molecule ( H₂O) , there are then 2 oxygen and 4 hydrogen in the product. We must have the 2 oxygen molecules in order to balance that oxygen is a paired gas. Therefore, we have to make sure that there is a coefficient of 2, to designate that 2 paired molecules of hydrogen are necessary for this reaction to progress.

If you need more review, you can look more into balancing equations here with a very informative youtube video.