Balancing Equations AVHS - Chemistry

The Law of Conservation of Mass states that in a chemical reaction there is no loss of mass. So, this means that every atom that is present in the beginning of a reaction must be present at the end of the reaction. Remember, reactions are simply moving atoms around. Balancing is just keeping track of them.

Let's now look at some examples of writing and balancing chemical equations. Problem: Hydrogen (H 2 ) burns in oxygen (O 2 ) to form water (H 2 O). Hmmm…. What kind of reaction is this?

Lets begin by writing a word description of the problem. From the information given above, we have hydrogen + oxygen > water This will not be necessary forever, but it is a nice Baby step in the beginning!

The next step is to replace each name with its corresponding chemical formula. The formulas were all given in the problem, so we have, H 2 + O > H 2 O

Looking at the equation we have so far, we can see it is not balanced. Keep in mind that a chemical reaction is just a rearrangement of atoms, so every atom you start out with has to be somewhere in the end. And you can't end up with any atoms you didn't have in the beginning. The equation above does not meet these requirements.

H 2 + O > H 2 O We can see that the left side of this equation has 2 atoms of hydrogen and 2 atoms of oxygen, while the right hand side has 2 atoms of hydrogen and only 1 atom of oxygen. At this point, hydrogen is ok, but oxygen is not.

H 2 + O > H 2 O Beginners are tempted to solve problems by adjusting the subscripts, but that is NEVER the thing to do! For example, one incorrect approach someone might try is to drop the subscript of 2 in O 2 and write H 2 + O > H 2 O

Why is this wrong?? This DOES balance the equation, but that's not our only objective. We also want our equation to be a correct description of what really happens in nature. Oxygen does not exist as a monatomic gas.

Another approach -- also incorrect – that someone might try -- is to balance oxygen by adding a subscript 2 to the oxygen in water and write the equation as H 2 + O > H 2 O 2

Why is this wrong? Again, we have a balanced equation, but things are still not right. Our original objective was to describe the formation of WATER, and H 2 O 2 is not water. Often, when you start changing subscripts, you get some non-sense formula that does not exist. In this case we do get a real substance. H 2 O 2 is known as hydrogen peroxide.

The bottom line is NEVER ADJUST SUBSCRIPTS! Every chemical substance has a definite chemical formula. When you change subscripts, what you are doing is changing the chemical formula, which is never the correct course of action.

Rather than adjusting the subscripts, you can adjust the numbers that go in front of the formulas. These numbers are called coefficients, and they tell us how many of those formula units we have. The correctly balanced equation for the formation of water from hydrogen and oxygen is 2H 2 + O > 2H 2 O