Bean Counting 101 (Balancing Chemical Equations)

No, this is not an accounting paper. I can’t even balance my check book.

The fundamental principle in chemistry is the Law of Conservation of Matter. Simply stated, stuff in an ordinary chemical reaction is neither created nor destroyed. (I can’t account for physicists who monkey around with matter and energy…….).

Matter, of course, can change in all sorts of ways; isn’t that why we do chemical reactions? No matter what happens, however, the same atoms present at the start will be there after the (dust clears?) reaction ends.

Some jargon: chemicals present at the start of the reaction are termed reactants, those formed as a result are termed products.

Here is a simple one: C + H₂ —–> CH₄ On the reactant (left) side of the reaction, we count one C and 2H’s. On the right, one C and 4H’s. If I double the H₂‘s on the reactant side the thing balances: C + 2H₂ —–> CH₄ Seems simple enough. A ground rule: You may change coefficients (the big #’s) but never the subscripts.

Let’s try another one: C₂H₆ +O₂—–> CO₂ + H₂O. This is a good way to keep warm. We burn (combine with oxygen) a hydrocarbon (C₂H₆). The products are carbon dioxide, water and heat (which is the only product we care about). OK, we start with:

2C’s

6H’s

2O’s

and end up with:

C

3O’s

2H’s

We can double the CO₂, giving us balanced carbons. If we put a 3 ahead of H₂O, this balances hydrogen. At this point, we end with 7 O’s on the product side. What’s on the reactant side? Oxygen, in the form of O₂. What’s this? Reality! There are several atoms which go through life as “diatomic atoms”. This is to pair up valence electrons (unpaired electrons are called “free radicals” a chemical no no. But I digress….). In any case, one is tempted to break up the diatom and balance by putting 7O’s there. DON’T DO IT!! Instead, multiply all the compounds by 2, and end up with:

2C₂H₆ +7O₂ —–> 4CO₂ + 6H₂O

Incidentally, the second equation represents a class of compounds referred to as “hydrocarbons”. Their general equation is C_N H_2N+2 . They all combine with oxygen producing carbon dioxide and water (and heat). Since digestable food consists mostly of carbon, our bodies manufacture carbon dioxide and water as waste products. Biologists call this “metabolism”. As Sam Cooke famously observed back in the day. I “don’t know much about biology” (too complicated), but along with the energy food produces we produce water (urine) and CO₂ in our exhalation.

Why do we need to balance equations? Because if we want to make new stuff, we need to be able to measure out the equivalent amounts to make a desired amount of product (and I don’t mean heat!). Next, we’ll look at counting atoms and molecules. Keep this in mind: one mole = 6.02 exp 23 atoms or molecules = X number of grams. We’ll check this out next.