Here's a video discussion of chemical equations (14 min): Khan Academy: Balancing Chemical Equations, on YouTube
Here's another video discussion of chemical equations and mole calculations (13 min): CrashCourse Chemistry, Stoichiometry: Chemistry for Massive Creatures on YouTube
Chemical equations are a way to show what happens in a chemical
reaction. A chemical equation looks something like this:
A → B
In this case, A represents a reactant or reagent, and B represents a
product. Usually one element doesn't turn into another element, so A
and B might represent different molecules that are too complicated
to just write the formulas. There might also be multiple reactants
and products, like this:
A + B → C + D
In this class, we will mostly study reactions of simple molecules, so
we will use their formulas in equations, like this:
Cl2 + Mg → MgCl2
Often, we might want to show the state of the reactants and products,
so we can use (g, l, s, or aq) to show if it is a gas, liquid,
solid, or in a water solution (solutions in water are called aqueous solutions). For the example above, this
becomes, assuming we do the reaction dry
Cl2(g) + Mg(s) → MgCl2(s)
Notice that the numbers showing how many atoms of each element are in
the formula go after the element, as a subscript like
this. When we write chemical equations, usually we want them
to be balanced equations, which means that
they have the same number of each kind of particle on each side of
the equation. Here's an example of an unbalanced equation:
Br2(l) + Na(s) → NaBr(s)
In this example, there are 2 bromine atoms on the left, and only 1 on
the right. We always have to balance the number of each type of
nucleus on each side, like this
Br2(l) + 2Na(s) → 2NaBr(s) OR
1/2Br2(l) + Na(s) → NaBr(s)
Notice that we have put the number of sodium atoms needed for the
reaction in front of the symbol for sodium, and not in a
subscript after the symbol. This is called a coefficient, and it is different because it tells
us how many of a molecule we need, not how many atoms are in the
molecule, like this:
2H2(g) + O2(g) → 2H2O(l)
We also have to balance the number of electrons on each side. The
easiest way to do this is usually to make sure the charges on both
sides add up to the same number. For example, here's an equation
that isn't balanced for electrons, even though it is balanced for nuclei:
Mg(s) + Ag+(aq) → Mg2+(aq) + Ag(s)
To balance this equation we have to make both the charges and the
nuclei balanced, like this:
Mg(s) + 2Ag+(aq) → Mg2+(aq) + 2Ag(s)
This means that the number of each type of particle must be the same on both sides of the equation, because particles (nuclei or electrons) can't appear or disappear (except under special circumstances, which we call nuclear chemistry, so don't worry about that right now). This is why equations need to be balanced.
The second important thing is that the formulas in the equation need to match the actual molecules that are used or produced in the reaction. So if you are given the formulas, and you change them instead of changing the coefficients when you balance the equations, the equation and the reaction it represents has changed! You can't do this. On the other hand, if you are trying to write a chemical equation but you aren't sure what the formulas are, you can definitely use balancing to help you decide, and in this case you could change the formulas and the coefficients, as long as the formulas you use match all the information you have.
Here's an example of how balancing equations can help you avoid
mistakes, based on some wrong answers my students gave on an exam last
year. The question was, what reaction happens between calcium ions and
carbonic acid in the ocean (a water solution)? To answer this, we have to translate
it into formulas. Calcium ions: this is an alkaline earth metal, so
Ca2+. Carbonic acid is H2CO3.
Carbonic acid will dissociate a little bit, making some hydrogen ion (H+),
some bicarbonate ion (HCO3–), and some
carbonate ion (CO32–). Calcium carbonate is
insoluble, so it will form a solid, ionic material. You could write
the reaction like this:
Ca2+(aq) + H2CO3(aq) → CaCO3(s)
+ 2H+(aq)
This equation is balanced. What if you wrote it like this, as many of
my students did?
Ca2+(aq) + H2CO3(aq) → CaCO3(s)
+ H2(aq)
Now it isn't balanced, because the charge on the left side is 2+ and
the charge on the right side is 0. This means that there are 2 more
electrons on the right than on the left, which is a problem! They
can't come from nowhere, so this isn't a complete equation: we are
missing the source of the electrons. The mistake may have been
confusing 2H+ with H2 (the difference between 2
before H and 2 after H is important!), or it might have been
confusing Ca2+ with Ca metal. For instance, this reaction
is fine:
Ca(s) + H2CO3(aq) → CaCO3(s)
+ H2(aq)
However, in the context of the question, it would be very surprising
to have Ca(s) in the ocean, because alkaline earth metals, like alkali
metals, react with water. So we have lots of Ca2+,
Mg2+, and Na+ ions in the ocean, but no atomic
Ca, Mg, or Na in the ocean.