A less wordy way to say it might be: "Matter is neither
created nor destroyed." Therefore, we must finish our
chemical reaction with as many atoms of each element as when we
started. We must account for as many atoms on the "reactant"
side of the equation as there are atoms on the "product"
side of the equation.
Here is the example equation for this lesson:
It is an unbalanced equation (sometimes also called a skeleton equation). This means that there are UNEQUAL numbers at least one atom on each side of the arrow.
In the example equation, there are two atoms of hydrogen on each side, BUT there are two atoms of oxygen on the left side and only one on the right side.
Remember this: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.
An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that only the coefficients can be changed, NEVER a subscript.
The coefficient times the subscript gives the total number of atoms.
Three quick examples before balancing the equation.
(a) 2 H2 - there are 2 x 2 atoms of hydrogen (a total of 4).
(b) 2 H2O - there are 2 x 2 atoms of hydrogen (a total of 4) and 2 x 1 atoms of oxygen (a total of 2).
(c) 2 (NH4)2S - there are 2 x 1 x 2 atoms of nitrogen (a total of 4), there are 2 x 4 x 2 atoms of hydrogen (a total of 16), and 2 x 1 atoms of sulfur (a total of 2).
So, now to balancing the example equation:
The hydrogen are balanced, but the oxygens are not. We have to get both balanced. We put a two in front of the water and this balances the oxygen.
However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side.
This balances the equation.
Two things you CANNOT do when balancing an equation.
1) You cannot change a subscript.You cannot change the oxygen's subscript in water from one to two, as in:
True, this balances the equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O. In, fact, H2O2 is hydrogen peroxide -- the stuff in a brown bottle in your medicine cabinet used for cleaning up cuts, etc. Don't think you'd want to drink it instead of water .... it is poisonous!
2) You cannot place a coefficient in the middle of a formula.
The subscript goes at the beginning of a formula, not in the middle, as in:
Water only comes as H2O and you can only use whole molecules of it.
There is another thing you should avoid. Make sure that your final set of coefficients are all whole numbers with no common factors other than one. For example, this equation is balanced:
However, all the coefficients have the common factor of two. Divide through to eliminate common factors like this. Always REDUCE your answer!
Balance this equation: H2 + Cl2 ---> HCl
Remember that the rule is: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.
The correctly balanced equation is:
1 H2 + 1 Cl2 ---> 2 HCl
Placement of a two in front of the HCl balances the hydrogen and chlorine at the same time.
Note: If you open a chemistry text, you will NOT typically see the coefficient "1" written in front of a molecule, atom or formula unit. When nothing is written there, the "1" is understood. For the purposes of learning how to balance equations for the beginner, it is not a bad idea to actually write the "1" so that you know that you have addressed the balancing of that particular substance.
Balance this equation: O2 ---> O3
Hint: think about what the least common multiple (LCM) is between 2 and 3. That's right - six.
The LCM tells you how many of each atom will be needed. Your job is to pick coefficients that get you to the LCM.
The correctly balanced equation is:
3 O2 ---> 2 O3
Practice Problems
Balance these equations:
C2H6 + O2 ---> CO2 + H2O
Zn + HCl ---> ZnCl2 + H2
KClO3 ---> KCl + O2
S8 + F2 ---> SF6
Fe + O2 ---> Fe2O3