Balancing Chemical Equations

Balancing Chemical Equations: Problems #1 - 10

Balancing Chemical Equations: Problems #11 - 25

Balancing Chemical Equations: Problems #26 - 45

The meaning of a modern chemical equation

Return to Equations Menu


Chemical equations usually do not come already balanced. Making sure they are balanced must be done before the equation can be used in any chemically meaningful way.

All chemical calculations you will see in other units must be done with a balanced equation.

IMPORTANT DEFINITION: A balanced equation has equal numbers of each type of atom on each side of the equation.

The Law of Conservation of Mass is the rationale for balancing a chemical equation. The law was discovered by Antoine Laurent Lavoisier (1743-94) and this is his formulation of it, translated into English in 1790 from the Traité élémentaire de Chimie (which was published in 1789):

"We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements."

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.


Example #1: Balance the following equation: H2 + O2 ---> H2O

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. By the way, a skeleton equation is not wrong, it just hasn't been balanced yet. Presenting it as being balanced would be wrong.

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.

Important point: the coefficient times the subscript gives the total number of atoms.

Three quick examples before balancing the equation.

(a) 2H2 - there are 2 x 2 atoms of hydrogen (a total of 4).

(b) 2H2O - 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).

And, one more: how many oxygens are indicated: 3Ca(NO3)2

Answer: 18. The 3 on the nitrate times 2 outside the parenthesis equals 6 oxygen in one formula unit. The coefficient of three times the 6 gives the final answer of 18.


So, now to balancing the example equation:

H2 + O2 ---> H2O

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.

H2 + O2 ---> 2H2O

However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side.

2H2 + O2 ---> 2H2O

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:

H2 + O2 ---> H2O2

True, this is a balanced equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O. So, it's not the answer to the question that was asked.

2) You cannot place a coefficient in the middle of a formula. <--- I actually had a student do this. I was gentle in my correction of his mistake.

The coefficient goes at the beginning of a formula, not in the middle, as in this example:

H2 + O2 ---> H22O

Water only comes as H2O and you can only use whole formula units of it.


Two more points:

1) 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:

4H2 + 2O2 ---> 4H2O

However, all the coefficients have the common factor of two. Divide through to eliminate common factors like this.

Technically, the equation just above is balanced, but only if you ignore the "no common factors other than one" rule. The correct answer has all common factors greater than one removed. If you were to answer a test question balanced as above, you will probably only get partial credit, if that.

2) NO fractions allowed in the final answer, only whole numbers. For example:

H2 + Cl2 ---> (1/2)HCl

is an allowable step along the way to the answer, but it is not the final answer.

The use of fractions in balancing is a powerful tool. Look for it in the solved examples.


From here on out, I will simply give the equation to be balanced.

Example #2: 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:

H2 + Cl2 ---> 2HCl

Placement of a two in front of the HCl balances the hydrogen and chlorine at the same time. This happens fairly often at the end of a balancing sequence, when the placement of one coefficient balances two different elements at the same time.


Example #3: O2 ---> O3

Hint: think about what the least common multiple 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:

3O2 ---> 2O3

Example #4: Na + H2O ---> NaOH + H2

In the skeleton equation as written, the Na and the O are already balanced. So, we look only at the H.

Notice that the H must come in twos on the left-hand side. That means we must have an even number of hydrogen on the right-hand side. We do this:

Na + H2O ---> 2NaOH + H2

to make an even number of hydrogens on the right. We then balance the H like this:

Na + 2H2O ---> 2NaOH + H2

Notice that the first placing of a 2 messed up the balance of the Na and the O. In addition, notice that the second placing of a 2 balances the oxygen and the hydrogen at the same time.

The last step is to balance the Na:

2Na + 2H2O ---> 2NaOH + H2

and it's done.

Here's another method. Go back to the skeleton equation and balance it like this:

Na + H2O ---> NaOH + (1/2)H2

What you did with (1/2)H2 is reduce the number of hydrogens on the right-hand side from a total of 3 to a total of 2. Since everything but the hydrogen was already in balance, the equation is now balanced.

Multiply through by 2 to get the final answer (which should not have any fractions in it).


Example #5: Fe + CuSO4 --> Cu + FeSO4

As you examine the equation, you see that it is already balanced. There is one Fe on each side, one Cu, one S and four oxygens (the last two can also be identified as one sulfate on each side).

Nothing needs to be done to this equation. As it is written, it is balanced.

You sometimes see this presented using the phrase "balanced as written."

Also, students are sometimes confused by this type of problem (sometimes even suspicious that a trick is being played on them). In many cases, a students has seen example after example where something is done to the equation to balance it. The above example needs nothing to be done; it is already balanced.


Example #6: Zn + HCl ---> ZnCl2 + H2

Upon examining this equation, you see that there is already one Zn on each side of the equation. We will attempt to leave it alone, if at all possible, since it is already balanced.

One the right side, we see two chlorines and two hydrogens, with only one of each on the left. Putting a two in front of the HCl doubles the number of chlorine and hydrogen on the left side.

This now leaves us with two chlorines and two hydrogens on each side of the arrow, making them both balanced.

Since the zinc was already balanced, the entire equation is now balanced.

Zn + 2HCl ---> ZnCl2 + H2


Example #7: KClO3 ---> KCl + O2

Start by noticing the the K and the Cl are ALREADY balanced in the skeleton equation. However, the oxygen is out of balance with three on the left and two on the right.

It is important to emphasize that the oxygen on the left will increase only in steps of three, while the oxygen on the right will increase only in steps of two. The question to ask yourself is "What is the least common multiple between 2 and 3?" The answer of course is six. We need six oxygens on each side of the equation. We use a two on the left side since 2 x 3 = 6 and we use a three on the right side since 3 x 2 = 6.

This causes the K and the Cl to become unbalanced, but putting a two in front of the KCl on the right side fixes that.

This problem is interesting because you focused on the oxygens first. Normally, oxygen is the last (or next-to-last) element to be balanced.

2KClO3 ---> 2KCl + 3O2

Another way to balance this equation is by using a fractional coefficient:

KClO3 ---> KCl + (3/2)O2

And, in one step, it's balanced. You then multiply through by 2 to get the whole number set of coefficients, the 2, 2, 3 just above.


Example #8: S8 + F2 ---> SF6

An eight in front of the SF6 will balance the sulfurs.

This now gives us 48 fluorines on the right-hand side, since 8 x 6 = 48. Use a 24 in front of F2 since 24 x 2 also equals 48.

S8 + 24F2 ---> 8SF6


Example #9: Fe + O2 ---> Fe2O3

In the unbalanced equation, there was only one Fe on the left and two on the right. Putting a two in front of the Fe on the left brings the irons into balance.

The situation balancing the oxygen is quite common. You saw it in a previous example. This time, I'll try to lay it out in steps.

  1. The oxygen on the left ONLY comes in twos, while the right-hand side oxygen comes only in threes.
  2. We have to get an equal number of oxygens. (Remember, we can only adjust the value of the coefficient. We cannot change the subscript.)
  3. The least common multiple between two and three is six. This means we will need six oxygens on each side of the equation.
  4. To get this, we put a three in front of the O2 since 3 x 2 = 6 and we put a two in front of the Fe2O3 since 2 x 3 = 6.

The Fe was balanced, but has become unbalanced as a consequence of our work with the oxygen. Putting a four in front of the Fe on the left solves this.

4Fe + 3O2 ---> 2Fe2O3

This equation could also have been balanced using a fractional coefficient:

2Fe + (3/2)O2 ---> Fe2O3


Example #10: C2H6 + O2 ---> CO2 + H2O

First, balance the carbons with a two in front of the CO2. Then balance the hydrogens with a three in front of the H2O. This leaves the following equation:
C2H6 + O2 ---> 2CO2 + 3H2O

Only the oxygens remain to be balanced, but there is a problem. On the right side of the equation, there are seven oxygen atoms, BUT oxygen only comes in a group of two atoms on the left side. Another way to say it - with O2 it is impossible to generate an ODD number of oxygen atoms.

However, that is true only if you were using whole number coefficients. It is allowable to use FRACTIONAL coefficients in the balancing process. That means I can use seven-halves as a coefficient to balance this equation, like this:

C2H6 + (7/2)O2 ---> 2CO2 + 3H2O

Generally, the fractional coefficient is not retained in the final answer. Multiplying the coefficients through by two gets rid of the fraction. here is the final answer:

2C2H6 + 7O2 ---> 4CO2 + 6H2O

Also, improper fractions like 7/2 should be used rather than a mixed number like 3 1/2.


Example #11-13: Balance these three equations using fractional coefficients:

S8 + F2 ---> SF6

C4H10 + O2 ---> CO2 + H2O

S8 + O3 ---> SO2

Solutions:

(1/8)S8 + 3F2 ---> SF6

C4H10 + (13/2)O2 ---> 4CO2 + 5H2O

(1/8)S8 + (2/3)O3 ---> SO2

Alternate answer for the third equation:

(3/8)S8 + 2O3 ---> 3SO2

Generally speaking, fractions are mostly used with diatomics (with O2 is the most common). In addition, as you delve deeper into balancing, you may even see something like (1/2)H2O used in a balancing step, but you will never see it as the final answer.


Example #14: H2 + O3 ---> H2O

However, you must balance it with one restriction: the coefficient in front of the water must be a one.

Solution:

1) No fraction to balance the hydrogen:

H2 + O3 ---> H2O

Two H on the left, two H on the right.

2) Fraction to balance the oxygen:

H2 + (1/3)O3 ---> H2O

(1/3)O3 means one O on the left side and there's one O on the right.

Balancing with a 1/3 as the fractional coefficient is unusual, but you do see it every now and then. Especially, if a teacher is trying to trip you up. Say by using 1/2 all the time, but then suddenly have a 1/3 or a 5/2 on the test. Teachers are sneaky! (Well, at least we think we are!)


Example #14: Balance this equation:

P4 + O2 ---> P2O5

However, balance it with one restriction: the coefficient in front of the P2O5 must be a one.

Solution:

1) Balance the P:

(1/2)P4 + O2 ---> P2O5

2) Balance the O:

(1/2)P4 + (5/2)O2 ---> P2O5

Balancing Chemical Equations: Problems #1 - 10

Balancing Chemical Equations: Problems #11 - 25

Balancing Chemical Equations: Problems #26 - 45

The meaning of a modern chemical equation

Return to Equations Menu


Want more balancing practice? Here's Balancing Worksheet #1 with 50 problems and answers.

Want even more? Here's Balancing Worksheet #2 with 60 more problems and answers.