Balancing
Chemical Equations
For AP Chemistry and those who know the
polyatomic ions
An equation is balanced when there are an equal number of
atoms/ ions on each side of the equation.
Why? CONSERVATION OF
MASS
Matter
is neither created nor destroyed!
Example: 2H2
+ O2 → 2H2O
There are two types of numbers in this equation
1. Subscripts:
example - O2
The subscripts can NEVER be changed.
2. Coefficients: example
2H2
This is the number in front of the atom or compound that
can be changed to balance the equation.
If there is no coefficient assume the number 1. There is one unit of O2 in the
above equation.
In order to calculate the numbers of atoms/ ions multiply
the subscript by the coefficient.
For polyatomic ions, multiply the subscript outside of the
brackets by the
coefficient. No brackets
means the subscript for the whole polyatomic ion is 1.
Examples: 2Na2SO4 or 2Na2(SO4) There are 4
Na and 2 SO4
2Ca(NO3)2 There are 2 Ca and 4 NO3
Steps
to Balance Equations
(that
usually work!)
1. If the equation is not written for you, write the
reactants on the left followed by the products on the right.
2. Look for matching polyatomic ions on both sides. If they are there, bracket them off and treat
them as a unit.
Note: A Polyatomic ion must exist on both sides of the
equation to be considered a unit!!!!
3. If there is and OH ion on one side and not on the other,
but there is an H2O on the
other side only (not on both sides), write water as H(OH) and treat the OH as an ion in
brackets.
Example for step 2 and 3
NaOH + H2SO4 → Na2SO4
+ H2O
Step 2 - Polyatomic
ions are on both sides. Bracket ions so there is no confusion.
Na(OH)
+ H2(SO4) → Na2(SO4) + H2O
Step 3 – There is and OH on one side and a water on the other so rewrite.
Na(OH)
+ H2(SO4) → Na2(SO4) + H(OH)
4. Use a chart to help balance. Write
each atom/ ion on each side with the number of atom next to it. One side is for the reactants; the other side
is for the products.
Na(OH) + H2(SO4)
→ Na2(SO4) +
H(OH)
Na 1 OH 1 H 2 SO4 1 |
Na 2 OH 1 H 1 SO4 1 |
5. Balance one atom/ ion at a time using coefficients. Start with metals. If you can, leave diatomic oxygen and hydrogen
(not in ions) for last. Change your numbers in the chart for each atom changed. Do this for each
step always checking both sides. You
may have to go back and rebalance some atoms/ions as you go along.
2 Na(OH) + H2(SO4)
→ Na2(SO4) +
H(OH)
Na OH SO4 1 H 2 |
Na 2 OH 1 SO4 1 H 1 |
6. Next, balance non-metals one at a time. Now we have 2 (OH) on one side
and not on the other. Fix that using coefficients, then change the numbers in
your chart. Always check both sides.
2 Na(OH) + H2(SO4)
→ Na2(SO4) + 2 H(OH)
Na OH SO4 1 H 2 |
Na 2 OH SO4
1 H |
7. Now we are balanced. We have
equal number of atoms/ions on the reactant side and product side of the
equation. Rewrite the equation as it should be.
Single polyatomic ions are never in brackets and water is written as H2O
2NaOH + H2SO4 → Na2SO4
+ 2H2O
8. Finally make sure the coefficients are in the
lowest whole number ratio.
The above coefficients are 2, 1, 1, 2 this is the
lowest whole number ratio for each coefficient.