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Balancing Chemical
Equations
A Chemical Reaction
Reactants
Products
A process in which at least one new
substance is produced as a result of
chemical change.
Writing a Chemical Equation
Chemical symbols give a “before-and-after”
picture of a chemical reaction
Reactants
MgO
+
Products
C
magnesium oxide
reacts with carbon
Bonds are broken
CO
to form
+
Mg
carbon monoxide
and magnesium
New bonds form
Reading A Chemical Equation
4 NH3 + 5 O2
bonds broken
4 NO + 6 H2O
new bonds
form
Four molecules of NH3 react with five
molecules O2 to produce four molecules NO
and six molecules of H2O
A Balanced Chemical Equation
Same numbers of each type of atom on each
side of the equation.
Aluminum reacts with sulfur to form aluminum
sulfide.
Al
+
S
2Al
+
3S
Al2S3
1Al2S3
Not Balanced
Balanced
If a coefficient is “1”, write the “1”. Don’t
leave it blank.
Translate word equation into formula
equation
Calcium chloride reacts with
aluminum hydroxide to produce
calcium hydroxide and aluminum
chloride.
“reacts with”
“to produce”
“and”
=>
=>
=>
+
+
Translate word equation into formula
equation
Calcium chloride reacts with aluminum
hydroxide to produce calcium hydroxide and
aluminum chloride.
CaCl2 + Al(OH)3
Ca(OH)2 +
Why is this Not
Balanced ?
AlCl3
Translate word equation into formula
equation
Hydrogen reacts with chlorine to form hydrochloric
acid.
H2 + Cl2
HCl
Not Balanced !
Dicarbon tetrahydride reacts with oxygen to form
carbon dioxide and water.
C2H4
+ O2
CO2 + H2O
Not Balanced !
Balance Equations with
Coefficients
Coefficients in front of formulas balance
each type of atom.
4NH3 +
5O2
4NO + 6H2O
4N
=
4N
12 H
=
12 H
10 O
=
10 O
DO NOT CHANGE SUBSCRIPTS TO BALANCE AN
EQUATION !
Balance Equations with
Coefficients
Do not put coefficients in the middle of a formula.
4NH3 is OK, but N4H3 is NOT OK.
4NH3 +
5O2
4NO + 6H2O
DO NOT CHANGE SUBSCRIPTS TO BALANCE AN
EQUATION !
Steps in Balancing An Equation
1)
Use coefficients to balance equations. Never change or add
subscripts.
2)
Place a coefficient of “1” in front of the most complex formula,
especially those with polyatomic ions.
3)
Balance each element in the formula. It’s helpful to leave oxygen
compounds and pure elements until last.
4)
Balance odd-even problems by placing a “2” in front of the “odd”
formula.
5) When a polyatomic ion occurs on both side, treat it as a group.
6)
Do a final check to make sure that there are the same number of
atoms for each element on each side of the equation.
If a coefficient is “1”, write the “1”. Don’t
leave it blank.
Steps in Balancing An Equation
Not on Handout
Fe3O4 + H2
__Fe3O4 + __H2
Fe + H2O
__Fe + __H2O
Fe:
1 Fe3O4 + __H2
3 Fe + __H2O
O:
1 Fe3O4 + __H2
3 Fe + 4 H2O
H:
1 Fe3O4 + 4 H2
3 Fe + 4 H2O
Treat PI’s as a Group
Mg
+ H3PO4
+ __H3PO4
__Mg
Mg3(PO4)2 + H2
__ Mg3(PO4)2 + __H2
__Mg
+ __H3PO4
1 Mg3(PO4)2 +
__H2
__Mg
+
2 H3PO4
1 Mg3(PO4)2 +
__H2
__Mg
+
2 H3PO4
1 Mg3(PO4)2 +
3 H2
3 Mg
+
2 H3PO4
1 Mg3(PO4)2 +
3 H2
Balancing: PI doesn’t stay as a group
Not on Handout
Sodium reacts with water to form sodium
hydroxide and hydrogen.
Translate into chemical equation:
Na +
H 2O
NaOH
+
H2
Hydroxide on the left does not appear as
hydroxide on the right.
Balance oxygens and hydrogen separately.
Also notice product side: hydrogen occurs in
both formulas.
Balancing: PI doesn’t stay as a group
__Na + __H2O
__NaOH
+ __H2
Use the “odd-even” strategy. There is an odd number of
hydrogens on the right side.
Make the “odd” side even by putting a coefficient of “2” in front of
NaOH
__Na + __H2O
2 NaOH
+ __H2
2 NaOH
+ __H2
Then balance oxygens on the left.
__Na + 2 H2O
Be careful counting the hydrogens: 4 on the left
On the right, 2 hydrogens in 2NaOH + ? H2
Balancing: PI doesn’t stay as a group
__Na + 2 H2O
2 NaOH
+ __H2
Be careful counting the hydrogens: 4 on the left
On the right, 2 hydrogens in 2NaOH + ? H2
You need 1 group of H2 to get a total of 4 on the right.
__Na + 2 H2O
2 NaOH + 1 H2
Finish with sodium
2 Na + 2 H2O
2 NaOH + 1 H2
If a coefficient is “1”, write the “1”. Don’t
leave it blank.
Strategy for Combustion Reactions
Combustion rxns have a hydrocarbon reacting
with oxygen to from carbon dioxide and water.
C8H18 + O2 → CO2 + H2O
Put a coefficient of “1” in front of the hydrocarbon.
Balance the carbons, then balance the hydrogens.
Save oxygen until last. If a “1” doesn’t work, erase
and try a “2”. See board.
More Practice Online:
http://www.sciencegeek.net/APchemistry/APtaters/EquationBalancing.htm