UNIT 8 - Chemical Equations
BALANCING EQUATIONS
Q: WHY DO
WE NEED TO BALANCE CHEMICAL EQUATIONS?
A: The LAW OF CONSERVATION OF MASS says that matter cannot be created or
destroyed. In other words, you
cannot end up with any more or any less
than you started with.
Q: HOW DO YOU BALANCE AN EQUATION?
A: (1) Count up the number of atoms of each element (or polyatomic ion) on each
side of the arrow in the
equation (eqn).
(2) Use coefficients (numbers in FRONT of the element or compound)
to balance the numbers on either side of
the eqn.
(3) Do not ever change subscripts (formulas) in a compound!
Q: WHAT ARE
"REACTANTS" & "PRODUCTS"?
A: In a chemical eqn, reactants
are on the left side of the arrow à products are
on the right
EXAMPLE
#1: ___ Na + ___ Br2
à ___ NaBr
STEP 1: Set up a chart with # of atoms of each element on each side of eqn.
Reactants | Products
Na 1 1
Br
2 1
STEP 2: Balance one of the elements that is not balanced. In this case, that is the
(Reactant side has more than product side, so coefficient should go on the
product side.)
___ Na + ___ Br2
à _2_ NaBr
* Reminder: the coefficient gets multiplied by subscripts of all elements
in the compound it
is in front of. *
Reactants | Products
Na 1 2
Br 2
2
STEP 3: Check all elements to see if they are balanced. Na is not balanced, so it needs a coefficient
of 2.
_2_ Na + ___
Br2 à _2_
NaBr
Reactants | Products
Na 2 2
Br 2 2
EXAMPLE #2:
___ Fe(NO3)2 + ___ Na3PO4
à ___ Fe3(PO4)2
+ ___ NaNO3
reactants
| products
Fe
1
3
NO3
2
1
Na
3
1
PO4
1 2
* because there is oxygen in every compound in the equation, it may be helpful
to count the number of a polyatomic ion, rather than splitting the polyatomic
ion into its elements and then counting.*
_3_ Fe(NO3)2 + ___ Na3PO4
à ___ Fe3(PO4)2
+ ___ NaNO3
reactants | products
Fe
3
3
NO3
6
1
Na
3
1
PO4
1 2
_3_ Fe(NO3)2 + ___ Na3PO4
à ___ Fe3(PO4)2
+ _6_ NaNO3
reactants | products
Fe
3
3
NO3
6
6
Na
3
6
PO4
1 2
_3_ Fe(NO3)2 + _2_ Na3PO4
à ___ Fe3(PO4)2
+ _6_ NaNO3
reactants | products
Fe
3
3
NO3
6
6
Na 6
6
PO4
2 2
Finished!
Now, you try these examples:
1.) ___ HgO + ___ Cl2 à
___ HgCl + ___ O2
2.) ___ C3H8 + ___ O2
à ___ CO2 + ___
H2O
**HINT:
Balance the H's and O's last.**
3.) ___ KClO3 à ___ KCl
+ ___ O2
4.) ___ Ca(OH)2 + ___ HNO3
à ___ Ca(NO3)2
+ ___ H2O
5.) ___ Al2O3 à
___ Al + ___ O2
6.) ___ CuCl2 + ___ H2S
à ___ CuS + ___ HCl
7.) ___ Cl2 + ___ NaBr à
___ NaCl + ___ Br2
8.) ___ NaOH + ___ HCl à
___ NaCl + ___ H2O
9.) ___ Na2O + ___ CO2
à ___ Na2CO3
10.) ___ H2O + ___ Fe à
___ Fe2O3 + ___ H2
TYPES
OF EQUATIONS
DECOMPOSITION:
a compound breaks apart into simpler substances
* To recognize a DECOMPOSITION reaction, look for only 1 REACTANT. *
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SYNTHESIS:
2 or more simple substances combine to form one compound; opposite of
decomposition
* To recognize a SYNTHESIS reaction, look for only 1 PRODUCT. *
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SINGLE
REPLACEMENT: an element reacts with a compound to form a new element
&
a new compound
* To recognize a SINGLE REPLACEMENT reaction, look for one element and one
compound as the reactants. *
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COMBUSTION:
an organic compound (usually a hydrocarbon) combines with oxygen to
produce carbon dioxide and water
* To recognize a COMBUSTION reaction, the reactants will be an organic compound
and
oxygen. *
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DOUBLE
REPLACEMENT: a compound reacts with another compound to form 2 new
compounds
* To recognize a DOUBLE REPLACEMENT reaction, look for 2 compounds as the
reactants. *
PREDICTING WHETHER A
SINGLE REPLACEMENT REACTION WILL OCCUR
The
reactants in a single replacement reaction are an element (by itself) and a
compound. Some single replacement
reactions will happen, others will not.
In order to determine if a single replacement reaction will occur, you must
use the Activity Series. (It is located
on page 7 of the Reference Tables, but I have included one here so you don’t
have to go searching for it.)