# 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 Br.
(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  +  ___ H2
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.)

# ACTIVITY SERIES OF HALOGENSF2                                Cl2                                Br2                                I2

EXAMPLE 1 – Will the following reaction happen or not?                             Al  +  PbCl2 à

Step 1 – Look at the element by itself.  Is this element a metal or a nonmetal?  Al (aluminum) is a metal because it is located to the left side of the staircase line on the Periodic Table.
Step 2 – You will compare the type of element by itself to the similar type of element in the compound.  In this case, aluminum is a metal, so I will compare it with the metal in the compound (which is Pb).

Step 3 – RULE:  The element that is by itself must be HIGHER on (closer to the top of) the Activity Series for the reaction to happen!  In this case…

Al             (by itself)
Mn
Zn
Cr

Fe
Cd                                                           Replace hydrogen from steam
--------------------------------------------------------------------------------------------------------------------
Co
Ni
Sn
Pb            (in compound)

Aluminum (Al) is higher than lead (Pb), so the reaction happens.

What are the products of the reaction?

Aluminum and lead “switch places”.  However, remember to use the oxidation numbers of the elements to write the formulas.  Then, balance the equation.

Al  +  PbCl2  à  Pb  +  AlCl3                  (aluminum has a +3 oxidation number)

Balanced…        2 Al  +  3 PbCl2  à  3 Pb  +  2 AlCl3

EXAMPLE 2 – Will the following reaction happen or not?                             Fe + Mg(NO3)2  à

Answer:  Iron (Fe) needs to be higher on the Activity Series than magnesium (Mg) in order for the reaction to occur.  It is not, so there is no reaction.

EXAMPLE 3 – Will the following reaction happen or not?                             NaCl  +  Br2  à

Answer:  Br2 is the element by itself.  It is a nonmetal, so I have to compare it to the nonmetal in the compound – which is Cl.  Br needs to be higher than Cl in order for the reaction to happen.  It is not, so there’s no reaction.

EXAMPLE 4 – Will the following reaction happen or not?                             AlBr3  +  F2  à

Answer:  F2 is the element by itself.  It is a nonmetal, so I have to compare it to the nonmetal in the compound – which is Br.  F needs to be higher than Br in order for the reaction to happen.  It is, so the reaction happens.  Fluorine and bromine “switch places”.  Remember to use the oxidation numbers of the elements to write the formulas.  Also, remember that fluorine, chlorine, bromine, and iodine are DIATOMIC – that means that these elements will have a subscript of 2 when they are by themselves.

AlBr3  +  F2  à  Br2  +  AlF3

Balanced…        2 AlBr3  +  3 F2  à  3 Br2  +  2 AlF3

Here are a few for you to try…

1.         Al  +  NiBr2  à
2.         Fe2(SO4) 3  +  Ag
à
3.         NaCl  +  F2
à
4.         Ba(NO3) 2  +  Ni
à
5.         I2  +  AlBr3
à
6.         Ca  +  KCl
à
7.         Mg  +  Zn3(PO4) 2
à
8.         Na  +  SnCO3
à