Balancing Equations
Chemical Reactions

Chemical rxns occur when bonds (between
electrons of atoms) are formed or broken

Chemical rxns involve:
 changes

making new materials with new properties
 energy


in the chemical composition of matter
changes:
Bond breaking absorbs Energy (endothermic)
Bond making releases Energy (exothermic)
Note:
The overall change in energy for certain rxns can be
found on Ref. Table I
Chemical Equations
Depict the kind of reactants and products
and their relative amounts in a reaction.
4 Al (s) + 3 O2 (g) → 2 Al2O3 (s)
The numbers in the front are called
stoichiometric coefficients
Symbols Used in Equations
Solid
(s), Liquid (l), Gas (g)
Aqueous
solution (aq) (dissolved in water)
Catalyst
H2SO4
Escaping
gas ()
Change
 (

or
Pt
of heat energy
or + 3kJ or – 3kJ)
*there is no subtraction…a negative sign means released/exothermic

Express a chemical equation as follows:
Reactants  Products

The arrow is equivalent to an “=“ math. When
describing the equation use the word “yields” or
“produces” instead of equals
Ex:
C + O2  CO2
This reads “carbon plus oxygen react to yield carbon dioxide”

Because of the principle of the
conservation of matter an
equation must be balanced.
 matter can’t be created or
destroyed

Must have the same number of
each type of atom on both sides.

Must have same # grams of
matter on both sides!!
Lavoisier, 1788
Coefficients not Subscripts

When balancing add coefficients in front of
compounds to balance the atoms in the
reaction
 DO NOT change the subscripts of the
formulas.
 Changing
subscripts changes the compound.


Subscripts: tell you how many atoms of a particular
element are in a compound.
Coefficient: tells you about the quantity, or number, of
molecules of the compound.
Balancing a chemical equation:
DO NOT CHANGE THE FORMULAS!


Find number of atoms for an element on left side.
Compare those against number of the atoms of the same
element on right side.

Place coefficients in front of formulas so that left side has
the same number of atoms as the right side for EACH
element

Check your answer to see if:
 The numbers of atoms on both sides of the equation
balanced.
 The coefficients are in the lowest possible whole
number ratios.
Helpful hints for balancing equations:
 Take
one element at a time, working left to right.
 Save
H for next to last, and O until last.
 IF
everything balances except for O, and there is no way
to balance O with a whole number, double all the
coefficients and try again.

Useful in combustion reactions!
 (Shortcut)
Polyatomic ions that appear on both sides of
the equation should be balanced as independent units

Just circle them and think of them as one thing
Balancing Equations
___ H2(g) + ___ O2(g) → ___ H2O(l)
What happened to the other Oxygen atom?????
This equation is not balanced!
Balance Equation
Na + Cl2
Na = 1
Cl = 2
NaCl
Na = 1
Cl = 1
The number of sodium atoms balance but
the chlorine does not. Use coefficients in
order to balance this equation.
Inserting Coefficients
Na + Cl2
2 NaCl
Na = 1
Cl = 2
Na = 2
Cl = 2
Now chlorine balances but the sodium does
not! So we go back and balance the sodium.
Finally balanced!
2Na + Cl2
2 NaCl
Na = 2
Cl = 2
Na = 2
Cl = 2
Since the number of each element on the
reactant side and the product side of the
equation are equal, the equation is balanced.
Balance Practice
(Page 2 in Chemical Equations Packet)
___Al2O3 + ___H2 → ___H2O + ___Al
___MgI2 + ___K → ___KI + ___Mg
___AlF3 → ___Al + ___F2
Balance Practice
(Page 2 in Chemical Equations Packet)
___Na2CO3 + ___Mg → ___MgCO3 + ___Na
___ZnO → ___Zn + ___O2
___Fe + ___O2 → ___Fe2O3
Balancing Practice

For more help go to:
http://richardbowles.tripod.com/chemistry/
balance.htm#part0

For some fun balancing equations go to:
http://www.mpcfaculty.net/mark_bishop/b
alancing_equations_tutorial.htm
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