Download How to Balance Chemical Equations

Dr. Chin Chu
River Dell Regional High School
The Principle
 Law of Conservation of Mass: mass is neither created
nor destroyed during any chemical reactions.
 No new atom is created nor old atom destroyed during
any chemical reactions.
Total number for each element involved in any chemical
reactions DOES NOT change, which means the total
number of any given element on the reactant side has to
match that on the product side.
The Application
 The goal – properly balanced chemical equations that
have:
 perfect match of total number of atoms for each element on
both reactant and product sides.
 Simplest set of integers as coefficients.
 For any given chemical reaction, elements and compounds
involved are set and their formulas can not be altered.
Hence, any change to the subscripts is NOT allowed.
 ONLY COEFFICIENTS ARE ALLOWED TO BE
CHANGED!!
The Process
 Write skeleton equation.
 Note: it is of critical importance that correct formulas
to be written for all compounds/elements.
 Make an atom inventory on each side (reactant or
product) for all elements involved in the chemical
reaction.
 Select the element that has different number of atoms
from one side to another.
 Find the least common factors for the two numbers.
The Process (cont.)
 Adjust proper coefficients to get the total number of
atoms match from both sides.
 Each time any coefficient is adjusted, re-do the atom
inventory on that side of the chemical equation.
 Repeat the process until total number of atoms for
each element perfectly matches on both sides of the
chemical equation.
Example I
Matched/Balanced
Skeleton Equation:
2H2(g) + O2(g)  2 H2O(g)
Atom
H 2 [4]
Inventory
O 2 [2]
2
[4]
1
[2]
Still unbalanced.
O is fine now. But H is still out.
BALANCED NOW!
Unbalanced.
Choose O to start.
Exercises
 Balance the following Chemical Equations:
a) N2 + O2  N2O
b) H2O2  H2O + O2
c) N2 + H2  NH3
Exercises
a) N2 + O2  N2O
Exercises (Answers)
 Balance the following Chemical Equations:
a) 2N2 + O2  2N2O
b) 2H2O2  2H2O + O2
c) N2 + 3H2  2NH3
Example II
O should be
balanced last here.
Balance H first.
Skeleton Equation:
C3H8(g) + 5 O2(g)  3CO2(g) + 4H2O(g)
Atom
Inventory
3 [3]
1
[3]
[3]
H 8 [8]
2
[2]
[8]
O 2 [10]
3
[7]
[10]
C
Trick #1: Any element in a chemical
equation is your dear friend.
Balance it last.
Unbalanced.
Which element to be
the first one?
Example III
C2H6(g) +
Atom
Inventory
7
2
O2(g) 
2 CO2(g) + 3H2O(g)
2 [2]
1
[2]
[2]
H 6 [6]
2
[2]
[6]
O 2 [7]
3
[3]
[7]
C
How could one get 7
O from the diatomic
O2 molecule?
C and H are balanced.
But how about O?
The Solution:
7
2
2
7
Example III
2 {
Atom
Inventory
C2H6(g) +
Wait a minute! How
could a coefficient be a
fraction not an integer?
7
2
O2(g)}{2 CO2(g) + 3H2O(g)}
C [4]
[4]
H [12]
[12]
O [14]
[14]
How to resolve this?
Remember, above is a balanced
chemical equations. Hence the
relation still holds after each side is
multiplied by the same factor.
2
Example III
2C2H6(g) + 7 O2(g)  4CO2(g) + 6 H2O(g)
Atom
Inventory
C [4]
[4]
H [12]
[12]
O [14]
[14]
Horary! The chemical equation now
is balanced!
Exercises
Balance the following Chemical Equations:
a) FeS2 + O2  Fe2O3 + SO2
b) C4H10 + O2  H2O + CO2
c) Si2H3 + O2  SiO2 + H2O
d) Fe2O3 + H2  Fe + H2O
e) Fe2O3 + CO  Fe + CO2
f) N2 + O2 + H2O  HNO3
Exercises (Answers)
Balance the following Chemical Equations:
a) 4FeS2 + 11O2  2Fe2O3 + 8SO2
b) 2C4H10 + 13O2  10H2O + 8CO2
c) 2Si2H3 + 11O2  8SiO2 + 6H2O
d) Fe2O3 + 3H2  2Fe + 3H2O
e) Fe2O3 + 3CO  2Fe + 3CO2
f) 2N2 + 5O2 + 2H2O  4HNO3
Example IV
Skeleton Equation:
Ba(ClO3)2(aq) + Na2SO4(aq)  BaSO4(p) + NaClO3(aq)
Ba 1
1
Cl 2
1
Na 2
1
S
1
1
O 10
7
Trick #2: Keep polyatomic ions that
do not change during the
reaction as single units.
Atom
Inventory
Unbalanced.
How to start? They
are a lot of elements!
Wait. Have you seen
the patterns here?
Polyatomic ions do
not change during
the reaction.
Example IV
Treat each
polyatomic ion
as a single unit.
Ba(ClO3)2(aq) + Na2SO4(aq)  BaSO4(p) +2NaClO3(aq)
Atom
Inventory
Ba
ClO3
Na
SO4
1
2
2
1
1
1
1
1
[1]
[2]
[2]
[1]
Now the equation is a
lot easier to balance!
Unbalanced.
Polyatomic ions.
Example V
Wait. Where’s OH
ion?
Skeleton Equation:
Al(OH)3(aq) + H2SO4(aq)  Al2(SO4)3(aq) + H2O(l)
2 Al(OH)3(aq) +3 H2SO4(aq)  Al2(SO4)3(aq) + 6H-OH(l)
Al
Atom
OH
Inventory
H
SO4
1
3
2
1
[2]
[6]
[2]
[1]
[2]
[6]
[6]
[3]
2
1
1
3
[2]
[6]
[6]
[3]
Trick #3: water molecule (H2O) can be treated as H-OH.
Exercises
Balance the following Chemical Equations:
a) AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
b) Fe(NO3)2(aq) + KOH(aq)  Fe(OH)2(s) + KNO3(aq)
c) Pb(NO3)2(aq) + KI(aq)  PbI2(s) + KNO3(aq)
d) FeCl3(aq) + (NH4)2CO3(aq)  Fe2(CO3)3(s) + NH4Cl(aq)
e) H3PO4(aq) + NaOH(aq)  Na3PO4(aq) + H2O(l)
f) Na2CO3(aq) + H2SO4(aq)  Na2SO4(aq) + CO2(g) + H2O(l)
Exercises (Answers)
Balance the following Chemical Equations:
a) AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
b) Fe(NO3)2(aq) + 2KOH(aq)  Fe(OH)2(s) + 2KNO3(aq)
c) Pb(NO3)2(aq) + 2KI(aq)  PbI2(s) + 2KNO3(aq)
d) 2FeCl3(aq) + 3(NH4)2CO3(aq)  Fe2(CO3)3(s) + 6NH4Cl(aq)
e) H3PO4(aq) + 3NaOH(aq)  Na3PO4(aq) + 3H2O(l)
f) Na2CO3(aq) + H2SO4(aq)  Na2SO4(aq) + CO2(g) + H2O(l)