How molecules are symbolized
Cl2
2Cl
2Cl2
• Molecules may also have brackets to
indicate numbers of atoms. E.g. Ca(OH)2
• Notice that the OH is a group
O Ca O
H
• The 2 refers to both H and O H
• How many of each atom are in the following?
a) NaOH
Na = 1, O = 1, H = 1
b) Ca(OH)2 Ca = 1, O = 2, H = 2
c) 3Ca(OH)2 Ca = 3, O = 6, H = 6
Balancing equations: MgO
Dalton: chemical reactions are the result of a
rearrangement of atoms.
Lavoisier: Conservation of Mass – “What goes in must
come out” Number and types of atoms must be
conserved.
Thus, the number of a particular atom is the same on
both sides of a chemical equation
• Example: Magnesium + Oxygen Mg + O2  MgO
Mg + O O
• However, this is not balanced
• Left:
Mg = 1, O = 2
• Right: Mg = 1, O = 1
 Mg O
Representing Reactions
Word Equation:
Magnesium + hydrochloric acid  magnesium
chloride + hydrogen
Skeleton Equation:
Mg (s) + HCl (aq)  H2 (g) + MgCl2 (aq)
When going from word equations to skeleton
equations, don’t forget about HOBrFINCl gases!
Balancing Equation
Guidelines:
1. Balance the element with the greatest
number of atoms (not H or O!)
2. Balance other elements in
compounds. If an element is
uncombined, usually leave it until last.
3. Balance H or O, if they are combined
in compounds
4. Balance uncombined elements
5. Check
a) P4 + 5 O2  P4O10
b) 2 Li + 2 H2O  H2 + 2 LiOH
c) 2 Bi(NO3)3 + 3 K2S  Bi2S3 + 6 KNO3
d) C2H6 +3.5 O2  2 CO2 + 3 H2O
2 C2H6 + 7 O2  4 CO2 + 6 H2O
Balance these skeleton equations:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Mg + HCl  MgCl2 + H2
Ca + N2  Ca3N2
NH4NO3  N2O + H2O
BiCl3 + H2S  Bi2S3 + HCl
C4H10 + O2  CO2 + H2O
O2 + C6H12O6  CO2 + H2O
NO2 + H2O  HNO3 + NO
Cr2(SO4)3+ NaOH  Cr(OH)3+ Na2SO4
Al4C3 + H2O  CH4 + Al(OH)3
Balance these skeleton equations:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Mg + 2HCl  MgCl2 + H2
3Ca + N2  Ca3N2
NH4NO3  N2O + 2H2O
2BiCl3 + 3H2S  Bi2S3 + 6HCl
2C4H10 + 13O2  8CO2 + 10H2O
6O2 + C6H12O6  6CO2 + 6H2O
3NO2 + H2O  2HNO3 + NO
Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
Al4C3 + 12H2O  3CH4 + 4Al(OH)3
Returning to reaction types
• We have looked at several types of
reactions without worrying about balancing
• However, all equations should be balanced
• Predict the products and balance these:
(recall, metals above replace metals below,
reactions with water yield metal hydroxides)
2 Fe + 3 CuSO4  3 Cu +
Fe2(SO
)3
Ni + NaCl  NR
(no 4reaction)
2 Al + 3 CuCl2  3 Cu + 2AlCl3
2 Li + ZnCO3  Zn +
Li2CO3+ ½H2
2 Li + 2 H2O
 2 LiOH
 2 Al2O
4 Al + 3O2
K
Na
Li
Ca
Mg
Al
Zn
Fe
Ni
Sn
Pb
H
Cu
Hg
Ag
Au
a)
b)
c)
d)
e)
f)
g)
h)
i)
Here are some more to complete &
balance:
KNO3 
Pb(NO3)2 
P4 + I2 
MgO + H3PO4 
Br2 + KI 
Ca(OH)2 + HNO3 
Bi2O3 + H2 
Fe + O2 
CaO + C 
Here are some more to balance:
a)
b)
c)
d)
e)
f)
g)
h)
i)
2KNO3  2KNO2 + O2
2Pb(NO3)2  2PbO + 4NO2 + O2
P4 + 6I2  4PI3
3MgO + 2H3PO4  Mg3(PO4)2 + 3H2O
Br2 + 2KI  I2 + 2KBr
Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O
Bi2O3 + 3H2  2Bi + 3H2O
3Fe + 2O2  Fe3O4
2CaO + 5C  2CaC2 + CO2