Ch.7– Chemical Reactions
I.
Intro to Reactions
I
II III IV V
A.Signs of a Chemical Reaction
heat and light produced
 Formation of a gas
 Formation of a precipitate
 Color change

B.Law of Conservation of Mass

mass is neither created nor destroyed
in a chemical reaction
total mass stays the same
 atoms can only rearrange

4H
36 g
2O
4H
2O
4g
32 g
C. Chemical Equations
A+B  C+D
REACTANTS
PRODUCTS
C. Chemical Equations
p. 246
D. Writing Equations
2H2(g) + O2(g)  2H2O(g)

Identify the substances involved.

Use symbols to show:
 How many? - coefficient
 Of what? - chemical formula
 In what state? - physical state

Remember the diatomic elements.
Example:
Two atoms of aluminum react with
three units of aqueous copper(II)
chloride to produce three atoms of
copper and two units of aqueous
aluminum chloride.
• How many?
• Of what?
• In what state?
2Al(s) + 3CuCl2(aq)  3Cu(s) + 2AlCl3(aq)
E. Describing Equations

Describing Coefficients:
 individual atom = “atom”
 covalent substance = “molecule”
 ionic substance = “unit”
3CO2  3 molecules of carbon dioxide
2Mg
 2 atoms of magnesium
4MgO  4 units of magnesium oxide
Example
Zn(s) + 2HCl(aq)  ZnCl2(aq) + H2(g)
• How many?
• Of what?
• In what state?
One atom of solid zinc reacts with
two molecules of aqueous
hydrochloric acid to produce one unit
of aqueous zinc chloride and one
molecule of hydrogen gas.
Chemical Reactions
II. Balancing Equations
I
II III IV V
A. Balancing Steps
1. Write the unbalanced equation.
2. Count atoms on each side.
3. Add coefficients to make #s equal.
Coefficient  subscript = # of atoms
4. Reduce coefficients to lowest
possible ratio, if necessary.
5. Check yourself before you wreck
yourself
B. Helpful Tips
one element at a time.
 Update ALL atom counts after adding
a coefficient.
 If an element appears more than
once per side, balance it last.

C. Balancing Example
Aluminum and copper(II) chloride react
to form copper and aluminum chloride.
2 Al + 3 CuCl2  3 Cu + 2 AlCl3
2 1
Al
1 2
3 1
Cu
1 3
6 2
Cl
3 6
Chemical Reactions
III. Types of
Chemical
Reactions
I
II III IV V
A. Combustion

the burning of any substance in O2 to
produce heat
A + O2  B
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)

Products:
 contain oxygen
 hydrocarbons form CO2 + H2O
4 Na(s)+ O2(g)  2 Na2O(s)
C3H8(g)+ 5 O2(g)  3 CO2(g)+ 4 H2O(g)
B. Synthesis

the combination of 2 or more
substances to form a compound

only one product
A + B  AB
H2(g) + Cl2(g)  2 HCl(g)

Products:
 ionic – cross charges
 covalent - hard to tell
2 Al(s)+ 3 Cl2(g)  2 AlCl3(s)
C. Decomposition

a compound breaks down into 2 or
more simpler substances

only one reactant
AB  A + B
2 H2O(l)  2 H2(g) + O2(g)

Products:
 binary - break into elements
 others - hard to tell
2 KBr(l)  2 K(s) +
Br2(l)
D. Single Replacement

one element replaces another in a
compound
 metal replaces metal (+)
 nonmetal replaces nonmetal (-)
A + BC  B + AC
Cu(s) + 2AgNO3(aq)  Cu(NO3)2(aq) + 2Ag(s)

Products:
 metal  metal (+)
 nonmetal  nonmetal (-)
 free element must be more active
(check activity series)
Fe(s)+ CuSO4(aq)  Cu(s)+ FeSO4(aq)
Br2(l)+ NaCl(aq)  N.R.
E. Double Replacement
ions in two compounds “change
partners”
 cation of one compound combines
with anion of the other

AB + CD  AD + CB
Pb(NO3)2(aq) + K2CrO4(aq)  PbCrO4(s) + 2KNO3(aq)

Products:
 switch negative ions
 one product must be insoluble
(check solubility table)
Pb(NO3)2(aq)+ 2KI(aq)  PbI2(s)+2KNO3(aq)
NaNO3(aq)+ KI(aq)  N.R.