CHEMISTRY 4
CHAPTER 9
CHEMICAL CHANGE
1 ELECTROLYTES AND SOLUTION CONDUCTIVITY
Strong electrolytes.
Strong electrolyte is a substance whose solution is a good conductor.
They are completely dissociated into ions. The solute is present entirely as ions.
Weak electrolytes
Weak electrolytes conduct electricity, but poorly.
Most acids like acetic acid are weak electrolytes and are considered weak acids.
Ammonia and its derivatives are weak electrolytes and are considered weak bases
Nonelectrolytes.
Solutions of nonelectrolytes are nonconductors: Sugar
2 SOLUTION OF IONIC COMPOUNDS
When ionic compounds are dissolved in water, the ions become separated from one
another by water molecules. The ions are not formed when an ionic solid dissolves but
become free to move apart in the presence of water. Each ion has a number of water
molecules closely associated with it. We say that each ion is hydrated.
NaCl (s) 
Na+(aq.)
NaNO3(s)  Na+(aq.)
MgCl2(s)  Mg2+(aq.)
+
Cl-(aq.)
+
NO3-(aq.)
+
2 Cl-(aq.)
3 STRONG AND WEAK ACID
Hydrogen chloride HCl, which is a molecular compound, when dissolved in water forms
chloride ions and hydronium ions. Solution of HCl in water is called hydrochloric acid.
All molecules of HCl are converted to ions and we say that hydrochloric acid is a strong
acid.
HCl + H2O  H3O+(aq.)
+ Cl-(aq.)
To simplify sometime we write the dissociation of HCl as
HCl
 H+(aq.)
+ Cl-(aq.)
Only seven commonly encountered acids are strong acids: HCl, HBr, HI, HNO3, H2SO4,
HClO4, HClO3.
Most acids like acetic acid are weak electrolytes and are considered weak acids.
A weak acid is only slightly ionized in solution. Because of the low concentration of
ions, weak acids are poor conductors of electricity.
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4 NET IONIC EQUATIONS
Let us consider the full formula equation (conventional equation) of the reaction
2 NaCl(aq.)
+ Pb(NO3)2(aq.)  2 NaNO3 (aq.)
+ PbCl2(s)
Let us replace the formulas of strong electrolytes by their ions. This produces the full
ionic equation
2 Cl-(aq.) + 2Na+(aq.) + Pb2+(aq.) + 2NO3 -(aq.)  + 2 Na+(aq.) + 2 NO3- (aq.) + PbCl2(s)
Ions that are present at the scene of the reaction but experience no chemical changes are
called spectators ions.
Because spectator ions remain unchanged, we can remove them.
This cancellation leaves the net ionic equation
Pb+2 (aq.)
+ 2 Cl- (aq.)

PbCl2(s)
When writing a net ionic equation be sure that the equation is balanced in both atoms and
charge.
5 SINGLE-REPLACEMENT OXIDATION-REDUCTION REACTION
Let us consider the reaction between zinc and hydrochloric acid. The full formula
equation is:
 ZnCl2(aq.) + H2(g)
Zn(s) + 2 HCl(aq.)
The net ionic equation is
Zn(s) + 2 H+(aq.)
 Zn+2(aq.) + H2(g)
In this reaction, electrons are transferred from zinc(s) to hydrogen ions (hydronium ions)
Electron-transfer reaction is called oxidation-reduction (redox) reaction. The zinc atoms,
which lost electrons, are said to have been oxidized, and hydrogen ions, the receivers of
electrons , have been reduced.
Let us consider the reaction between zinc and copper sulfate. The conventional equation
is:
Zn(s) + CuSO4 (aq.)
 ZnSO4(aq.) + Cu(s)
The net ionic equation is
Zn(s) + Cu+2(aq.)

Zn+2 (aq.)
+ Cu(s)
2
To predict whether or not a redox reaction will take place we can use the following :
activity series
Li > K > Ba > Ca > Na > Mg > Al > Zn > Fe > Ni > Sn > Pb > H2> Cu > Ag > Au
6 OXIDATION-REDUCTION REACTIONS OF SOME ORGANIC COMPOUNDS
When organic compounds react with oxygen to form water and carbon dioxide, we say
they are oxidized. Electrons are transferred from organic compounds to oxygen.
Example 1 Reaction of methane and oxygen
CH4 (g) + 2O2(g) 
CO2(g)
+
2H2O(l)
Example 2 The burning of octane in gasoline
25O2(g)  16CO2(g) + 18H2O (g) + energy
2C8H18 (l) +
Example 3 The burning of sugar in blood
C6H12O6 (aq)
+
6O2(g) 
6CO2(g) + 6H2O (l) + energy
7 DOUBLE-REPLACEMENT PRECIPITATION REACTION
The reaction occurs when the cation from one reactant combines with the anion from the
another to form a precipitate.
NaCl(aq.)
+ AgNO3(aq.)  NaNO3 (aq.) + AgCl(s)
Ag+ (aq.)
+ Cl- (aq.)

AgCl(s)
Solubility rules: The following compounds are insoluble:
Chlorides, bromides, iodides of Ag+, Hg2+2, Pb+2
Sulfates of Ba+2, Sr+2, Ca+2, Pb+2 , Ag+, Hg2+2, Hg+2
Most carbonates, phosphates, hydroxides, sulfides except salt of alkali metals and NH4+
8 DOUBLE-REPLACEMENT MOLECULE-FORMATION REACTION
Water and weak acids are the most common kinds of molecular products of reaction
Example:
HCl(aq.) + NaOH(aq.) H2O(l) + NaCl(aq.)
H+(aq.) +
OH-(aq.) 
H2O(l)
3
+ NaCH3 COO(aq.)
HCl(aq.)
HCH3COO-(aq.)
CH3 COO-(aq.) 
H+(aq.) +
+
NaCl(aq.)
HCH3COO-(aq.)
9 DOUBLE REPLACEMENT REACTION THAT FORM UNSTABLE PRODUCTS
Ion combination may produce an unstable product that will further decompose into a gas
Examples
+
CO3-2 (aq.) 
H2CO3 (aq.)  CO2 (g) + H2O(l)
2 H+ (aq.) +
SO3-2(aq.) 
H2SO3(aq.)  SO2 (g) + H2O(l)
2 H+(aq.)
NH4+ (aq.) +
OH-(aq.) 
NH3 (aq.) + H2O(l)
10 DOUBLE REPLACEMENT REACTION WITH UNDISSOLVED REACTANTS
Hydrochloric acid can dissolve aluminum hydroxide
3 HCl (aq.) +
3 H+(aq.) +
Al(OH)3 (s) 
3 H2O (l) + AlCl3 (aq.)
Al(OH)3 (s)  3 H2O (l) + Al3+ (aq.)
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