1. No category

Mark S. Cracolice
Edward I. Peters
www.cengage.com/chemistry/cracolice
Chapter 9
Chemical Change
Mark S. Cracolice • The University of Montana
Electrolytes & Conductivity
Electrolytes & Conductivity
Electrolytes & Conductivity
Electrolytes & Conductivity
Conducting solutions contain ions.
Strong electrolyte: A substance whose solution is a good
conductor.
Weak electrolyte: A substance whose solution conducts
electricity poorly.
Nonelectrolyte: A substance whose solution is a nonconductor.
Solutions of Ionic Compounds
Sodium Chloride Dissolves in Water
The solution that results consists of water molecules,
sodium ions, and chloride ions:
Solutions of Ionic Compounds
Copper(II) Chloride Dissolves in Water
The solution that results consists of water molecules,
copper(II) ions, and chloride ions:
Solutions of Ionic Compounds
When an ionic compound dissolves in water, its solution consists
of water molecules and ions.
KOH(s) ® K+(aq) + OH–(aq)
(NH4)2SO4(s) ® 2 NH4+(aq) + SO42–(aq)
Strong and Weak Acids
Acid
A molecule or ion that releases a hydrogen ion in water solution.
Solution of HCl in water is called hydrochloric acid.
All molecules of HCl are converted to ions
HCl
+
H2O (l)

H3O+(aq.)
+ Cl-(aq.)
To simplify sometime we write the dissociation of HCl as
HCl(aq)  H+(aq) + Cl–(aq)
For nitric acid:
HNO3(aq)  H+(aq) + NO3–(aq)
Strong and Weak Acids
Acids are classified as strong or weak.
Strong acid
All molecules of a strong acid are converted to ions:
Only seven common acids are strong acids:
HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3.
Strong and Weak Acids
Weak acid
• Most acids like acetic acid, benzoic acid are weak electrolytes
and are considered weak acids.
• A weak acid is only slightly ionized in solution.
HAc(aq)
H+(aq) + Ac–(aq)
• Because of the low concentration of ions, weak acids are poor
conductors of electricity.
Strong and Weak Acids
Strong and Weak Acids
Identifying the Major Species in a Solution
Ions are the major species in the solutions of :
All soluble ionic compounds
Strong acids: HCl, HBr, HI, HNO3, H2SO4, HClO4, HClO3.
Neutral molecules are the major species in solutions of
everything else, primarily
Water, Weak acids, Weak bases
Net Ionic Equations
Consider the reaction of solutions of silver nitrate and sodium chloride.
AgNO3(aq) + NaCl(aq) 
AgCl(s) + NaNO3(aq)
Silver nitrate solution, AgNO3(aq):
Silver ions, Ag+(aq)
Nitrate ions, NO3–(aq)
Water molecules
Sodium chloride solution:
Sodium ions, Na+(aq)
Chloride ions, Cl–(aq)
Water molecules
Net Ionic Equations
How are these represented in a conventional equation?
AgNO3(aq) + NaCl(aq)
The problem with this conventional representation is that it
disguises the true composition of the solutions.
Net Ionic Equations
To avoid this misrepresentation, we can write the formulas of the
reactants as they actually occur in solution:
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq)
Net Ionic Equations
We can perform a similar analysis
with the products of the reaction.
Silver chloride precipitate:
Solid silver chloride, AgCl(s)
Sodium nitrate solution:
Sodium ions, Na+(aq)
Nitrate ions, NO3–(aq)
Water molecules
Net Ionic Equations
Conventional representation of products:
AgCl(s) + NaNO3(aq)
Products as they actually occur:
AgCl(s) + Na+(aq) + NO3–(aq)
Net Ionic Equations
Combining reactants and products:
AgNO3(aq) + NaCl(aq) ® AgCl(s) + NaNO3(aq)
This is the conventional equation.
It is good for stoichiometry problems (Ch. 10),
but it lacks in revealing what really happens in solution.
Net Ionic Equations
Writing actual reactants and products:
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) ®
AgCl(s) + Na+(aq) + NO3–(aq)
This is the total ionic equation.
It represents each species as it actually occurs in solution.
Net Ionic Equations
Note that some species appear as both
reactant and product in the total ionic equation:
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) ®
AgCl(s) + Na+(aq) + NO3–(aq)
NO3–(aq) is both reactant and product;
Na+(aq) is both reactant and product.
Neither ion undergoes a chemical change.
Net Ionic Equations
If we eliminate these non-reacting spectator ions,
Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) ®
AgCl(s) + Na+(aq) + NO3–(aq)
What remains is the net ionic equation:
Ag+(aq) + Cl–(aq) ® AgCl(s)
Net Ionic Equations
A net ionic equation describes
what happens in a chemical change:
Ag+(aq) + Cl–(aq) ® AgCl(s)
Aqueous silver ion reacts with aqueous chloride ion
to yield solid silver chloride.
A net ionic equation ignores non-reacting species:
Sodium ion
Nitrate ion
Water molecules
Net Ionic Equations
Writing a Net Ionic Equation
1.
Write the conventional equation, including state symbols—(g), (l), (s), and
(aq). Balance the equation.
2.
Write the total ionic equation by replacing each aqueous (aq) substance
that is a strong acid or a soluble ionic compound with its major species.
Do not separate a weak acid into ions, even though its state is aqueous
(aq). Also, never change solids (s), liquids (l), or gases (g) into ions. Be
sure the equation is balanced in both atoms and charge.
3.
Write the total ionic equation by removing the spectators from the total
ionic equation. Reduce coefficients to lowest terms, if necessary. Be sure
the equation is balanced in both atoms and charge.
Single-Replacement Redox
Single-Replacement Reactions
Reactants:
Element (A) plus a solution of either an acid
or an ionic compound (BX)
Reaction type:
Single-replacement
Equation type:
A + BX ® AX + B
Products:
An ionic compound (usually in solution) (AX)
plus an element (B)
Single-Replacement Redox
Example:
Let us consider the reaction between iron and hydrochloric acid.
The full formula equation is:
Fe(s) + 2 HCl(aq.)
 FeCl2(aq.)
+ H2(g)
Single-Replacement Redox
The iron atoms, which lost electrons, are said to have been
oxidized
Fe(s) – 2e
 Fe+2(aq.)
and hydrogen ions, the receivers of electrons , are said to
have been reduced.
2 H+(aq.) + 2e  H2(g)
Single-Replacement Redox
Let us consider the reaction between zinc and copper sulfate.
The conventional equation is:
Zn(s)
 ZnSO4(aq.)
+ CuSO4 (aq.)
+ Cu(s)
The net ionic equation is
Zn(s) + Cu+2(aq.)
Oxidation:
Reduction:
Zn(s)
Cu+2(aq.)

Zn+2 (aq.)
– 2e 
+ 2e 
+
Zn+2 (aq.)
Cu(s)
Cu(s)
Single-Replacement Redox
Let us consider the reaction between copper and silver nitrate.
The conventional equation is:
Cu(s)
 Cu(NO3)2(aq.)
+ 2 AgNO3(aq.)
The net ionic equation is
Cu(s) + 2 Ag+(aq.)
Oxidation:
Reduction:
Cu(s)
2 Ag+(aq.)

+
2e
2e
Cu+2 (aq.)


+ 2 Ag(s)
+
2Ag(s)
Cu+2 (aq.)
2Ag(s)
Single-Replacement Redox
The activity series lists the relative
reactivity of species that form
positive ions in a singlereplacement redox reaction.
Note that Cu is above Ag and under
Zn on the activity series.
This means that Cu replaces Ag in a
single-replacement reaction.
If solid copper was placed in a zinc
chloride solution, there would be
no reaction.
Double Replacement Precipitation Reactions
These are double-replacement reactions:
AX + BY  AY + BX
When one product is an insoluble ionic compound, the reaction is
called a precipitation reaction and the solid is called a
precipitate.
Double Replacement Precipitation Reactions
• The reaction occurs when the cation from one reactant
combines with the anion from the another to form a precipitate.
• Example 1: reaction of sodium chloride and silver nitrate
NaCl(aq.)
+ AgNO3(aq.)
Ag+ (aq.)
+
 NaNO3 (aq.)
Cl- (aq.)
.

+ AgCl(s)
AgCl(s)
Double Replacement Precipitation Reactions
Example 2: reaction of sodium sulfate and barium chloride
Na2SO4 (aq) + BaCl2(aq)  BaSO4 (s) + 2 NaCl(aq)
Net ionic reaction
Ba2+ (aq.)

+ SO4 2-(aq.)
.
BaSO4 (s)
Double Replacement Precipitation Reactions
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+
Double Replacement Precipitation Reactions
Double Replacement Precipitation Reactions
Double Replacement Molecule-Formation
Reactions
A double-replacement reaction
AX + BY  AY + BX
can lead to the formation of a molecular compound such as
water, a weak acid or a weak base.
Double Replacement Molecule-Formation
Reactions: Formation of water
Example 1: Reaction of a strong acid and a base to form water
HCl(aq.) + NaOH(aq.) H2O(l) + NaCl(aq.)
H+(aq.)
+
OH-(aq.) 
H2O(l)
Example 2 : Reaction of a weak acid and a base to form water
HCH3 COO(aq.) + NaOH(aq.)  NaCH3COO(aq.) + H2O(l)
HCH3COO(aq.) + OH-(aq.) 
CH3 COO-(aq.) + H2O(l)
Double Replacement Molecule-Formation
Reactions: Formation of a weak acid
Example 3 : Reaction of hydrochloric acid with sodium benzoate
HCl(aq) + NaC7H5O2(aq) ® HC7H5O2(aq) + NaCl(aq)
Total ionic equation:
H+(aq) + Cl–(aq) + Na+(aq) + C7H5O2–(aq) ®
HC7H5O2(aq) + Na+(aq) + Cl–(aq)
Net ionic equation:
H+(aq) + C7H5O2–(aq) ® HC7H5O2(aq)
The H+(aq) combines with the C7H5O2–(aq) to form the molecular
product benzoic acid HC7H5O2(aq).
Double Replacement Molecule-Formation
Reactions
Example 4 : Reaction to form a weak acid: acetic acid
HCl(aq.) + NaCH3 COO(aq.) HCH3COO(aq.) + NaCl(aq.)
H+(aq.)
+
CH3 COO-(aq.) 
HCH3COO(aq.)
Example 5: Reaction to form a weak base: ammonia NH3
NH4 Cl(aq.) + NaOH(aq.) NH3(aq.) + H2O(l) + NaCl(aq.)
NH4+(aq.)
+
OH-(aq.)  NH3(aq.) + H2O(l)
Double Replacement Reactions That Form
Unstable Products.
A double-replacement reaction
AX + BY  AY + BX
can lead to the formation of a molecular compound that will
further decompose into a gas
H2CO3(aq)  H2O(l) + CO2(g)
H2SO3(aq)  H2O(l) + SO2(g)
2 HNO2(aq)  H2O(l) + N2O3(g
Double Replacement Reactions That Form
Unstable Products.
Example: The reaction that occurs when hydrochloric acid and
sodium carbonate solutions are combined.
Conventional double-replacement equation:
2 HCl(aq) + Na2CO3(aq)  H2CO3(aq) + 2 NaCl(aq)
Recognize H2CO3(aq) as an unstable product, replace it with the
actual products:
2 HCl(aq) + Na2CO3(aq)  H2O(l) + CO2(g) + 2 NaCl(aq)
Net ionic equation
2 H+(aq) + CO32–(aq)  H2O(l) + CO2(g)
Double Replacement Reactions That
Form Unstable Products.
Write the net ionic equation for the reaction that occurs when
hydrochloric acid and sodium sulfite solutions are combined.
Conventional double-replacement equation:
2 HCl(aq) + Na2SO3(aq) ® H2SO3(aq) + 2 NaCl(aq)
Recognize H2SO3(aq) as an unstable product, replace it with the
actual products:
2 HCl(aq) + Na2SO3(aq) ® H2O(l) + SO2(aq) + 2 NaCl(aq)
Double Replacement Reactions That Form
Unstable Products.
2 HCl(aq) + Na2SO3(aq) ® H2O(l) + SO2(aq) + 2 NaCl(aq)
Total ionic equation:
2 H+(aq) + 2 Cl–(aq) + 2 Na+(aq) + SO32–(aq) ®
H2O(l) + SO2(aq) + 2 Na+(aq) + 2 Cl–(aq)
Net ionic equation: Eliminate the spectator ions:
2 H+(aq) + SO32–(aq) ® H2O(l) + SO2(aq)
Hydrogen ion combines with sulfite ion to form liquid water and
aqueous sulfur dioxide.
Double Replacement Reactions That
Form Unstable Products.
Three ion combinations yield molecular products
that are not the products you would expect.
You must be alert and catch them when they appear:
2 H+(aq) + CO32–(aq) ® H2CO3(aq) ® H2O(l) + CO2(g)
2 H+(aq) + SO32–(aq) ® H2SO3(aq) ® H2O(l) + SO2(aq)
NH4+(aq) + OH–(aq) ® “NH4OH” ® H2O(l) + NH3(aq)
Double Replacement Reactions with
Undissolved Reactants
Write the net ionic equation for the reaction that occurs when
solid sodium hydroxide is added to a hydrochloric acid
solution.
Conventional equation:
HCl(aq) + NaOH(s)  H2O(l) + NaCl(aq)
Total ionic equation
H+(aq) + Cl–(aq) + NaOH(s)  H2O(l) + Na+(aq) + Cl–(aq)
Net ionic equation
H+(aq) + NaOH(s)  H2O(l) + Na+(aq)
Double Replacement Reactions with
Undissolved Reactants
Sometimes, in a double-replacement reaction,
a reactant is a solid.
Hydrochloric acid can dissolve insoluble aluminum hydroxide
3 HCl (aq.) +
Al(OH)3 (s) 
3 H2O (l) +
AlCl3 (aq.)
When writing ionic equation do not replace solid by its ions
3 H+(aq.)
+
Al(OH)3 (s)  3 H2O (l) +
Al3+ (aq.)
Double Replacement Reaction that
will not happen
Write the net ionic equation for the reaction that would occur
when sodium chloride and potassium nitrate solutions are
mixed.
Conventional equation:
NaCl(aq) + KNO3(aq)  NaNO3 + KCl
Reactants and products are strong electrolytes and soluble:
NaCl(aq) + KNO3(aq)  NaNO3(aq) + KCl(aq)
Total ionic equation:
Na+(aq) + Cl–(aq) + K+(aq) + NO3–(aq) 
Na+(aq) + NO3–(aq) + K+(aq) + Cl–(aq)
All ions are spectators. There is no reaction
Double Replacement Reaction that will
not happen
Example:
Write the net ionic equation for the reaction that occurs when
iron(II) sulfate and magnesium nitrate solutions are mixed.
Solution:
Conventional equation:
FeSO4(aq) + Mg(NO3)2(aq) ® Fe(NO3)2( ) + MgSO4( )
From the solubility table and guidelines, both products are
soluble:
FeSO4(aq) + Mg(NO3)2(aq) ® Fe(NO3)2(aq) + MgSO4(aq)
Double Replacement Reaction that will
not happen
FeSO4(aq) + Mg(NO3)2(aq) ® Fe(NO3)2(aq) + MgSO4(aq)
Total ionic equation:
Fe2+(aq) + SO42–(aq) + Mg2+(aq) + 2 NO3–(aq) ®
Fe2+(aq) + 2 NO3–(aq) + Mg2+(aq) + SO42–(aq)
Net ionic equation: Eliminate the spectator ions:
All ions are spectators!
There is no reaction when these two solutions are combined.
The conventional equation disguises this fact, but the net ionic
equation helps to make it more apparent.
Summary