10/14/2011
Chapter 9
Chemical Change
Electrolytes and Solution Conductivity
TOP Image
The lightbulb glows when
electrons flows through the
apparatus. The metal strips
are in contact with one
another.
BOTTOM Image
If the metal strips are not in
contact, the electrons can’t
flow through the apparatus
and the bulb does not light.
Figure 9.1-A lightbulb conductivity apparatus
Solutions
Electrolytic Classification
Conductive solutions contain ions.
The extent of the concentration of the ions is
dependent on its relative concentration of each ion.
STRONG Electrolytes is a solution which is a good
conductor. Lots of ions are free in water.
WEAK Electrolytes is a solution which is a poor
conductor. Some ions are free in water.
NONElectrolytes is a solution which is a nonconductor.
No free ions are present in water.
1
10/14/2011
Illustration
Conductivity
Figure 9.2-Nonelectrolytes, strong electrolytes, and weak electrolytes.
Solutions of Ionic Compounds
If an ionic compound is
soluble in water, the
compound will breakdown to
its corresponding ions and
then interact with water.
REACTION:
H2O
NaCl (s) → Na+ (aq) + Cl- (aq)
Figure 9.4-Sodium chloride dissolves
in water. The solution that results
consists of water molecules, sodium
ions, and chloride ions.
Solutions of Ionic Compounds
If an ionic compound is
soluble in water, the
compound will breakdown to
its corresponding ions and
then interact with water.
REACTION:
Figure 9.5-Copper(II) chloride is
dissolved in water. The solution is
made up of water molecules,
copper(II) ions, and chloride ions. The
copper (II) ions and chloride ions are
in 1:2 ratio.
H2O
CuCl2 (s) → Cu2+ (aq) + 2Cl- (aq)
2
10/14/2011
Other EXAMPLES
Salts involving polyatomic ions which can
dissolve in water undergo similar processes.
H 2O
NaOH(s)
→ Na+ (aq) + OH- (aq)
H 2O
K2SO4 (s)
→ 2K+ (aq) + SO42- (aq)
H 2O
(NH4)2CO3 (s)
→ 2NH4+ (aq) + CO32- (aq)
Strong and Weak Acids
An acid is defined as a molecule which releases a
hydrogen ion in water solution.
Acids are in HX, H2X, or H3X.
X may be either a monatomic ion or a polyatomic
ion.
EXAMPLES:
HCl (aq) → H+ (aq) + Cl- (aq)
HNO3 (aq) → H+ (aq) + NO3- (aq)
Strong and Weak Acids
A strong acid is classified as a molecule which is nearly
ionized in solution leading to mostly ions with little unionized molecules in solution.
EXAMPLE:
HCl (aq) → H+ (aq) + Cl- (aq)
A weak acid is classified as a molecule which is slightly
ionized in solution leading to few ions being present in
solution.
HF (aq) → H+ (aq) + F- (aq)
3
10/14/2011
Figure 9.6-Strong and weak acids at the macroscopic and particulate levels.
Strong Acids
Seven Common Strong Acids
Two Well-Known Acids
Three Group 7A/17 Acids
Nitric, HNO3
Hydrochloric, HCl
Two Chlorine Oxyacids
Chloric, HClO3
Sulfuric, H2SO4
Hydrobromic, HBr
Perchloric, HClO4
Hydroiodic, HI
4
10/14/2011
SUMMARY
Identifying the Major Species in a Solution
Ions are the major species in the solutions of two kinds of
substances:
All soluble ionic compounds
The seven strong acids
Neutral molecules are the major species in solutions of
everything else, primarily
Weak acids
Weak bases
Water
Net Ionic Reactions
Typically, we wrote in chapter 8 in a doublereplacement reaction that for a precipitation reaction,
two soluble compounds mix together and after
exchanged ions, the precipitate is identified by the
substance having a state of matter of solid.
EXAMPLE: An aqueous solution of lead(II) nitrate is
added to an aqueous solution of sodium chloride.
REACTION: Pb(NO3)2 (aq) + 2NaCl (aq) →
PbCl2 (s) + 2NaNO3 (aq)
QUESTION: How do we write the net ionic reaction?
Procedure
Writing Net Ionic Reactions
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.
5
10/14/2011
Solution
1st: Write the conventional equation (MOLECULAR Equation)
Pb(NO3)2 (aq) + 2NaCl (aq) →PbCl2 (s) + 2NaNO3 (aq)
2nd : Write all applicable ions in preparing for the total ionic equation.
Pb2+ (aq) + 2NO3- (aq) + 2Na+ (aq) + 2Cl- (aq) → PbCl2 (s) + 2Na+ (aq)+ 2NO3- (aq)
3rd: Write the net ionic equation. (Eliminate all spectator ions and
reduce all coefficients to the lowest terms, if necessary)
Pb2+ (aq) + 2Cl- (aq) → PbCl2 (s)
Practice Problems
Write the net ionic equation for each of the following descriptions.
a)
hydrochloric acid mixes with sodium hydroxide to form liquid
water and an aqueous salt
b)
an aqueous solutions of iron(III) chloride when combined
with silver nitrate forms a white precipitate and an
aqueous solution of iron(III) nitrate
c)
aqueous solutions of nickel(II) nitrate and potassium
carbonate to yield a greenish solid and aqueous potassium
nitrate
d)
solid potassium when dissolved in a solution aluminum nitrate
forms solid aluminum and a solution of potassium nitrate
Single Replacement Reactions
Redox Reactions
When metals dissolves in a solution (i.e. an
aqueous solution like HCl or CuCl2), a transfer
of electrons will take place. Question is will a
chemical reaction occur.
Let’s look at an illustration!!!
6
10/14/2011
Figure 9.8-The reaction between iron and hydrochloric acid.
Single Replacement Reactions
(i.e. Redox 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)
To determine if a chemical
reaction occurs, we need to
look at this activity series to
see if a chemical reaction
will be observed.
The activity series illustrate
reactivity of elements who
easily lose electrons from
strongest to weakest.
Reactivity
Prediction of Products
7
10/14/2011
Sample Example
Will a chemical reaction
take place?
If so, write the net ionic
equation and if not, write
N.R. or no reaction.
Let’s see!!!
Reactivity
Solid calcium is placed in a
solution of aluminum
chloride.
We will approach this
problem in a similar
fashion as we did
earlier with net ionic
equations.
Let’s look at the
results.
Reactivity
Sample Example
Solution
Solution (cont’d)
Conventional Equation “MOLECULAR”
3Ca (s) + 2AlCl3 (aq) → 3CaCl2 (aq) + 2Al (s)
Total Ionic Equation
3Ca (s) + 2Al3+ (aq) + 6Cl- (aq) → 3Ca2+ (aq) + 6Cl- (aq) + 2Al (s)
Net Ionic Equation
3Ca (s) + 2Al3+ (aq) → 3Ca2+ (aq) + 2Al (s)
8
10/14/2011
Sample Example
Solution
Now to confirm that a
reaction will take place,
compare Ca with Al.
Since, Ca > Al in reactivity,
this reaction will take place.
Reactivity
3Ca (s) + 2Al3+ (aq) →
3Ca2+ (aq) + 2Al (s)
All of these reactions do take
place. Please provide me
the net ionic equation for
the following systems using
the activity series shown
here.
a) Solid calcium dissolved
in hydrochloric acid.
b) Solid magnesium is
inserted in an aqueous
solution of zinc nitrate.
Reactivity
Practice Problems
Combustion Reactions
Redox Reactions
Organic molecules which contains carbon, hydrogen and/or
oxygen when burned in oxygen undergo a combustion
reaction.
KEY: When you hear a substance is burn (ing) or oxidized
and the identity of the molecule has C, H, and/or O, you
must know the following is true:
General Equation:
CxHyOz + O2 (g) → CO2 (g) + H2O (l) [or H2O (g)]
EXAMPLE: Burning of methane
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
9
10/14/2011
Practice Problems
Write the complete “burning” of the following
substances:
a) C2H6 (g)
b) C6H6 (l)
c) C4H10 (l)
Double Replacement Reactions
(Precipitation, Gas, etc.)
Reactants:
Solutions of two compounds,
each with positive and negative
ions (AX + BY)
Reaction type:
Double-replacement
Equation type:
AX + BY → AY + BX
Products:
Two new compounds (AY + BX),
which may be a solid, water, an
acid, or an aqueous ionic
compound
Double Replacement Reactions
with Precipitation Reactions
When the cation (positive charged ion) mixes with an anion
(negative charged ion) of another compound, an ioncombination reaction occurs.
This is an example of double-replacement reactions in
which the ions appear to “change partners”:
AX + BY → AY + BX
If a product is comprised of an insoluble ionic compound
that settles at the bottom of a container of mixed solutions,
one calls this solid a “precipitate”, otherwise known as a
precipitation reaction.
Let’s look at an illustration and the solubility table!!!
10
10/14/2011
Figure 9.12-Sodium chloride and silver nitrate solutions are mixed, producing a white
precipitate of silver chloride. Sodium ions and nitrate ions remain in solution.
11
10/14/2011
Practice Problem
Aqueous solutions of calcium acetate and
potassium phosphate are mixed.
Write the net ionic reaction for this reaction, if
any.
SOLUTION
Write the conventional equation first.
REACTION:
Ca(C2H3O2)2 (aq) + K3PO4 (aq) →
Ca3(PO4)2 ( ) + KC2H3O2 ( )
Now, balance the equation.
3Ca(C2H3O2)2 (aq) + 2K3PO4 (aq) →
Ca3(PO4)2 ( ) + 6KC2H3O2 ( )
Let’s look at the solubility table.
SOLUTION
From the solubility table,
3Ca(C2H3O2)2 (aq) + 2K3PO4 (aq) →
Ca3(PO4)2 (s) + 6KC2H3O2 (aq)
Because most phosphates are insoluble except
for ammonium and group IA ions. All acetate
ions are soluble ions.
Now, let’s write the total ionic and net ionic
equations for this system.
12
10/14/2011
SOLUTION
Conventional Equation: “MOLECULAR”
3Ca(C2H3O2)2 (aq) + 2K3PO4 (aq) →
Ca3(PO4)2 (s) + 6KC2H3O2 (aq)
Total Ionic Equation:
3Ca2+ (aq) + 6C2H3O2- (aq) + 6K+ (aq) + 2PO43- (aq)
→ Ca3(PO4)2 (s) + 6K+ (aq) + 6C2H3O2 (aq)
Net Ionic Equation:
3Ca2+ (aq) + 2PO43- (aq) → Ca3(PO4)2 (s)
Practice Problems
Write the net ionic equation, if any, for the
mixing of the following aqueous solutions. If
no net ionic equation is present (no
precipitate), write N.R. (i.e.-no reaction).
a) ammonium carbonate and calcium
carbonate
b) zinc (II) chlorate and sodium sulfide
Double Replacement Reactions with
Molecule-Formation Reactions
When an acid (with H+ ions) dissolves with a
base (with OH- ions), water molecules are
produced instead of a precipitate.
When water is produced, we call this a
neutralization reaction.
Neutralization reaction the most common
molecular-product reactions.
13
10/14/2011
Sample Problem
Write the conventional, ionic, and net ionic
equations between hydrochloric acid and
barium hydroxide.
Solution (cont’d)
Conventional Equation “MOLECULAR”
2HCl (aq) + Ba(OH)2 (aq) → BaCl2 (aq) + 2HOH (l)
Total Ionic Equation
2H+ (aq) + 2Cl- (aq) + Ba2+ (aq) + 2OH- (aq) →
Ba2+ (aq) + 2Cl- (aq) + HOH (l)
Net Ionic Equation
H+ (aq) + OH- (aq) → HOH(l)
Sample Problem
Write the conventional, ionic, and net ionic
equations between acetic acid and potassium
hydroxide.
14
10/14/2011
Solution (cont’d)
Conventional Equation “MOLECULAR”
HC2H3O2 (aq) + KOH (aq) → KC2H3O2 (aq) + HOH (l)
Total Ionic Equation
HC2H3O2 (aq) + K+ (aq) + OH- (aq) →
K+ (aq) + C2H3O2- (aq) + HOH (l)
Net Ionic Equation
HC2H3O2 (aq) + OH- (aq) → C2H3O2- (aq) + HOH(l)
SUMMARY
For a strong acid with a strong base:
NET Ionic Equation:
H+ (aq) + OH- (aq) → HOH(l)
For a weak acid with a strong base:
NET Ionic Equation:
HnA (aq) + OH- (aq) → An- (aq) + HOH(l)
Double Replacement Reactions
with Unstable Reactants
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(g)
NH4+(aq) + OH–(aq) → “NH4OH” → H2O(l) + NH3(g)
15
10/14/2011
Double Replacement Reactions
with Unstable Reactants
EXAMPLE:
Write the conventional, total ionic, and net
ionic equations for the reaction between
potassium carbonate and hydrobromic acid.
Solution (cont’d)
Conventional Equation “MOLECULAR”
K2CO3 (aq) + 2HBr (aq) → H2O (l) + CO2 (g) + 2KBr (aq)
Total Ionic Equation
2K+ (aq) + CO32- (aq) + 2H+ (aq) + 2Br- (aq) →
H2O (l) + CO2 (g) + 2K+ (aq) + 2Br- (aq)
Net Ionic Equation
CO32- (aq) + 2H+ (aq) → H2O (l) + CO2 (g)
Practice Problems
Write the conventional, total ionic, and net ionic
equations to describe the reaction between the
following reactants.
a) Aqueous solutions of ammonium chloride
and sodium hydroxide.
b) Aqueous solutions of lithium sulfite and
sulfuric acid.
c) Aqueous solutions of nitric acid and sodium
carbonate.
16
10/14/2011
Double Replacement Reactions
with Unstable Reactants
Also, keep in mind that in a double
displacement reaction, it is possible that
products might have a combination (s), (l),
(aq), & (g) as to their state in the system.
Remember, consider the solubility table,
and/or these unstable substances (NH4OH,
H2SO3, and H2CO3).
Double Replacement Reactions
with Undissolved Reactants
When you have an acid and a solid component
is placed in the acid, one can still illustrate a
chemical reaction of the process.
EXAMPLE:
Write the net ionic reaction to describe the
the process between nitric acid and solid
lead(II) hydroxide.
Solution (cont’d)
Conventional Equation “MOLECULAR”
2HNO3 (aq) + Pb(OH)2 (s) → Pb(NO3)2 (aq) + 2HOH (l)
Total Ionic Equation
2H+ (aq) + 2NO3- (aq) + Pb(OH)2 (s) →
Pb2+ (aq) + 2NO3- (aq) + HOH(l)
Net Ionic Equation
2H+ (aq) + Pb(OH)2 (s) → Pb2+ (aq) + HOH(l)
17
10/14/2011
Double Replacement Reactions
with Others
We have observed that a chemical reaction
occurs with these type of system but is it possible
no net ionic reaction can occur. Let’s observe an
example.
EXAMPLE:
Write the net ionic reaction to see if aqueous
solutions of potassium carbonate and sodium
chloride will take place.
Solution (cont’d)
Conventional Equation “MOLECULAR”
K3PO4 (aq) + 3NaCl (aq) → Na3PO4 (aq) + 3KNO3 (aq)
Total Ionic Equation
3K+ (aq) + PO43- (aq) + 3Na+ (aq) + 3Cl- (aq) →
3Na+ (aq) + PO43- (aq) + 3K+ (aq) + 3Cl- (aq)
Net Ionic Equation
NONE
SUMMARY
18