Solutions

1
Solutions
Solution A homogeneous mixture of two or more
pure substances.
Solute The substance being dissolved
Solvent The substance doing the dissolving.

• Aqueous solutions The solvent is water.

2
The Water Molecule
.

3
The Water Molecule
Water is a polar molecule that forms hydrogen
bonds

4
The Crystalline Lattice Structure of Sodium
Chloride
5
Aqueous Sodium Chloride

• The polar nature of water allows it to insert
  itself between the Na cations and the Cl-
  anions in NaCl (s)

6
Aqueous Dissociation of Ionic Salts
NaCl (s) -----gt Na (aq) Cl- (aq)
CaCl2 (s) -----gt
Ca2 (aq) 2 Cl- (aq)
Mg(NO3)2 (s) ----gt
Mg2 (aq) 2 NO3-1 (aq)
Solutions that contain aqueous ions will conduct
electricity and are called electrolytes
Molecular solids (like sugar) do not dissociate
into aqueous ions. They are called non-
electrolytes
7
Solubility The maximum amount of solute that
will dissolve in a given volume of solvent at a
given temperature.

• Not all pure substances will dissolve to the
  same extent in water

Solubility is a function of Temperature
A saturated solution contains the maximum amount
of a substance soluble at that temperature
8
Reactions in Aqueous Solution
We have already seen two types of reactions in
aqueous solution
1. Precipitation reactions
2 AgNO3 (aq) CaCl2 (aq) ----gt 2 AgCl (s)
Ca(NO3 )2 (aq)
Ion transfer
2. Acid - Base Reactions
HNO3 (aq) H 2O (l) ----gt H 3O (aq)
NO3- (aq)
HNO3 (aq) NaOH (aq) ----gt NaNO3 (aq) H 2O
(l)
H (proton) transfer
9
Reactions in Aqueous Solution
3. Redox reactions
2Ca (s) O2 (g) ----gt 2 CaO (s)
Electron transfer
Ca and O2 are neutral elemental species
CaO is an ionic compound made up of Ca2 and O-2
ions
Electrons must be transferred in order to form
ions
Oxidation-reduction reactions involve electron
transfer
10
Oxidation-Reduction Reactions
The half-reaction concept
Can be split into two half-reactions
2 Ca (s) O2 (g) ----gt 2 CaO (s)
2 Ca0 ----gt 2 Ca 2 4e-
Electrons are lost by Ca0
O20 4e- -----gt 2 O -2
Electrons are gained by O20
2 Ca0 O20 4e- ----gt 2 Ca 2 2 O -2
4e-
2 Ca0 O20 ----gt 2 Ca 2 2 O -2
The ions combine to give
2 Ca (s) O2 (g) ----gt 2 CaO (s)
11
Oxidation-Reduction Reactions
The half-reaction concept
Can be split into two half-reactions
2 Ca (s) O2 (g) ----gt 2 CaO (s)
2 Ca0 ----gt 2 Ca 2 4e-
Oxidation half-reaction
Reduction half-reaction
O20 4e- -----gt 2 O -2
2 Ca0 O20 ----gt 2 Ca 2 2 O -2
Overall redox reaction
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Oxidation-Reduction Reactions
Definitions
Oxidation The half-reaction that involves the
loss of electrons
2 Ca0 ----gt 2 Ca 2 4e-
Reduction The half-reaction that involves the
gain of electrons
O20 4e- -----gt 2 O -2
Overall redox reaction
2 Ca0 O20 ----gt 2 Ca 2 2 O -2
All electrons lost in the oxidation process must
be gained in the reduction process.
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Oxidation-Reduction Reactions
Definitions
Oxidizing Agent The substance that is reduced
(gains electrons) causing the oxidation (loss of
electrons) by another substance.
O20 4e- -----gt 2 O -2
Oxygen is a strong oxidizing agent.
Reducing Agent The substance that is oxidized
(loses electrons) causing the reduction (gain of
electrons) by another substance.
2 Ca0 -----gt 2 Ca2 4e-
Calcium metal is a good reducing agent.
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Oxidation-Reduction Reactions
2 Fe2O3 (s) 3C (s) ------gt 4 Fe (s) 3 CO2
(g)
Carbon donates the electrons to reduce the Fe3
ions to Fe0
reducing
Carbon is the ________________ agent
oxidized
Carbon is ___________________ to CO2
Fe2O3 accepts electrons to oxidize C to CO2
oxidizing
Fe2O3 is the ________________ agent
reduced
Fe2O3 is ___________________ to Fe
15
Oxidation Numbers
A set of rules used to determine whether an
element is oxidized or reduced in a chemical
reaction.
These rules result in the assignment of an
oxidation number for each element.
Oxidation number The charge an atom would have
in a molecule (or ionic compound) if the
electrons were transferred completely.
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Rules for Assigning Oxidation Numbers
1. The oxidation number of an atom in a pure
element is zero.
2. The total of the oxidation numbers of all
atoms in a molecule is zero. For an ion this
total is equal to the charge on the ion.
3. In their compounds the alkali metals (group
1A) have an oxidation number of 1 and the
alkaline earth metals (group 2A) have an
oxidation number of 2
4. In its compounds the oxidation number of
hydrogen is 1 that of fluorine, -1
5. In its compounds, oxygen has an oxidation
number of -2.
6. In their binary compounds with metals, the
halogens (group 7A) have an oxidation number of
-1 those of group 6A, -2 and those of group
5A, -3.
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Oxidation Numbers
Find the oxidation number of each element in SO3
Oxidation Number For a single O atom
Oxidation Number For a single S atom
-2
6
SO3
Total charge on sulfur
Total charge on oxygen
-6
6
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Oxidation Numbers
Find the oxidation number of each element in NO2-
Oxidation Number For a single O atom
Oxidation Number For a single N atom
-2
3
NO2-1
Total charge on nitrogen
Total charge on oxygen
-4
3
The sum of the total charges must equal the
overall charge on the species
19
Oxidation Numbers
Find the oxidation number of each element in N2H4
Oxidation Number For a single H atom
Oxidation Number For a single N atom
1
-2
N2H4
Total charge on nitrogen
Total charge on hydrogen
4
-4
Rule 4 states that H 1
Rule 6 says that N should be N-3
Rule 4 is higher on the list and therefore is
primary.
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Balancing Redox Reactions
In Acidic Media
MnO4-

SO2
-----gt
Mn2

HSO4-
Note Spectator ions are not included
Step 1
Separate into half reactions.
MnO4-
-----gt
Mn2
SO2
-----gt
HSO4-
Step 2
Balance all atoms except O and H in each half
reaction
Its not necessary in this example--but you
should always check.
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Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 3
Balance O atoms using H2O.
MnO4-
-----gt
Mn2

4 H2O
SO2
-----gt
HSO4-

2 H2O
Step 4
Balance H atoms using H
MnO4-
-----gt
Mn2

4 H2O
8H

SO2
-----gt
HSO4-

2 H2O

3H
22
Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 5
Balance charges using e-
MnO4-
-----gt
Mn2

4 H2O
5e-

8H

SO2
-----gt
HSO4-

2 H2O

3H

2e-
23
Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 6
Equalize charges
MnO4-
-----gt
Mn2

4 H2O)
(5e-

2
8H

SO2
-----gt
HSO4-

(2 H2O

3H

2e- )
5
24
Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 6
Equalize charges
2 MnO4-
-----gt
2 Mn2

8 H2O
16H

10 e-

5 SO2
-----gt
5 HSO4-

H2O

15H

10e-
10
25
Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 7
Cancel
2 MnO4-
-----gt
2 Mn2

8 H2O
16H

10 e-

5 SO2
-----gt
5 HSO4-

H2O

15H

10e-
10
26
Balancing Redox Reactions
In Acidic Media
MnO4-
-----gt
Mn2

HSO4-

SO2
Step 7
Cancel
2 MnO4-
-----gt
2 Mn2
H

5 SO2
-----gt
5 HSO4-

H2O
2
5 SO2
H2O
2

H

2 MnO4-

-----gt
5 HSO4-

2 Mn2
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Preparation of a Standard Solution
28
Solution Composition

• 1. Molarity (M)
• 2. Mass (weight) percent
• 3. Mole fraction (?A)
• 4. Molality (m)