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Reactions in Aqueous Solution
Chapter 4
4.1
4.2
4.4
4.5
General properties of aqueous solutions
Precipitation reactions
Oxidation-reduction reactions
Concentration of solutions
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4.1 General properties of aqueous solutions
A solution is a homogenous mixture of 2 or more substances
The solute is(are) the substance(s) present in the smaller
amount(s)
The solvent is the substance present in the larger amount
Solution
Solvent
Solute
Soft drink (l)
H2O
Sugar, CO2
Air (g)
N2
O2, Ar, CH4
Soft Solder (s)
Pb
Sn
aqueous solutions
of KMnO4
2
An electrolyte is a substance that, when dissolved in
water, results in a solution that can conduct electricity.
A nonelectrolyte is a substance that, when dissolved,
results in a solution that does not conduct electricity.
nonelectrolyte
weak electrolyte
strong electrolyte
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Conduct electricity in solution?
Cations (+) and Anions (-)
Strong Electrolyte – 100% dissociation
NaCl (s)
H 2O
Na+ (aq) + Cl- (aq)
Weak Electrolyte – not completely dissociated
Ionization of acetic acid:
CH3COOH
CH3COO- (aq) + H+ (aq)
A reversible reaction. The reaction can
occur in both directions.
Acetic acid is a weak electrolyte because its
ionization in water is incomplete.
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Conduct electricity in solution?
Cations (+) and Anions (-): the cations will be attracted
to the negative electrode & the anions will be attracted
to the positive electrode. This movement sets up an
electric current that is equivalent to the flow of
electrons along a metal wire.
Strong Electrolyte – 100% dissociation (more ions)
NaCl (s)
H 2O
Na+ (aq) + Cl- (aq)
Weak Electrolyte – not completely dissociated (less
amount of ions)
CH3COOH
CH3COO- (aq) + H+ (aq)
4.1
Hydration is the process in which an ion is surrounded
by water molecules arranged in a specific manner.
d-
d+
H2O
6
Nonelectrolyte does not conduct electricity?
No cations (+) and anions (-) in solution
C6H12O6 (s)
H 2O
C6H12O6 (aq)
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4.3 Precipitation Reactions
Precipitate – insoluble solid that separates from solution
precipitate
Pb(NO3)2 (aq) + 2NaI (aq)
PbI2 (s) + 2NaNO3 (aq)
molecular equation
Pb2+ + 2NO3- + 2Na+ + 2I-
PbI2 (s) + 2Na+ + 2NO3-
ionic equation
Pb2+ + 2IPbI2
PbI2 (s)
net ionic equation
Na+ and NO3- are spectator ions
8
Precipitation of Lead Iodide
Pb2+ + 2I-
PbI2 (s)
PbI2
9
Solubility is the maximum amount of solute that will dissolve
in a given quantity of solvent at a specific temperature.
10
Examples of Insoluble Compounds
CdS
PbS
Ni(OH)2
Al(OH)3
11
Question: Classify the following ionic compounds as
‘soluble’ or ‘insoluble’ in water:
Answer:
1. Na2CO3
2. PbSO4
3. Co(OH)2
4. Ba(NO3)2
5. (NH4)3PO4
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Writing Net Ionic Equations
1. Write the balanced molecular equation.
2. Write the ionic equation showing the strong electrolytes
completely dissociated into cations and anions.
3. Cancel the spectator ions on both sides of the ionic
equation
4. Check that charges and number of atoms are balanced in
the net ionic equation
Write the net ionic equation for the reaction of silver nitrate
with sodium chloride.
AgNO3 (aq) + NaCl (aq)
AgCl (s) + NaNO3 (aq)
Ag+ + NO3- + Na+ + Cl-
AgCl (s) + Na+ + NO3-
Ag+ + Cl-
AgCl (s)
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Question: Write the net ionic equation for the
precipitation reaction that occurs when solution of
calcium chloride and sodium carbonate are mixed.
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4.4 Oxidation-Reduction Reactions
(electron transfer reactions)
2Mg
O2 + 4e-
2Mg2+ + 4e- Oxidation half-reaction (lose e-)
2O2Reduction half-reaction (gain e-)
2Mg + O2 + 4e2Mg2+ + 2O2- + 4e15
2Mg + O2
2MgO
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Zn (s) + CuSO4 (aq)
ZnSO4 (aq) + Cu (s)
Zn2+ + 2e- Zn is oxidized
Zn
Cu2+ + 2e-
Zn is the reducing agent
Cu Cu2+ is reduced Cu2+ is the oxidizing agent
Copper wire reacts with silver nitrate to form silver metal.
What is the oxidizing agent in the reaction?
Cu (s) + 2AgNO3 (aq)
Cu
Ag+ + 1e-
Cu(NO3)2 (aq) + 2Ag (s)
Cu2+ + 2eAg Ag+ is reduced
Ag+ is the oxidizing agent
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Oxidation number
The charge the atom would have in a molecule (or an
ionic compound) if electrons were completely transferred.
1. Free elements (uncombined state) have an oxidation
number of zero.
Na, Be, K, Pb, H2, O2, P4 = 0
2. In monatomic ions, the oxidation number is equal to
the charge on the ion.
Li+, Li = +1; Fe3+, Fe = +3; O2-, O = -2
3. The oxidation number of oxygen is usually –2. In H2O2
and O22- it is –1.
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4.4
4. The oxidation number of hydrogen is +1 except when
it is bonded to metals in binary compounds. In these
cases, its oxidation number is –1.
5. Group IA metals are +1, IIA metals are +2 and fluorine is
always –1.
6. The sum of the oxidation numbers of all the atoms in a
molecule or ion is equal to the charge on the
molecule or ion.
7. Oxidation numbers do not have to be integers.
Oxidation number of oxygen in the superoxide ion,
O2-, is –½.
-
HCO3
What are the oxidation numbers
of all the elements in HCO3- ?
O = –2
H = +1
3x(–2) + 1 + ? = –1
C = +4
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The Oxidation Numbers of Elements in their Compounds
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What are the oxidation numbers of
all the elements in each of these
compounds?
NaIO3
IF7
K2Cr2O7
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Question: Determine the oxidation number of sulphur
in each of the following:
1. H2S
2. S8
3. SCl2
4. Na2SO3
5. SO42−
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Types of Oxidation-Reduction
Reactions
Combination Reaction
A+B
0
C
+3 -1
0
2Al + 3Br2
2AlBr3
Decomposition Reaction
C
+1 +5 -2
2KClO3
A+B
+1 -1
0
2KCl + 3O2
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Types of Oxidation-Reduction Reactions
Combustion Reaction
A + O2
B
0
0
S + O2
0
0
2Mg + O2
+4 -2
SO2
+2 -2
2MgO
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Types of Oxidation-Reduction Reactions
Displacement Reaction
A + BC
0
+1
+2
Sr + 2H2O
+4
0
TiCl4 + 2Mg
0
AC + B
-1
Cl2 + 2KBr
0
Sr(OH)2 + H2 Hydrogen Displacement
0
+2
Ti + 2MgCl2
-1
Metal Displacement
0
2KCl + Br2
Halogen Displacement
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The Activity Series for Metals
Hydrogen Displacement Reaction
M + BC
MC + B
M is metal
BC is acid or H2O
B is H2
Ca + 2H2O
Ca(OH)2 + H2
Pb + 2H2O
Pb(OH)2 + H2
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The Activity Series for Halogens
F2 > Cl2 > Br2 > I2
Halogen Displacement Reaction
0
-1
Cl2 + 2KBr
I2 + 2KBr
-1
0
2KCl + Br2
2KI + Br2
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Types of Oxidation-Reduction Reactions
Disproportionation Reaction
The same element is simultaneously oxidized
and reduced.
Example:
reduced
+1
0
Cl2 + 2OH-
-1
ClO- + Cl- + H2O
oxidized
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Classify each of the following reactions.
Ca2+ + CO32Zn + 2HCl
Ca + F2
CaCO3
ZnCl2 + H2
CaF2
Precipitation
Redox (H2 Displacement)
Redox (Combination)
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Question: Classify the following reactions.
For redox reaction, identify the oxidizing and
reducing agents.
1. Barium chloride(aq) and ammonium sulfate(aq)
2. Manganese metal and tin IV chloride (aq)
3. Cobalt metal and nitrogen gas to yield cobalt (II) nitride
4. Sodium peroxide to give sodium oxide and oxygen gas
Solution Stoichiometry
The concentration of a solution is the amount of solute
present in a given quantity of solvent or solution.
M = molarity =
moles of solute
liters of solution
What mass of KI is required to make 500. mL of a
2.80 M KI solution?
M KI
volume of KI solution
500. mL x
1L
1000 mL
moles KI
x
2.80 mol KI
1 L soln
x
M KI
166 g KI
1 mol KI
grams KI
= 232 g KI
31
Preparing a Solution of Known Concentration
32
Question: Calculate the molarity of a solution made
by dissolving 23.4 g of sodium sulphate in enough
water to form 125 mL of solution.
Question: What are the molar concentrations of each
of the ions present in a 0.025 M aqueous solution of
calcium nitrate?
Question: How many grams of Na2SO4 are required
to make 0.350 L of 0.500 M aqueous solution of
Na2SO4?
33
Dilution is the procedure for preparing a less concentrated
solution from a more concentrated solution.
Dilution
Add Solvent
Moles of solute
before dilution (i)
=
Moles of solute
after dilution (f)
MiVi
=
MfVf
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How would you prepare 60.0 mL of 0.200 M HNO3
from a stock solution of 4.00 M HNO3?
MiVi = MfVf
Mi = 4.00 M Mf = 0.200 M Vf = 0.0600 L
Vi =
MfVf
Mi
Vi = ? L
= 0.200 M x 0.0600 L = 0.00300 L = 3.00 mL
4.00 M
Dilute 3.00 mL of acid with water to a total volume
of 60.0 mL.
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Question: How many mL of 3.0 M H2SO4 are needed
to make 450 mL of 0.10 M H2SO4?
Question: If 10.0 mL of a 10.0 M stock solution of
NaOH is diluted to 250 mL, what is the concentration
of the resulting solution?
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