Chapter 7

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Chemistry 101
Chapter 7
Reactions in Aqueous Solutions
Reactions in Aqueous Solutions
Ionic compounds (Salt)
Aqueous solution: solvent is water
Reactions in Aqueous Solutions
Chemical reactions that occur in water.
In our body reactions occur in the aqueous solution.
Water in our body
1. About 60% of our body.
2. Most of the reactions occur in aqueous solution.
3. Participates in many biochemical reactions.
4. Transports reactants and products from one place in our
body to another.
5. Eliminates the waste materials from cells and our body (urine).
Why does a chemical reaction occur?
Several driving forces:
1. Formation of a solid
2. Formation of water
3. Transfer of electrons
4. Formation of a gas
Why does a chemical reaction occur?
Several driving forces:
1. Formation of a solid
2. Formation of water
3. Transfer of electrons
4. Formation of a gas
Formation of a solid
Precipitation reactions
Precipitate
KI + Pb(NO3)2  ?
Ionic Compounds
When an ionic compound dissolves in water, ions are produced.
Each ion is surrounded by water molecules.
Ions Hydrated by H2O
Hydration
Ionic Compounds
1. Soluble solid: it completely dissolves in water (ions are formed).
2. Slightly soluble solid: it partially dissolves in water.
3. Insoluble solid: it does not dissociate in water (almost).
•
Note: the terms insoluble and slightly soluble mean
such a miniscule amount dissolves that you can’t see
any decrease in the amount of solid present.
Solubility Rules
Memorize
these
RULES!
Soluble
Insoluble
Preceding rules trump following rules.
Solubility Rules
• Another way of showing the same rules.
Electrolytes
bulb
Electrolyte: conducts an electric current.
Na+
Clelectrolyte
+
-
Ionization (Dissociation)
NaCl → Na+ + Cl-
strong electrolytes: molecules dissociate completely into ions (NaCl).
weak electrolytes: molecules dissociate partially into ions (CH3COOH).
nonelectrolytes: molecules do not dissociate into ions (DI water).
Formation of a solid
Precipitation reactions
Precipitate
KI + Pb(NO3)2  ?
Aqueous Solution
(ionic compounds)
aqueous solution: solvent is water
KI(s)
H2O
K+(aq) + I-(aq)
Dissociation
(Ionization)
KI
K+
Pb(NO3)2(s)
H2O
Pb2+(aq) + 2NO3-(aq)
I-
Pb(NO3)2
Pb2+ NO3NO3-
Aqueous Solution
(ionic compounds)
sometimes the ions react with each other.
Positive ions will interact with negative ions.
KI
Pb(NO3)2
K+
I-
Pb2+
NO3-
Sometimes they stick together to form a solid (precipitate).
2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
Molecular equation: 2KI(aq) + Pb(NO3)2(aq)  PbI2(s) + 2KNO3(aq)
Complete
Ionic equation:
2K+(aq) + Pb2+(aq) + 2I-(aq) + 2NO3-(aq)  PbI2(s) + 2K+(aq) + 2NO3-(aq)
2K+(aq) + Pb2+(aq) + 2I-(aq) + 2NO3-(aq)  PbI2(s) + 2K+(aq) + 2NO3-(aq)
Spectator ions
Net ionic equation:
Pb2+(aq) + 2I-(aq)  PbI2(s)
total charge on left side = total charge on right side
balanced equation
2As3+(aq) + 3s2-(aq)  As2S3(s)
Example
Pb(NO3)2(aq) + Na2SO4(aq)  ?
Pb(NO3)2(aq) + Na2SO4(aq)  PbSO4(s) + NaNO3(aq)
Balance it:
Pb(NO3)2(aq) + Na2SO4(aq)  PbSO4(s) + 2 NaNO3(aq)
Pb2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + SO42-(aq)  PbSO4(s) + 2 Na+(aq) + 2 NO3-(aq)
Complete ionic equation
Example
Pb2+(aq) + 2 NO3-(aq) + 2 Na+(aq) + SO42-(aq)  PbSO4(s) + 2 Na+(aq) + 2 NO3-(aq)
•
Net ionic equation:
The ions that do not react are called
spectator ions.
Pb2+(aq) + SO42-(aq)  PbSO4(s)
Practice
1. Molecular equation
3 KOH(aq) + Fe(NO3)3(aq)  ?
2. Balancing
3. Complete ionic equation
4. Net ionic equation
Why does a chemical reaction occur?
Several driving forces:
1. Formation of a solid
2. Formation of water
3. Transfer of electrons
4. Formation of a gas
Acids and Bases
Acids: sour
Bases: bitter or salty
Acids and Bases
Arrhenius definition:
Acid: produces H3O+ (H+) in water.
CH3COOH(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
H3O+ (Hydronium ion): H+(aq) + H2O(l)
H3O+(aq)
Base: produces OH- in water.
NaOH(s)
H2O
NH3(aq) + H2O(l)
Na+(aq) + OH-(aq)
NH4+(aq) + OH-(aq)
Acids and Bases
Weak acid and base: it is partially ionized in aqueous solution.
produces less H+ and OH-
CH3COOH(aq) + H2O(l)
NH3(aq) + H2O(l)
CH3COO-(aq) + H3O+(aq)
NH4+(aq) + OH-(aq)
Strong acid and base: it is completely ionized in aqueous solution.
produces more H+ and OH-
HCl(aq) + H2O(l)
Cl-(aq) + H3O+(aq)
NaOH(aq) + H2O(l)
Na+(aq) + OH-(aq)
Acid-Base Reactions
Neutralization
Strong acid + Strong base → Salt + H2O
NaOH(aq) + HCl(aq)
NaCl(aq) + H2O(l)
Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) +  Na+(aq) + Cl-(aq) + H2O(l)
H+(aq) + OH-(aq)
H2O(l)
The only chemical change is the formation of water.
Why does a chemical reaction occur?
Several driving forces:
1. Formation of a solid
2. Formation of water
3. Transfer of electrons
4. Formation of a gas
Oxidation and Reduction reactions (redox)
e-
2Na(s) + Cl2(g)
2NaCl(s)
Na → Na+ + eCl + e- → Cl-
Oxidation and Reduction reactions (redox)
oxidation: it is the loss of electrons.
Na → Na+ + ereduction: it is the gain of electrons.
Cl + e- → Cl-
Remember – LEO says GER.
Loss of Electrons is Oxidation
Gain of Electrons is Reduction.
Oxidation and Reduction reactions (redox)
Metal + Nonmetal : Transfer of electrons
Oxidation and Reduction reactions (redox)
Oxidation and reduction always occur together.
(The lost e- must go somewhere!)
Oxidation and Reduction reactions (redox)
oxidation: it is the loss of electrons.
reduction: it is the gain of electrons.
Zn(s) + Cu2+(aq)  Zn2+(aq) + Cu(s)
redox reaction
Zn(s)  Zn2+(aq) + 2e-
Zn is oxidized (reducing agent)
Cu2+(aq) + 2e-  Cu(s)
Cu2+ is reduced (oxidizing agent)
Oxidation and Reduction reactions (redox)
oxidation: is the gain of oxygen / loss of hydrogen.
reduction: is the loss of oxygen / gain of hydrogen.
CH4(s) + 2O2(g)  CO2(g) + 2H2O(g)
C gains O and loses H
is oxidized
O gains H
Is reduced
(reducing agent)
(oxidizing agent)
redox reaction
single replacement reaction and combustion reactions  redox reactions
double replacement reactions  non redox
Oxidation and Reduction reactions (redox)
Example 2:
• 2 Al(s) + Fe2O3(s)  2 Fe(s) + Al2O3(s)
is oxidized
is reduced
• Called the Thermite reaction.
• Let’s just say it’s vigorous!
Oxidation and Reduction reactions (redox)
Example 3:
Cu(s) + 2 Ag+(aq)  2 Ag(s) + Cu2+(aq)
is oxidized
is reduced
Oxidation and Reduction reactions (redox)
Example 4:
Zn(s) + 2 HCl(aq)  H2(g) + ZnCl2(aq)
Zn(s) + 2 H+(aq)  H2(g) + Zn2+(aq)
is oxidized
is reduced
Note: this reaction also shows the fourth driving force of a
reaction, namely, the formation of a gas.
Oxidation and Reduction reactions (redox)
Practice:
2 Mg(s) + O2(g)  2 MgO(s)
Classification of chemical reactions
Chemical reactions
Precipitation
Reactions
Combustions
Reactions
Oxidation-Reduction
Reactions
Acid-Base
Reactions
Synthesis (combination)
Reactions
Decomposition (Analysis)
Reactions
(Reactants are elements.)
(Products are elements.)
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