Transcript Physical Properties of Solutions

Chapter
4
Reactions in Aqueous Solution
Chemistry, 5th Edition
McMurry/Fay
Some Interesting Chemical Reactions
1. Production of Smog
N2 + O2 + heat → 2 NO
2 NO + O2 → 2 NO2 (brown gas)
2. The Greenhouse Effect
2 C8H18 + 25 O2 → 18 H2O + 16 CO2
CO2 transmits visible light but absorbs heat
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Some Interesting Chemical Reactions
3. Reduction of Iron Ore
Fe2O3 + 3 CO → 2 Fe + 3 CO2
4. Depletion of Ozone
O3 + uv rays → O2 + O
CF2Cl2 + O3 → O2 + O
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Some Interesting Chemical Reactions
5. Photosynthesis
6 CO2 + 6 H2O → C6H12O6 + 6 O2
6. Acid Rain
S + O2 + heat → SO2
SO2 + H2O → H2SO3 (acid)
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Types of Chemical Reactions
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Precipitation Reactions: The chemical reaction
that occurs when two solutions are mixed and a
solid precipitate is formed.
AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl (s)
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Precipitation Reactions
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Net Ionic Equations: Break the reaction down into
aqueous ions and precipitates. Eliminates
“spectator ions,” thus simplifying the equation.
AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) + AgCl (s)
Ag+ + NO3- + Na+ + Cl- → AgCl (s) + Na+ + NO3Net Ionic Equation: Ag+ + Cl- → AgCl (s)
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Precipitation Reactions
Solubility Rules
Always form soluble compounds:
Group 1 cations, NH4+, NO3Often form precipitates:
Ag+, Hg22+, Pb22+
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Types of Chemical Reactions
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Acid–Base Reactions: A reaction of an acid with a
base, forming water plus a salt.
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The driving force of this reaction is the formation of
the stable water molecule.
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
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Acid-Base Reactions
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Arrhenius Definition:
1. Acid releases H+ in water.
HCl → H+ + Cl2. Base releases OH- in water
NaOH  Na+ + OH9
Acid-Base Reactions
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Brønsted Definition:
Acid donates a proton (H+), base accepts a proton.
HCl + H2O → H3O+ + ClNH3 + H2O → NH4+ + OH-
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Acid-Base Reactions
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Lewis Definition:
Acid accepts 2 electrons, base donates 2 electrons.
(later)
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pH - A Measure of Acidity
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The pH of a solution is defined as the negative
logarithm of the hydrogen ion concentration (in
mol/L).
pH = –log[H+]
pH + pOH = 14
Acidic solutions: [H+] > 1.0 x 10–7 M,
Basic solutions:
[H+] < 1.0 x 10–7 M,
Neutral solutions: [H+] = 1.0 x 10–7 M,
pH < 7.00
pH > 7.00
pH = 7.00
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pH - A Measure of Acidity
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Calculate the pH of a HNO3 solution having a
hydrogen ion concentration of 0.76 M.
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The OH– ion concentration of a blood sample is
2.5 x 10–7 M. What is the pH of the blood?
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Types of Chemical Reactions
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Oxidation–Reduction (Redox) Reaction: A
reaction in which one or more electrons are
transferred between reaction partners.
Mg (s) + Cl2 (g)  MgCl2(s)
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Redox Reactions
Electrolyte: a solution that conducts electricity
- or –
An ionic compound dissolved in water
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Redox Reactions
Strong electrolyte: almost all molecules are
ionized.
HCl, HNO3, NaCl
Weak electrolyte: only a small proportion of the
ions are formed.
H2SO3, HC2H3O2
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Electrolytes in Solution
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Why do ionic
compounds conduct
electricity when
molecular ones
generally do not?
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Electrolytes in Solution
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Electrolytes:
Dissolve in water to
produce ionic
solutions.
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Nonelectrolytes: Do
not form ions when
they dissolve in water.
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Electrolytes in Solution
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Dissociation:
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The process by
which a compound
splits up to form ions
in the solution.
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Redox Reactions
How are electrons transferred??
Observe charges on elements:
Mg0 (s) + Cl20 (g)  MgCl2(s)
Mg0 (s) + Cl20 (g)  Mg2+ + 2ClMg0 → Mg2+ + 2 e-
2 e- + Cl20 → 2 Cl-
So electrons are TRANSFERRED from Mg to Cl2
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Redox Reactions
How are electrons transferred??
2 e- + Cl20 → 2 ClCl2 is REDUCED, since the charge on each Cl is lowered
Mg0 → Mg2+ + 2 eMg is OXIDIZED, electrons are removed
REDuction + OXidation = REDOX
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Redox Reactions
Reducing Agent: causes reduction (donates electrons)
Oxidizing Agent: causes oxidation (accepts electrons)
2 Mg + O2 → 2 MgO
Oxidizing Agent = O2
Reducing Agent = Mg
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Redox Reactions
Electrochemical Series: A table showing the relative
oxidizing strengths of different species.
Helps to predict the outcome of unknown redox reactions
Zn + Cu2+ → ??
Check Table…
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Activity Series of Elements
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Redox Reactions
Zn + Cu2+ → ??
From Table: Zn is a stronger reducing agent than Cu.
Therefore the electrons are more likely to reside with Cu
in the product:
Zn + Cu2+ → Zn2+ + Cu
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Redox Reactions
OXIDATION NUMBER
Effective charge on any atom
NaCl:
Na +1, Cl -1
MgCl2:
Mg +2, Cl -1
H2O:
H +1, O -2
MgO:
Mg +2, O -2
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Redox Reactions
CALCULATING OXIDATION NUMBER
1. The sum of the O.N.s of all of the atoms in a molecule
or ion is equal to its total charge.
NaCl:
ONNa + ONCl = 0
SO42-:
ONS + 4 x ONO = -2
+1 + (-1) = 0
6 + 4x(-2) = -2
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Redox Reactions
CALCULATING OXIDATION NUMBER
2. For atoms in the elemental form the oxidation number
is zero.
Mg (s):
ON = 0
O2 (g):
ON = 0
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Redox Reactions
CALCULATING OXIDATION NUMBER
3. Remember some common oxidation numbers:
Group 1 → +1
Group 2 → +2
Halogens → -1
Oxygen → -2
Hydrogen → +1, -1
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Redox Reactions
CALCULATING OXIDATION NUMBER
HSO3-
ONS = ??
ONH + ONS + 3(ONO) = -1
ONH = +1, ONO = -2
So ONs = -1 – (1) – (3 x (-2)) = +4
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Redox Reactions
HALF REACTIONS
Mg + Cl2 → MgCl2
Find ON of each atom:
Mg0 + Cl20 → Mg2+Cl21-
Break reaction into two separate (HALF) reactions
showing electrons
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Redox Reactions
HALF REACTIONS
Mg + Cl2 → MgCl2
Break reaction into two separate (HALF) reactions
showing electrons
Mg0 → Mg2+ + 2 eCl20 + 2 e- → 2 Cl-
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Balancing Redox Reactions
HALF REACTION METHOD
1. Break reaction into half reactions.
2. Balance each half reaction separately.
a) use H+ and H2O as needed in ACIDIC solution.
b) use OH- and H2O as needed in BASIC solution.
3. Balance charge in each half reaction by adding
electrons to one side of the equation.
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Balancing Redox Reactions
HALF REACTION METHOD
4. Obtain equal number of electrons in each half reaction
by multiplying entire equation by a constant.
5. Add the 2 half reactions back together so that the
electrons cancel.
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Balancing Redox Reactions
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Balance the following for acidic and basic solution:
ClO–(aq) + Cr(OH)4–(aq)  CrO42–(aq) + Cl–(aq)
NO3–(aq) + Cu(s)  NO(g) + Cu2+(aq)
MnO4–(aq) + IO3–(aq)  MnO2(s) + IO4–(aq)
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