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4/28/2010
Chapter 4
Aqueous Reactions and
Solution Stoichiometry
Today’s Goals
Water as a unique solvent
Properties of aqueous solutions
Dissolution/Disassociation
Electrolytes/Non-electrolytes
Acids, bases and ionization
Some reaction types: precipitation, metathesis, neutralization
Ionic Equations
Water: where the action is
• Water is found in all three
states on Earth.
• Most common liquid solvent
found in nature.
• Unique properties of water
make it a perfect
environment for chemical
and biological reactions
• Without water, life as we
know it could not exist.
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What makes water so special?
• Polar nature of covalent bond
–
–
–
–
Ability to form hydrogen bonds
Relatively high boiling point
Strong solvent for ionic compounds
Low density of ice
Dissolution of Ionic compounds
• Water molecules surround the NaCl crystal.
• The – side of the H2O dipole is attracted to the Na+ ions
while the + side of the H2O dipole heads for the Cl- ions.
• The H2O molecules surround and carry off each ion, until
the crystal is completely dissolved.
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Dissolution/Dissociation
• When ionic compounds dissolve in water,
the anions and cations are separated from
each other. This is called dissociation.
– However, not all ionic compounds are
soluble in water!
• When compounds containing polyatomic
ions dissociate, the polyatomic group stays
together as one ion.
• When molecular compounds dissolve in
water, the only ones that can form ions in
solution are acids and bases. The rest will
just dissolve as the complete molecule (if
that molecule is soluble in water)
Solutions
There are hundreds of
aromatic compounds
in coffee
• Solutions are defined as
homogeneous mixtures of two or
more pure substances.
• The solvent is present in greatest
abundance.
• All other substances are solutes.
• Aqueous solutions such as coffee,
seawater and wine have many
solutes, both liquid and solid
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Properties of aqueous Solutions:
Electrical conductivity
eCl-
Na
Na++
• For electric current to be conducted, charged, mobile
carriers such as free e- or free ions must be present.
– Metals have mobile electrons to carry current.
– Aqueous solutions which contains ions can conduct electricity.
• Strong electrolytes produce a high concentration of
ions and are good conductors
• Weak electrolytes produce fewer ions and are poor
conductors.
• Non-electrolytes don’t form ions or conduct electricity
Electrolytes & Non-electrolytes
Ionic compound →
Complete dissociation
(after dissolving)
CH3COOH ↔CH3COO- + H3O+
↔
+
Non-acid molecular →
No dissociation
(methanol in water)
-
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Strong ElEctrolytES ArE…
• Soluble ionic
compounds
• Strong acids
• Strong bases
Weak Electrolytes:
• Weak acids – incomplete ionization
– Memorize the 7 strong acids; the rest are weak
• Weak Bases – incomplete ionization
– Usually contain N-H in the compound (like NH4OH)
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Reaction Types
• Precipitation reactions
– Two soluble reactants combine to form an insoluble product
• Metathesis (transpose) or displacement/
replacement
– Exchange ions in reactants to form new products
• Acid-base reactions
– Neutralization reaction forming ionic salt and water (usually)
• Acid reactions with gas formation
– Carbonic acid decomposition
– Formation of H2S
Solution Chemistry
• We need to pay attention to exactly what species are
present in a reaction mixture (i.e., solid, liquid, gas,
aqueous solution).
• If we are to understand reactivity, we must be aware of
just what is changing during the course of a reaction.
• Sometimes there is no visible change in the solution, but
the reaction still occurred
2Al(s) + 3Br2(l) → 2AlBr3(s)
Precipitation reactions
2 AgNO3 + Cu
Cu(NO3)2 + 2 Ag
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Precipitation Reactions
• Some reactions involve the combining of
ions in solution resulting in formation of a
material that is insoluble in water. These
are called precipitation reactions.
AgNO3 (aq) + NaCl (aq) g AgCl (s) + NaNO3
Pb(NO3)2(aq) + 2KI(aq)
PbI2(s) + 2KNO3(aq)
• Precipitation reactions are generally done
with the reactants dissolved in water to
allow the ions to move more freely.
2+
2-
Mg 2+ + CO32- g MgCO3 i
Precipitation Reaction with Free Ions Combining
Precipitation Reactions
2 KI(aq) + Pb(NO3)2(aq)
Dissociated ions in
solution (both beakers)
2 KNO3(aq) + PbI2(s)
PbI2 precipitate with K+
and NO3- ions in solution
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No Precipitate Formation g No Reaction
KI(aq) + NaCl(aq)
KCl(aq) + NaI(aq)
All ions still present in solution no reaction.
Predicting Precipitation
• Need to know solubility of reaction products
– Low solubility in reaction media (usually water) means product
will precipitate
• Solubilities for many ions can be predicted based on
historical data
• Solubility (a physical property) can be used to predict
whether a reaction will occur and can be used to
separate the reaction products
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Solubility Rules:
Compounds that Are Generally Soluble in Water
Compounds containing the
following ions are generally
soluble
Exceptions
(when combined with ions on the
left the compound is insoluble)
Li+, Na+, K+, NH4+
none
NO3–, C2H3O2–
none
Cl–, Br–, I–
Ag+, Hg22+, Pb2+
SO42–
Ca2+, Sr2+, Ba2+, Pb2+
Solubility Rules:
Compounds that Are Generally Insoluble
Compounds containing the
following ions are generally
insoluble
OH–
S2–
CO32–, PO43–
Exceptions
(when combined with ions on the
left the compound is soluble or
slightly soluble)
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+,
Ca2+, Sr2+, Ba2+
Li+, Na+, K+, NH4+
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Metathesis (Exchange) Reactions
• Metathesis comes from a Greek word that
means “to transpose.”
• It appears the ions in the reactant compounds
exchange, or transpose, ions.
• Also known as double displacement or
replacement reactions
AgNO3 (aq)+KCl (aq) → AgCl(s)+KNO3 (aq)
acetic
acid
Properties of Acids
• Formula starts with H (hydrogen),
acids are H+ donors.
• Sour taste.
• Change color of vegetable dyes.
• React with bases to form ionic salts
and often H2O.
• React with “active” metals, not
noble metals.
– i.e., Al, Zn, Fe, but not Cu, Ag or Au.
– Zn + 2 HCl ZnCl2 + H2
– Corrosive.
phosphoric
acid
Tartaric
acid
Citric
acid
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Acids
There are only seven
strong acids:
Cl: icks!
•
•
•
•
•
•
•
Properties of Bases
•
•
•
•
Hydrochloric (HCl)
Chloric (HClO3)
Perchloric (HClO4)
Hydrobromic (HBr)
Hydroiodic (HI)
Nitric (HNO3)
Sulfuric (H2SO4)
Ammonium
hydroxide
Bases are H+ acceptors
Not so tasty - bitter
Feel slippery.
sodium hydroxide
Change color of vegetable dyes.
– Different color than acid.
• Strong bases contain hydroxide
ion (OH-)
• React with acids to form ionic
salts and water (usually)
– Neutralization.
quinine
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Strong Bases
The strong bases are the
soluble metal salts of
hydroxide ion (OH-)
• Alkali metals
• Calcium
• Strontium
• Barium
Acid–Base Reactions
• Also called neutralization reactions because the acid
and base neutralize each other’s properties.
• In the reaction , the H+1 from the acid combines with
the OH-1 from the base to make water.
• The cation from the base combines with the anion
from the acid to make the salt.
acid + base
salt + water
2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)
HC2H3O2(aq) + NH4OH (aq)
NH4C2H3O2 (aq)+ H2O(l)
Think of water as H-OH
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Predicting Acid–Base Reaction
Products
1. Determine what ions each aqueous reactant has.
2. Exchange ions.
– (+) ion from one reactant with (-) ion from the other.
– H+ combines with OH− to make water.
3. Balance charges of combined ions to get formula of
the salt.
4. Balance the equation.
5. Determine solubility of the salt.
– Use the solubility rules.
– If the salt is insoluble or slightly soluble, it will precipitate.
Acid reactions w/ Gas Evolution
• Some reactions form a gas directly:
K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g)
• Other reactions form a gas by the decomposition of one
of the products into a gas and water.
NaHCO3 + HC2H3O2
H2CO3
NaC2H3O2 + H2CO3 exothermic
CO2(g) + H2O endothermic
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Gas-Forming Reactions
When a carbonate or bicarbonate reacts with an
acid, the products are a salt, carbon dioxide, and
water.
CaCO3 (s) + HCl (aq)
NaHCO3 (aq) + HBr (aq)
CaCl2 (aq) + CO2 (g) + H2O (l)
NaBr (aq) + CO2 (g) + H2O (l)
Similarly, when a sulfite reacts with an acid, the
products are a salt, sulfur dioxide, and water.
SrSO3 (s) + 2 HI (aq)
SrI2 (aq) + SO2 (g) + H2O (l)
WhAt’S rEAlly going on?
• Molecular equations lists the reactants and products in
“molecular” or formula unit form:
2 KI(aq) + Pb(NO3)2(aq)
2 KNO3(aq) + PbI2(s)
• In ionic equations, all strong electrolytes (strong acids,
strong bases, and soluble ionic salts) are shown
dissociated into their ions:
2K+(aq) + 2I-(aq)+ Pb+(aq)+2NO3-(aq)
2K+(aq)+2NO3-(aq) + PbI2(s)
• This shows what’s really going on in that water
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Net Ionic Equation
• The net ionic equation shows us what reaction actually
happened – if any.
• To make the net ionic equation, start with the ionic
equation and cross out anything that does not change
from the left side of the equation to the right.
2K+(aq) + 2I-(aq)+ Pb+(aq)+2NO3-(aq)
2I-(aq)+ Pb+(aq)
2K+(aq)+2NO3-(aq) + PbI2(s)
PbI2(s)
• Those things that didn’t change (and were deleted from
the net ionic equation) are called spectator ions.
Writing Net Ionic Equations
1.
Write a balanced molecular equation.
AgNO3 (aq) + KCl (aq)
AgCl (s) + KNO3 (aq)
2.
Dissociate all strong electrolytes – write as
individual ions (keep the correct coefficients).
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)
AgCl (s) + K+(aq) + NO3-(aq)
3. Cross out anything that is unchanged on the left
side and right sides of the equation (the spectator
ions
)
4. Write the net ionic equation with the species that
remain. Ag+(aq) + Cl-(aq)
AgCl (s)
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Neutralization Reactions
When a strong acid reacts with a strong base, the
reaction is…
Acid + Base → Salt + Water
HCl (aq) + NaOH (aq)
NaCl (aq) + H2O (l)
The ionic equation is:
H+ (aq) + Cl
x- (aq) + Na
x + (aq) + OH-(aq) Na
x + (aq) + Cl
x - (aq) + H2O (l)
The net ionic equation is:
H+ (aq) + OH- (aq)
H2O (l)
Some practice
Write the Molecular Equation, Total Ionic Equation
and Net Ionic Equation for each reaction:
#1 CaCO3(s) + HCl (aq) → ?
#2 Fe(s) + AgNO3(aq) →?
#3 K3PO4(aq) + 3HNO3(aq) →?
#4 KNO3(aq) + HCl(aq) →?
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Some Practice
Example 1: CaCO3(s) + HCl → ?
Molecular: CaCO3(s) + 2HCl(aq) gCaCl2(aq) + H2O(l) + CO2(g)
Total Ionic : CaCO3(s) + 2H+(aq) + 2Cl-(aq) gCa2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g)
Net Ionic : CaCO3(s) + 2H+(aq) gCa2+(aq) + H2O(l) + CO2(g)
Example 2: Fe(s) + AgNO3(aq) g?
Molecular : Fe(s) + 2AgNO3(aq) g2Ag(s) + Fe(NO3)2(aq)
Total Ionic : Fe(s) + 2Ag+(aq) + 2NO3-(aq) g2Ag(s) + Fe2+(aq) + 2NO3-(aq)
Net Ionic : Fe(s) + 2Ag+(aq) g2Ag(s) + Fe2+(aq)
Example 3: K3PO4(aq) + 3HNO3(aq) g?
Molecular : K3PO4(aq) + 3HNO3(aq) ® H3PO4(aq) + 3KNO3(aq)
Total Ionic :
3K+(aq) + PO43-(aq) + 3H+(aq) + 3NO3-(aq) ® H3PO4(aq) + 3K+(aq) + 3NO3-(aq)
Net Ionic : PO43-(aq) + 3H+(aq) ® H3PO4(aq)
Example 4: KNO3(aq) + HCl(aq) ® ?
Molecular : KNO3(aq) + HCl(aq) ® KCl(aq) + HNO3(aq)
Total Ionic : K+(aq) + NO3-(aq) + H+(aq)+ Cl-(aq) ® K+(aq) + Cl-(aq) + H+(aq) + NO3-(aq)
Net Ionic : There is no net because all species remained as aqueous ions
For thursday
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Keep reading in Chapter 4
Lab Quiz 1
Purity of Hydrate Lab due
Work on Chapter 4 Mastering Chemistry
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