chapter 4 aqueous reactions and solution stoichiometrycheminnerweb.ukzn.ac.za/files/chem 110...
TRANSCRIPT
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Chapter 3
Aqueous Reactions and
Solution Stoichiometry
John D. BookstaverSt. Charles Community College
Cottleville, MO
Chemistry, The Central Science,
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Solutions
• Solutions are defined as
homogeneous mixtures
of two or more pure
substances.
• The solvent is present in
greatest abundance.
• All other substances are
solutes.
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Aqueous
Reactions
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Dissociation
• When an ionic
substance dissolves
in water, the solvent
pulls the individual
ions from the crystal
and solvates them.
• This process is called
dissociation.
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Aqueous
Reactions
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Dissociation
• An electrolyte is a
substances that
dissociates into ions
when dissolved in
water.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Electrolytes
• An electrolyte is a
substances that
dissociates into ions
when dissolved in
water.
• A nonelectrolyte may
dissolve in water, but
it does not dissociate
into ions when it does
so.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Electrolytes and
Nonelectrolytes
Soluble ionic
compounds tend
to be electrolytes.
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Aqueous
Reactions
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Electrolytes and
Nonelectrolytes
Molecular
compounds tend to
be nonelectrolytes,
except for acids and
bases.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Electrolytes
• A strong electrolyte
dissociates completely
when dissolved in
water.
• A weak electrolyte
only dissociates
partially when
dissolved in water.
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Aqueous
Reactions
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Strong Electrolytes Are…
• Strong acids
• Strong bases
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Aqueous
Reactions
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Strong Electrolytes Are…
• Strong acids
• Strong bases
• Soluble ionic salts
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Aqueous
Reactions
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Precipitation Reactions
When one mixes ions
that form compounds
that are insoluble (as
could be predicted by
the solubility
guidelines), a
precipitate is formed.
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Aqueous
Reactions
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Metathesis (Exchange) Reactions
• Metathesis comes from a Greek word that
means “to transpose.”
AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
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.
AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Solution Chemistry
• It is helpful 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.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Molecular Equation
The molecular equation lists the reactants
and products in their molecular form.
AgNO3 (aq) + KCl (aq) AgCl (s) + KNO3 (aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Ionic Equation
• In the ionic equation all strong electrolytes (strong
acids, strong bases, and soluble ionic salts) are
dissociated into their ions.
• This more accurately reflects the species that are
found in the reaction mixture.
Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq)
AgCl (s) + K+ (aq) + NO3- (aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Net Ionic Equation
• To form the net ionic equation, cross out anything
that does not change from the left side of the
equation to the right.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)
AgCl (s) + K+(aq) + NO3-(aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Net Ionic Equation
• To form the net ionic equation, cross out anything
that does not change from the left side of the
equation to the right.
• The only things left in the equation are those things
that change (i.e., react) during the course of the
reaction.
Ag+(aq) + Cl-(aq) AgCl (s)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Net Ionic Equation
• To form the net ionic equation, cross out anything
that does not change from the left side of the
equation to the right.
• The only things left in the equation are those things
that change (i.e., react) during the course of the
reaction.
• Those things that didn’t change (and were deleted
from the net ionic equation) are called spectator ions.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq)
AgCl (s) + K+(aq) + NO3-(aq)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Writing Net Ionic Equations
1. Write a balanced molecular equation.
2. Dissociate all strong electrolytes.
3. Cross out anything that remains
unchanged from the left side to the
right side of the equation.
4. Write the net ionic equation with the
species that remain.
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Aqueous
Reactions
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Acids
• Arrhenius defined acids
as substances that
increase the
concentration of H+
when dissolved in water.
• Brønsted and Lowry
defined them as proton
donors.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Acids
There are only seven
strong acids:
• Hydrochloric (HCl)
• Hydrobromic (HBr)
• Hydroiodic (HI)
• Nitric (HNO3)
• Sulfuric (H2SO4)
• Chloric (HClO3)
• Perchloric (HClO4)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Bases
• Arrhenius defined bases
as substances that
increase the
concentration of OH−
when dissolved in water.
• Brønsted and Lowry
defined them as proton
acceptors.
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Aqueous
Reactions
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Bases
The strong bases
are the soluble
metal salts of
hydroxide ion:
• Alkali metals
• Calcium
• Strontium
• Barium
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Aqueous
Reactions
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Acid-Base Reactions
In an acid-base
reaction, the acid
donates a proton
(H+) to the base.
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Aqueous
Reactions
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Neutralization Reactions
Generally, when solutions of an acid and a base are
combined, the products are a salt and water.
CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)
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Aqueous
Reactions
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Neutralization Reactions
When a strong acid reacts with a strong base, the net
ionic equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Neutralization Reactions
When a strong acid reacts with a strong base, the net
ionic equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)
Na+ (aq) + Cl- (aq) + H2O (l)
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Neutralization Reactions
When a strong acid reacts with a strong base, the net
ionic equation is…
HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l)
H+ (aq) + Cl- (aq) + Na+ (aq) + OH-(aq)
Na+ (aq) + Cl- (aq) + H2O (l)
H+ (aq) + OH- (aq) H2O (l)
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Aqueous
Reactions
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Gas-Forming Reactions
• Some metathesis reactions do not give the
product expected.
• In this reaction, the expected product (H2CO3)
decomposes to give a gaseous product
(CO2).
CaCO3 (s) + HCl (aq) CaCl2 (aq) + CO2 (g) + H2O (l)
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Aqueous
Reactions
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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) CaCl2 (aq) + CO2 (g) + H2O (l)
NaHCO3 (aq) + HBr (aq) NaBr (aq) + CO2 (g) + H2O (l)
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Aqueous
Reactions
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Gas-Forming Reactions
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)
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Aqueous
Reactions
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Gas-Forming Reactions
• This reaction gives the predicted product, but
you had better carry it out in the hood, or you
will be very unpopular!
• But just as in the previous examples, a gas is
formed as a product of this reaction.
Na2S (aq) + H2SO4 (aq) Na2SO4 (aq) + H2S (g)
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Aqueous
Reactions
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Oxidation-Reduction Reactions
• An oxidation occurs
when an atom or ion
loses electrons.
• A reduction occurs
when an atom or ion
gains electrons.
• One cannot occur
without the other.
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Aqueous
Reactions
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Oxidation Numbers
To determine if an oxidation-reduction
reaction has occurred, we assign an
oxidation number to each element in a
neutral compound or charged entity.
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Aqueous
Reactions
Rules for Assigning Oxidation
Numbers
1.For an atom in its elemental form, the
oxidation number is always zero.
e.g. Na = 0
and Cl2 = 0
2.For any monatomic ion the oxidation number
equals the charge on the ion.
e.g. Na+ = +1
and Cl- = -1
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Aqueous
Reactions
Oxidation Numbers3. Nonmetals usually have negative oxidation
numbers:
a) The oxidation number of oxygen is usually -2
except in the peroxide ion in which it has an
oxidation number of −1.
b) The oxidation number of hydrogen is +1 when bonded to a metal and -1 when bonded to a nonmetal.
c) The oxidation number of fluorine is -1 in all
compounds. The other halogens usually have an oxidation number of -1 unless combined with oxygen (oxyanions) where they have a positive oxidation number.
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Aqueous
Reactions
Oxidation Numbers
4.The sum of the oxidation numbers of all
atoms in a neutral compound is zero.
5.The sum of the oxidation numbers in a
polyatomic ion atom equals the charge
of the ion.
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Aqueous
Reactions
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Displacement Reactions
• In displacement reactions,
ions oxidize an element.
• The ions, then, are
reduced.
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Aqueous
Reactions
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Displacement Reactions
In this reaction,
silver ions oxidize
copper metal.
Cu (s) + 2 Ag+ (aq) Cu2+ (aq) + 2 Ag (s)
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Aqueous
Reactions
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Displacement Reactions
The reverse reaction,
however, does not
occur.
Cu2+ (aq) + 2 Ag (s) Cu (s) + 2 Ag+ (aq)x
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Activity Series
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Molarity
• Two solutions can contain the same
compounds but be quite different because the
proportions of those compounds are different.
• Molarity is one way to measure the
concentration of a solution.
moles of solute
volume of solution in litersMolarity (M) =
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Mixing a Solution
• To create a solution of a
known molarity, one
weighs out a known mass
(and, therefore, number of
moles) of the solute.
• The solute is added to a
volumetric flask, and
solvent is added to the line
on the neck of the flask.
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Aqueous
Reactions
Mixing a Solution
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Dilution
• One can also dilute a more concentrated solution by– Using a pipet to deliver a volume of the solution to
a new volumetric flask, and
– Adding solvent to the line on the neck of the new flask.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Dilution
The molarity of the new solution can be determined from the equation
Mc Vc = Md Vd,
where Mc and Md are the molarity of the concentrated and dilute solutions, respectively, and Vc and Vd are the volumes of the two solutions.
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Using Molarities in
Stoichiometric Calculations
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Aqueous
Reactions
© 2009, Prentice-Hall, Inc.
Titration
Titration is an
analytical
technique in
which one can
calculate the
concentration
of a solute in
a solution.