QA Aula 2 Equilíbrio Químico e Iônico

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Profa Dra Silvania Maria Netto
2017
QUÍMICA ANALÍTICA 
QUALITATIVA
 Eletrólito
 Ácidos e Bases
 Equilíbrio Químico
PARTE II
Capítulo 9
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INTRODUÇÃO
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A substance, such as sodium chloride, that dissolved in water and
produces ions to give an electrically conducting solution is called
an electrolyte.
A substance, such as sucrose, or table sugar (C12H22O11), that is
not ionized and does not conduct electric current when dissolved in
water is called a Nonelectrolyte.
ELETRÓLITO
(a) Electrolyte solutions (b) Nonelectrolyte solutions
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Strong electrolyte: is a substance that is almost completely ionized in
aqueous solution, and the solution is a good electrical conductor.
Weak electrolyte: is partially ionized in aqueous solution, and the
solution is only a fair conductor of electricity. (is an electrolyte that
dissolves in water to give an equilibrium between a molecular
substance and a relatively small quantity of ions.)
NaCl(s) Na
+
(aq) + Cl
-
(aq)
 
NH3(aq) + H2O(l) NH4
+
(aq) + OH
-
(aq)
Strong electrolyte
Weak electrolyte
~ Essentially all soluble ionic compounds and only a relatively few
molecular compounds are strong electrolytes.
~ Most molecular compounds are either nonelectrolytes or weak
electrolytes.
ELETRÓLITO
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 Examples of strong electrolytes – they ionize
completely:
 NaCl(aq)  Na+(aq) + Cl-(aq)
 H2SO4(aq)  H+(aq) + HSO4
-(aq)
 Ca(NO3)2(aq)  Ca2+(aq) + 2NO3
-(aq);
 Examples of weak electrolytes – they do not ionize
completely:
 HC2H3O2(aq)  H+(aq) + C2H3O2
-(aq)
 NH4OH(aq)  NH4+(aq) + OH
-(aq);
 Mg(OH)2(s)  Mg2+(aq) + 2 OH-(aq)
ELETRÓLITO
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Electrolyte Classification of Some Common Substances
Strong Electrolytes Weak Electrolytes Nonelectrolytes
HCl, HBr, HI CH3COOH H2O
HClO4 HF CH3OH
HNO3 C2H5OH
H2SO4 C12H22O11(sucrose)
KBr Most organic compounds
NaCl
NaOH, KOH
Other soluble 
ionic compounds
ELETRÓLITO
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ELETRÓLITO
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Aqueous reactions can be grouped into three general
categories, each with its own kind of driving force:
precipitation reactions, acid base neutralization
reactions, and oxidation-reduction reactions.
REAÇÕES AQUOSAS
Precipitation Reactions
Precipitation reactions are process in which soluble
reactants yield an insoluble solid product that falls out
of solution
Most precipitations take place when certain cations
and anions combined to produce an insoluble ionic
solid called a precipitate.
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 Reactions that produce insoluble products (or
precipitates) when two aqueous solutions are mixed.
 Examples:
1) AgNO3(aq) + KBr(aq)  AgBr(s) + KNO3(aq)
2) Pb(NO3)2(aq) + K2CrO4  PbCrO4(s) + 2KNO3(aq)
3) BaCl2(aq) + H2SO4(aq)  BaSO4(s) + 2NaCl(aq)
4) 3Hg(NO3)2 (aq) + 2Na3PO4 (aq)  Hg3(PO4)2(s) + 6NaNO3(aq)
Precipitation Reactions
REAÇÕES AQUOSAS
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 Soluble salts:
 All compounds of alkali metals and NH4+
 All compounds containing nitrate, NO3-, and acetate. C2H3O2, 
except silver acetate, which is sparingly soluble;
 Most chlorides, bromides, and iodides, except AgX, Hg2X2, 
PbX2, and HgI2; where X = Cl-, Br-, or I-.
 Most sulfates, except CaSO4, SrSO4, BaSO4, PbSO4 and 
Hg2SO4.
 Insoluble or slightly soluble salts:
 Most hydroxides (OH-), sulfides (S2-), carbonates (CO3
2-), 
chromates (CrO4
2-), and phosphate (PO4
3-), except those 
associated with the Group 1 metals or NH4
+.
REAÇÕES AQUOSAS
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 Complete and balance the following reactions in aqueous 
solution and identify the precipitate, if formed.
a) CaCl2(aq) + Na2CO3(aq)  ?
b) NH4NO3(aq) + MgCl2(aq)  ?
c) Pb(NO3)2(aq) + KI(aq)  ?
d) AgNO3(aq) + Na3PO4(aq)  ?
REAÇÕES AQUOSAS
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E.g. the reaction of silver nitrate and sodium iodide in an
aqueous water solution yields sodium nitrate in solution and a
yellow precipitate of silver iodide. We can write the equation
for the reaction as follow:
AgNO3(aq) + NaI(aq) AgI(s) + NaNO3(aq)
We called this a molecular equation, an equation in
which the substances are written as if they were molecular
substances, even though they may exist as ions in the
solution. Molecular equation is useful because it is explicit
about what solutions have been added and what products
are obtained; and to calculate the amount of reactants or
products.
REAÇÕES AQUOSAS
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The molecular equation does not tell us that the reaction
actual involves ions in solution. However, we know that
soluble ionic substances in solution should be represented
by their separate ions.
To represent this, the above reaction as an ionic equation,
in which all the ions are explicitly shown:
An ionic equation:
Ag+(aq) + NO3
-
(aq) + Na
+
(aq) + I
-
(aq) AgI(s) + Na
+
(aq)+NO3
-
(aq)
Thisis an example of ionic equation, which is a chemical equation for
a reaction involving ions in solution in which soluble substances are
represented by the formulas of the predominant species in that
solution
REAÇÕES AQUOSAS
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Note that some ions appear on both side of equation.
These ions go through the reaction unchanged- does not
take part in the reaction. We called them spectator ions.
We can cancel them from the equation. The resulting
equation is a net ionic equation.
Ag+(aq) + I
-
(aq)  AgI(s)
Net ionic Equation
A net ionic equation is an equation that includes only the actual
participants in a reaction, with each participant denoted by the symbol
or formula that best represent it.
REAÇÕES AQUOSAS
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Some reactions are driven to products by the formation of
a gas.
E.g carbonates react with acids to produce gases product
NaHCO3(s) + HC2H3O2(aq)  NaC2H3O2 (aq)+ H2CO3(aq)
CaCO3(s) + 2H
+
(aq)  Ca
2+
(aq) + H2CO3(aq)
Reactions with Gases Formation
REAÇÕES AQUOSAS
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REAÇÕES ÁCIDO BASE
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Consider the production of ions in pure water. It produces a small
percentage of ions (about 2x10-7 % of the molecules react to gives
ions)
H2O(l)+ H2O(l)  H3O
+
(aq) + OH
-
(aq)
Arrhenius definition
An acid is a substance that provides hydrogen ions (H+) (increase
the concentration of H+) in aqueous solution. The symbol H+(aq)
does not really represent the structure of the ion present in
aqueous solution. H+ is too reactive to exit by itself, it attaches to
water to give the more stable hydronium ion, H3O
+
A base is a substance that produces hydroxide ions (OH-) (increase
the conc. of hydroxide ions) in aqueous solution.
REAÇÕES ÁCIDO BASE
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HA(aq)  H
+
(aq) + A
-
(aq)
HA is a general formula for an acid
MOH(aq)  M
+
(aq) + OH
-
(aq)
MOH is a general formula for a base
REAÇÕES ÁCIDO BASE
 Water is a amphiprotic solvent (acid and base)
 A useful definition of acids and bases is that independently
introduced by Johannes Brønsted (1879–1947) and Thomas Lowry
(1874–1936) in 1923.
 In the Brønsted-Lowry definition, acids are proton donors, and
bases are proton acceptors.
 Note that these definitions are interrelated. Defining a base as a
proton acceptor means an acid must be available to provide the
proton.
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O CH3COOH(aq) + NH3(aq) CH3COO
–
(aq) + NH4
+
(aq)
REAÇÕES ÁCIDO BASE
For example, acetic acid, CH3COOH, donates a proton
to ammonia, NH3, which serves as the base.
NH3 + H2O  NH4
+ + OH-
Base1 Ácido 2 Ácido 1 Base 2
H2O + CH3COOH(aq)  H3O
+ + CH3COO
–
(aq)
Base1 Ácido 2 Ácido 1 Base 2
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A strong acid is an acid that is almost completely ionized in
aqueous solution.
A weak acid is an acid that only partially ionized (as result of an
equilibrium reaction with water) in aqueous solution
E.g. HCl(aq)  H+(aq) + Cl-(aq) strong acid
HC2H3O2(aq)  H
+(aq) + C2H3O2
-(aq) weak acid
Acids such as HCl and HNO3 that have only one acdic hydrogen
atom per acid molecule are called monoprotic acids.
A polyprotic acids such as H2SO4 and H3PO4 are acids that yield
two or more acidic hydrogens per molecules.
E.g. sulfuric acid, dissociate twice
H2SO4(aq) + H2O(l)  HSO4
-
(aq) + H3O
+
(aq)
HSO4
-
(aq) + H2O(l)  SO4
2
-(aq) + H3O
+
(aq)
FORÇA dos ÁCIDOS e BASES
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A strong base is a base that dissociate nearly completely in
aqueous solution.
A weak base is a base that is only partially ionized (as result of 
an equilibrium reaction with water) in aqueous solution. 
NaOH(s) Na
+
(aq) + OH
-
(aq) strong base
NH3(aq) + H2O(l)  NH4
+
(aq) + OH
-
(aq) weak base
As polyprotic acids can give more than one H+, some base yield 
more than one hydroxide ions.
FORÇA dos ÁCIDOS e BASES
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FORÇA dos ÁCIDOS e BASES
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 Weak acids, of which aqueous acetic acid is one example, cannot
completely donate their acidic protons to the solvent.
 Instead, most of the acid remains undissociated, with only a small
fraction present as the conjugate base.
CH3COOH(aq) + H2O(l)  H3O
+
(aq) + CH3COO
–
(aq)
 The equilibrium constant for this reaction is called an acid
dissociation constant
FORÇA dos ÁCIDOS e BASES
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Some acids, such as phosphoric acid, can donate more than one
proton and are called polyprotic weak acids. Polyprotic acids are
described by a series of acid dissociation steps, each characterized
by it own acid dissociation constant.
FORÇA dos ÁCIDOS e BASES
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Amphiprotic Species
 Some species can behave as either an acid or a base. For example,
the following two reactions show the chemical reactivity of the
hydrogencarbonate ion, HCO3
–, in water
Kb2  Ka2 we expect that aqueous solutions of HCO3
– will be basic
 A species that can serve as both a proton donor anda proton
acceptor is called amphiprotic.
 Whether an amphiprotic species behaves as an acid or as a base
depends on the equilibrium constants for the two competing
reactions
FORÇA dos ÁCIDOS e BASES
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 When a base is added to an acid solution, the acid is said to
be neutralized. In a neutralization reaction, an acid and a
base react to form water and an aqueous solution of an
ionic compound called a salt
HA(aq) + MOH(aq)  MA(aq) +H2O(l)
HCl(aq) + NaOH(aq)  NaCl(aq) +H2O(l)
H+(aq) + Cl
-
(aq) +Na
+
(aq) + OH
-
(aq)  Na
+
(aq) +Cl
-
(aq) + H2O(l)
REAÇÕES ÁCIDO BASE
Acid-Base Neutralization Reaction
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The net ionic equation:
H2O(l)  H
+
(aq) + OH
-
(aq)
H2O(l) + H2O(l)  H3O
+
(aq) + OH
-
(aq) 
Kw = [H3O
+] [OH-] = 1.0x10-14
A value of 1.0x10–14 at a temperature of 24 °C. The value of Kw
varies substantially with temperature. For example, at 20 °C, Kw is
6.809x10–15, but at 30 °C Kw is 1.469x10–14
At the standard state temperature of 25 °C, Kw is 1.008x10–14, which
is sufficiently close to 1.0x10–14 that the latter value can be used
with negligible error.
DISSOCIAÇÃO da ÁGUA
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H2O(l) + H2O(l)  H3O
+
(aq) + OH
-
(aq) 
Kw = [H3O
+] [OH-] = 1.0x10-14
 What is the [OH–] if the [H3O
+] is 6.12x10–5 mol/L?
DISSOCIAÇÃO da ÁGUA
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EXERCÍCIOS
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EXERCÍCIOS
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EXERCÍCIOS
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EXERCÍCIOS