Common acid-base theories

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Common acid-base theories

• By Dr. O. Rajabi (Pharm.D.- Ph.D.)
• Associate Professor of Chemistry
• Department of Medicinal chemistry
• Mashad University of Medical Sciences

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Acid and Bases
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Acid and Bases
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Acid and Bases
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1- Lavoisier definition
August 26, 1743
May 8, 1794
Lavoisier's knowledge of strong acids was mainly
restricted to oxyacids, which tend to contain
central atoms in high oxidation states surrounded
by oxygen, such as HNO3 and H2SO4
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An oxoacid is an acid which contains oxygen. More
specifically, it is an acid which contains
oxygen contains at least one other element
has at least one hydrogen atom bound to oxygen
and forms an ion by the loss of one or more
protons
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The name oxyacid is sometimes used, although this
is not recommended
Generally, oxoacids are simply polyatomic ions
with a hydrogen cation
Although carboxylic acids fulfill the criteria
above, they are not generally considered as
oxoacids
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2- Liebig definition
This definition was proposed by Justus von Liebig
circa 1838, based on his extensive works on the
chemical composition of organic acids.
(May 12, 1803 April 18, 1873)
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This finished the doctrinal shift from
oxygen-based acids to hydrogen-based acids,
started by Davy
According to Liebig, an acid is a
hydrogen-containing substance in which the
hydrogen could be replaced by a metal. Liebig's
definition, while completely empirical, remained
in use for almost 50 years until the adoption of
the Arrhenius definition.
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3- Arrhenius definition
The Arrhenius definition of acid-base reactions
is a more simplified acid-base concept devised by
Svante Arrhenius, which was used to provide a
modern definition of bases that followed from his
work with Friedrich Wilhelm Ostwald in
establishing the presence of ions in aqueous
solution in 1884
Nobel prize in chemistry in 1903
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As defined at the time of discovery, acid-base
reactions are characterized by Arrhenius acids,
which dissociate in aqueous solution form
hydrogen or the later-termed oxonium (H3O) ions,
and Arrhenius bases which form hydroxide (OH-)
ions.
More recent IUPAC recommendations now suggest the
newer term "hydronium be used in favor of the
older accepted term "oxonium.
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Arrhenius acids form hydrogen ions in aqueous
solution with Arrhenius bases forming hydroxide
ions.
acid base- ? salt water
2NaOH H2SO4 ? 2 H2O Na2SO4
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4- Brønsted-Lowry definition
1923
professor of inorganic and physical chemistry at
Copenhagen
English physical chemist
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The Brønsted-Lowry definition, formulated
independently by its two proponents Johannes
Nicolaus Brønsted and Martin Lowry in 1923 is
based upon the idea of protonation of bases
through the de-protonation of acids -- more
commonly referred to as the ability of acids to
"donate" hydrogen ions (H) or protons to bases,
which "accept" them.
CH3COOH H2O CH3COO- H3O NH3 H2O
NH4 OH- CH3COO- H2O CH3COOH OH- NH4
H2O NH3 H3O
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In contrast to the Arrhenius definition, the
Brønsted-Lowry definition refers to the products
of an acid-base reaction as conjugate acids and
bases to refer to the relation of one proton, and
to indicate that there has been a reaction
between the two quantities, rather than a
"formation" of salt and water, as explained in
the Arrhenius definition
AH B ? BH A- General formula for
representing Brønsted-Lowry reactions.
HCl (aq) H2O ? H3O (aq) Cl- (aq)
Hydrochloric acid completely reacts with water
to form the hydronium and chloride ions
CH3COOH NH3 ? NH4 CH3COO- Acetic acid
reacts incompletely with ammonia, no hydronium
ions being produced
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In differentiation from the Arrhenius definition,
the Brønsted-Lowry definition postulates that for
each acid, there is a conjugate acid and base or
"conjugate acid-base pair" that is formed through
a complete reaction, which also includes water,
which is amphoteric
For example, zinc oxide (ZnO) reacts differently
depending on the pH of the solution In acids
ZnO 2H ? Zn2 H2O In bases ZnO H2O 2OH-
? Zn(OH)42-
Base (Proton Acceptor) H2O HCl ? H3O
Cl- Acid (Proton Donor) H2O NH3 ? NH4
OH- (Indeed, it can do both at once 2H2O ? H3O
OH-)
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• O (e.g. H2SO4) was originally thought to cause
  acidic properties. Later, H was implicated, but
  it was still not clear why CH4 was neutral.
• Arrhenius made the revolutionary suggestion that
  some solutions contain ions that acids produce
  H3O (hydronium) ions in solution.

• The more recent Bronsted-Lowry concept is that
  acids are H (proton) donors and bases are proton
  acceptors

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The Bronsted-Lowry concept

• In this idea, the ionization of an acid by water
  is just one example of an acid-base reaction.

acid
base
conjugate acid
conjugate base
conjugate acid-base pairs

• Acids and bases are identified based on whether
  they donate or accept H.
• Conjugate acids and bases are found on the
  products side of the equation. A conjugate base
  is the same as the starting acid minus H.

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Practice problems
Identify the acid, base, conjugate acid,
conjugate base, and conjugate acid-base pairs
HC2H3O2(aq) H2O(l) ? C2H3O2(aq) H3O(aq)
acid
base
conjugate acid
conjugate base
conjugate acid-base pairs
OH (aq) HCO3(aq) ? CO32(aq) H2O(l)
acid
base
conjugate acid
conjugate base
conjugate acid-base pairs
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• Acid Base Strength
• Strong Acid - Transfers all protons to water
  completely ionizes
  strong electrolyte
  conjugate base is weaker and has
  negligible tendency to be
  protonated.
• Weak Acid - Fraction of protons transferred to
  water partly ionized weak
  electrolyte conjugate base
  is stronger readily
  accepting protons from water
• As acid strength decreases, base strength
  increases.
• The stronger the acid, the weaker its conjugate
  base
• The weaker the acid, the stronger its conjugate
  base

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Leveling effect
The term leveling effect refers to a solvent's
ability to level the effect of a strong acid or
base dissolved in it.
When a strong acid is dissolved in water, it
reacts with it to form H3O in the following
reaction (where "HA" is a generic strong acid
such as "HCl") HA H2O -gt A- H3O Any acid
that is stronger than H3O reacts with H2O to
form H3O therefore, no acid can be stronger
than H3O in H2O. This is true with any solvent,
although usually to a lesser extent than with
water. Bases may be also leveled in solvents
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Which one is stronger? A) HCl B) HClO4
Becouse of this leveing effect, HClO4 and HCl
behave as if they had the same acid strength
both are leveled to H3O HClO4 H2O ---- H3O
ClO- HCl H2O ---- H3O Cl-
In acetic acid solvent, which is less basic than
H2O, HClO4 and HCl are not leveled to the same
strength HClO4 CH3COOH CH3COOH2 ClO4_
K 1.3 10-5 HCl CH3COOH CH3COOH2 Cl-
K 2.8 10-9
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5-Lewis definition
a famous American physical chemist known for his
1902 Lewis dot structures, his paper "The Atom
and the Molecule", which is the foundation of
modern valence bond theory, developed in
coordination with Irving Langmuir .
his 1923 textbook Thermodynamics and the Free
Energy of Chemical Substances, written in
coordination with Merle Randall, one of the
founding books in chemical thermodynamics
Gilbert Newton Lewis (October 23, 1875 - March
23, 1946)
In 1926, Lewis coined the term "photon" for the
smallest unit of radiant energy.
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The Lewis definition of acid base reactions,
devised by Gilbert N. Lewis in 1923 is an
encompassing theory to the Brønsted-Lowry and
solvent-system definitions with regards to the
premise of a donation mechanism, which conversely
attributes the donation of electron pairs from
bases and the acceptance by acids, rather than
protons or other bonded substances and spans both
aqueous and non-aqueous reactions
Ag 2 NH3 ? H3NAgNH3
A silver cation reacts as an acid with ammonia
which acts as an electron-pair donor, forming an
ammonia-silver adduct
Acid - An electron pair acceptor Base - An
electron pair donor
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6-Solvent-system definition
This definition is based on a generalization of
the earlier Arrhenius definition to all
autodissociating solvents. In all such solvents
there is a certain concentration of a positive
species, solvonium cations and negative species,
solvate anions, in equilibrium with the neutral
solvent molecules. For example
2H2O ? H3O (hydronium) OH- (hydroxide) 2NH3
? NH4 (ammonium) NH2- (amide) or even some
aprotic systems N2O4 ? NO (nitrosonium) NO3-
(nitrate) 2SbCl3 ? SbCl2 (dichloroantimonium)
SbCl4- (tetrachloroantimonate)
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A solute causing an increase in the concentration
of the solvonium ions and a decrease in the
solvate ions is an acid and one causing the
reverse is a base.
Thus, in liquid ammonia, KNH2 (supplying NH2-) is
a strong base, and NH4NO3 (supplying NH4) is a
strong acid.
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Other acid-base theories
Usanovich definition
Lux-Flood definition
Pearson definition
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What is a strong Acid?
An Acid that is 100 ionized in water.
Strong Acids 100 ionized (completely
dissociated) in water. HCl H2O ? H3O
Cl-
often written as HCl ? H Cl-
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Strong Acids 100 ionized (completely
dissociated) in water. HCl H2O ? H3O
Cl-
Strong Acids Perchloric HClO4 Chloric,
HClO3 Hydrobromic, HBr Hydrochloric,
HCl Hydroiodic, HI Nitric,
HNO3 Sulfuric, H2SO4
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What is a strong Base?
A base that is completely dissociated in water
(highly soluble).
NaOH(s) ? Na OH-
Strong Bases Group 1A metal hydroxides (LiOH,
NaOH, KOH, RbOH, CsOH) Heavy Group 2A metal
hydroxides Ca(OH)2, Sr(OH)2, and Ba(OH)2
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Strong Acids 100 ionized (completely
dissociated) in water. HCl H2O ? H3O
Cl-
Note the one way arrow.
Weak Acids Only a small (dissociated) in
water. HC2H3O2 H2O ? H3O C2H3O2-
Note the 2-way arrow.
Why are they different?
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Strong Acids
HCl HCl HCl HCl HCl
(H2O)
ADD WATER to MOLECULAR ACID
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Strong Acids
Cl-
H3O
(H2O)
Cl-
H3O
H3O
Cl-
Cl-
H3O
H3O
Cl-
Note No HCl molecules remain in solution, all
have been ionized in water.
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Weak Acid Ionization
HC2H3O2
HC2H3O2
(H2O)
HC2H3O2
HC2H3O2
?
HC2H3O2
Add water to MOLECULES of WEAK Acid
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Weak Acid Ionization
HC2H3O2
HC2H3O2
H30 C2H3O2-
HC2H3O2
(H2O)
HC2H3O2
?
H30 C2H3O2-
HC2H3O2
HC2H3O2
Note At any given time only a small portion of
the acid molecules are ionized and since
reactions are running in BOTH directions the
mixture composition stays the same.
This gives rise to an Equilbrium expression, Ka
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Chemical equilibrium
In a chemical process, chemical equilibrium is
the state in which the chemical activities or
concentrations of the reactants and products have
no net change over time. Usually, this would be
the state that results when the forward chemical
process proceeds at the same rate as their
reverse reaction.
The reaction rates of the forward and reverse
reactions are generally not zero but, being
equal, there are no net changes in any of the
reactant or product concentrations. This process
is called as dynamic equilibrium
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Berthollet was one of the first chemists to
recognize the characteristics of a reverse
reaction, and hence, chemical equilibrium.
Potassium chlorate (KClO3), a strong oxidizer, is
known as Berthollet's Salt. Non-stoichiometric
compounds are also named berthollides in his
honor.
Claude Louis Berthollet.
Berthollet was one of several scientists who went
with Napoleon to Egypt !
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The concept of chemical equilibrium was developed
after Berthollet (1803) found that some chemical
reactions are reversible. For any reaction such
as
aA ßB ?C dD
to be at equilibrium the rates of the forward and
backward (reverse) reactions have to be equal. In
this chemical equation with harpoon arrows
pointing both ways to indicate equilibrium, A and
B are reactant chemical species, C and D are
product species, and a, ß, s, and ? are the
stoichiometric coefficients of the respective
reactants and products.
Forward reaction rate KAaBß Backward
reaction rate K-C?Dd
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where A, B, C and D are active masses and k and
k- are rate constants. Since forward and backward
rates are equal
KAaBß K-C?Dd
and the ratio of the rate constants is also a
constant, now known as an equilibrium constant
K K/K- C?Dd/AaBß
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