Common acid-base theories Common acid-base theories

By: Dr. O. Rajabi (Pharm.D.- Ph.D.)By: Dr. O. Rajabi (Pharm.D.- Ph.D.)

Associate Professor of ChemistryAssociate Professor of Chemistry

Department of Medicinal chemistryDepartment of Medicinal chemistry

Mashad University of Medical SciencesMashad University of Medical Sciences

Acid and BasesAcid and Bases

Acid and BasesAcid and Bases

Acid and BasesAcid and Bases

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

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

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

2- Liebig definition

(May 12, 1803 – April 18, 1873)

This definition was proposed by Justus von Liebig circa 1838, based on his extensive works on the chemical composition of organic acids.

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.

3- Arrhenius definition

Nobel prize in chemistry in 1903

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

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“.

Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions.

acid+ + base- → salt + water

2NaOH + H2SO4 → 2 H2O + Na2SO4

4- Brønsted-Lowry definition

1923

professor of inorganic and physical chemistry at Copenhagen English physical chemist

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+

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

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+ + H2OIn bases: ZnO + H2O + 2OH- → [Zn(OH)4]2-

Base (Proton Acceptor): H2O + HCl → H3O+ + Cl−

Acid (Proton Donor): H2O + NH3 → NH4+ + OH−

(Indeed, it can do both at once: 2H2O → H3O+ + OH−)

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

Ionization

+Cl HH

HO

+H

HH O Cl+

• 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 Bronsted-Lowry conceptThe Bronsted-Lowry concept• In this idea, the ionization of an acid by water

is just one example of an acid-base reaction.

• 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+.

+Cl HH

HO

+H

HH O Cl+

acid base conjugate acid conjugate base

conjugate acid-base pairs

Practice problemsPractice problemsIdentify the acid, base, conjugate acid, conjugate base, and conjugate acid-base pairs:

acid base conjugate acidconjugate baseHC2H3O2(aq) + H2O(l) C2H3O2

–(aq) + H3O+(aq)

conjugate acid-base pairs

acidbase conjugate acidconjugate baseOH

–(aq) + HCO3–(aq) CO3

2–(aq) + H2O(l)

conjugate acid-base pairs

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

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 -> 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

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

5-Lewis definition

Gilbert Newton Lewis (October 23, 1875 - March 23, 1946)

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

In 1926, Lewis coined the term "photon" for the smallest unit of radiant energy.

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 → [H3N:Ag:NH3]+

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 acceptorBase - An electron pair donor

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 H⇌ 3O+ (hydronium) + OH- (hydroxide) 2NH3 NH⇌ 4

+ (ammonium) + NH2− (amide)

or even some aprotic systems:N2O4 NO⇌ + (nitrosonium) + NO3

− (nitrate) 2SbCl3 SbCl⇌ 2

+ (dichloroantimonium) + SbCl4- (tetrachloroantimonate)

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.

Other acid-base theories

Usanovich definition

Lux-Flood definition

Pearson definition

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-

Strong Acids:Perchloric HClO4

Chloric, HClO3

Hydrobromic, HBrHydrochloric, HClHydroiodic, HINitric, HNO3

Sulfuric, H2SO4

Strong Acids:100% ionized (completely dissociated) in water.

HCl + H2O H3O+ + Cl-

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, andBa(OH)2]

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?

Strong Acids:

HCl HCl HClHCl HCl

ADD WATER to MOLECULAR ACID

(H2O)

Strong Acids:

(H2O)H3O+

H3O+

H3O+

H3O+

H3O+

Cl-

Cl-

Cl-

Cl-

Cl-

Note: No HCl molecules remain in solution, all have been ionized in water.

HC2H3O2

HC2H3O2

HC2H3O2

HC

2H3O

2

HC2H3O2

(H2O)

Weak Acid Ionization:

Add water to MOLECULES of WEAK Acid

HC2H3O2

HC2H3O2

HC2H3O2

HC

2H3O

2

HC2H3O2

H30

+ C2H3O2-

(H2O)

Weak Acid Ionization:

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

H30+ C2H3O2

-HC2H3O2

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

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 !

The concept of chemical equilibrium was developed after Berthollet (1803) found that some chemical reactions are reversible. For any reaction such as:

αA + βB === γC +δD

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 α, β, σ, and γ are the stoichiometric coefficients of the respective reactants and products.

Forward reaction rate = K+[A]α[B]β

Backward reaction rate = K-[C]γ[D]δ

where A, B, C and D are active masses and k+ and k− are rate constants. Since forward and backward rates are equal:

K+[A]α[B]β= K-[C]γ[D]δ

and the ratio of the rate constants is also a constant, now known as an equilibrium constant

K= K+/K- = [C]γ[D]δ/[A]α[B]β