Acid




Acids and bases:
edit

An acid (often represented by the generic formula HA [H+A-]) is traditionally considered any oxidation state.

Definitions

Main article: acid-base reaction theories

The word "acid" comes from the Latin acidus meaning "sour," but in chemistry the term acid has a more specific meaning. There are four common ways to define an acid:

  • Arrhenius: According to this definition developed by the Swedish chemist Sir Humphry Davy at the same time believed all acids contained hydrogen. Arrhenius used this belief to develop this definition of acid.
  • conjugate acid-base pairs. Brønsted and Lowry independently formulated this definition, which includes water-insoluble substances not in the Arrhenius definition.
  • solvent-system definition: According to this definition, an acid is a substance that, when dissolved in an autodissociating solvent, increases the concentration of the solvonium cations, such as H3O+ in water, NH4+ in liquid ammonia, NO+ in liquid N2O4, SbCl2+ in SbCl3, etc. Base is defined as the substance that increases the concentration of the solvate anions, respectively OH-, NH2-, NO3-, or SbCl4-. This definition extends acid-base reactions to nonaqueous systems and even some aprotic systems, where no hydrogen nuclei are involved in the reactions. This definition is not absolute, a compound acting as acid in one solvent may act as a base in another.
  • Lewis: According to this definition developed by HOMO) of a base. That is, the HOMO from the base and the LUMO from the acid combine to a bonding molecular orbital.

Although not the most general theory, the Brønsted-Lowry definition is the most widely used definition. The strength of an acid may be understood by this definition by the stability of hydronium and the solvated conjugate base upon dissociation. Increasing or decreasing stability of the conjugate base will increase or decrease the acidity of a compound. This concept of acidity is used frequently for carboxylic acid. The molecular orbital description, where the unfilled proton orbital overlaps with a lone pair, is connected to the Lewis definition.

Properties

Bronsted-Lowry acids:

  • Are generally sour in taste
  • Strong or concentrated acids often produce a stinging feeling on mucous membranes
  • React to indicators as follows: turn blue litmus and phenolphthalein
  • Will react with metals to produce a metal salt and hydrogen
  • Will react with metal carbonates to produce water, CO2 and a salt
  • Will react with a base to produce a salt and water
  • Will react with a metal oxide to produce water and a salt
  • Will conduct electricity, depending on the degree of dissociation
  • Will produce solvonium ions, such as hydronium (H3O+) ions in water
  • Will denature proteins

MSDS for more detailed information.

Nomenclature

In the classical naming system, acids are named according to their anions. That ionic suffix is dropped and replaced with a new suffix (and sometimes prefix), according to the table below. For example, HCl has hydrochloric acid. In the IUPAC naming system, "aqueous" is simply added to the name of the ionic compound. Thus, for hydrogen chloride, the IUPAC name would be aqueous hydrogen chloride.

Classical naming system:

Anion Prefix Anion Suffix Acid Prefix Acid Suffix Example
per ate per ic acid perchloric acid (HClO4)
ate ic acid chloric acid (HClO3)
ite ous acid chlorous acid (HClO2)
hypo ite hypo ous acid hypochlorous acid (HClO)
ide hydro ic acid hydrochloric acid (HCl)

Chemical characteristics

In water the following equilibrium occurs between a weak acid (HA) and water, which acts as a base:

HA(aq) + H2O ⇌ H3O+(aq) + A-(aq)

The acidity constant (or acid dissociation constant) is the equilibrium constant for the reaction of HA with water:

K_a = {[\mbox{H}_3\mbox{O}^+]\cdot[\mbox{A}^-] \over [\mbox{HA}]}

Strong acids have large Ka values (i.e. the reaction equilibrium lies far to the right; the acid is almost completely dissociated to H3O+ and A-). Strong acids include the heavier hydrohalic acids: hydrofluoric acid, HF, is relatively weak.) For example, the Ka value for hydrochloric acid (HCl) is 107.

hypochlorous acid are all weak.

Note on terms used:

  • The terms "hydrogen ion" and "proton" are used interchangeably; both refer to H+.
  • In aqueous solution, the water is protonated to form hydronium ion, H3O+(aq). This is often abbreviated as H+(aq) even though the symbol is not chemically correct.
  • The strength of an acid is measured by its acid dissociation constant (Ka) or equivalently its pKa (pKa= - log(Ka)).
  • The pH of a solution is a measurement of the concentration of hydronium. This will depend on the concentration and nature of acids and bases in solution.

Polyprotic acids

Polyprotic acids are able to donate more than one proton per acid molecule, in contrast to monoprotic acids that only donate one proton per molecule. Specific types of polyprotic acids have more specific names, such as diprotic acid (two potential protons to donate) and triprotic acid (three potential protons to donate).

A monoprotic acid can undergo one dissociation (sometimes called ionization) as follows and simply has one acid dissociation constant as shown below:

HA(aq) + H2O(l) ⇌ H3O+(aq) + A−(aq)         Ka

A diprotic acid (here symbolized by H2A) can undergo one or two dissociations depending on the pH. Each dissociation has its own dissociation constant, Ka1 and Ka2.

H2A(aq) + H2O(l) ⇌ H3O+(aq) + HA−(aq)       Ka1
HA−(aq) + H2O(l) ⇌ H3O+(aq) + A2−(aq)       Ka2

The first dissociation constant is typically greater than the second; i.e., Ka1 > Ka2 . For example, carbonate anion (CO32−). Both Ka values are small, but Ka1 > Ka2 .

A triprotic acid (H3A) can undergo one, two, or three dissociations and has three dissociation constants, where Ka1 > Ka2 > Ka3 .

H3A(aq) + H2O(l) ⇌ H3O+(aq) + H2A−(aq)        Ka1
H2A−(aq) + H2O(l) ⇌ H3O+(aq) + HA2−(aq)       Ka2
HA2−(aq) + H2O(l) ⇌ H3O+(aq) + A3−(aq)         Ka3

An citrate ion. Even though the positions of the protons on the original molecule may be equivalent, the successive Ka values will differ since it is energetically less favorable to lose a proton if the conjugate base is more negatively charged.

Neutralization

water; for example, hydrochloric acid and sodium hydroxide form sodium chloride and water:

HCl(aq) + NaOH(aq) → H2O(l) + NaCl(aq)

Neutralization is the basis of pH indicator shows equivalence point when the equivalent number of moles of a base have been added to an acid. It is often wrongly assumed that neutralization should result in a solution with pH 7.0, which is only the case with similar acid and base strengths during a reaction.

Neutralization with an base weaker than the acid results in an weakly acidic salt. An example is the weakly acidic sodium hydroxide.

Weak acid/weak base equilibria

Main article: Henderson-Hasselbalch equation

In order to lose a proton, it is necessary that the pH of the system rise above the pKa of the protonated acid. The decreased concentration of H+ in that basic solution shifts the equilibrium towards the conjugate base form (the deprotonated form of the acid). In lower-pH (more acidic) solutions, there is a high enough H+ concentration in the solution to cause the acid to remain in its protonated form, or to protonate its conjugate base (the deprotonated form).

Solutions of weak acids and salts of their conjugate bases form buffer solutions.

Applications of acids

There are numerous uses for acids. Acids are often used to remove rust and other corrosion from metals in a process known as pickling. They may be used as an electrolyte in a alkylation process to produce gasoline.

Common acids

  • Citric Acid

Mineral acids

Sulfonic acids

  • Methanesulfonic acid (aka mesylic acid) (MeSO3H)
  • Ethanesulfonic acid (aka esylic acid) (EtSO3H)
  • Benzenesulfonic acid (aka besylic acid) (PhSO3H)
  • Toluenesulfonic acid (aka tosylic acid, or (C6H4(CH3)(SO3H))

Carboxylic acids

References

    • Listing of strengths of common acids and bases
    • Zumdahl, Chemistry, 4th Edition.

    See also

    Chemistry
    Environment
     
    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Acid". A list of authors is available in Wikipedia.