Acid information

An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H⁺), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.^[1]

The first category of acids are the proton donors, or Brønsted–Lowry acids. In the special case of aqueous solutions, proton donors form the hydronium ion H₃O⁺ and are known as Arrhenius acids. Brønsted and Lowry generalized the Arrhenius theory to include non-aqueous solvents. A Brønsted or Arrhenius acid usually contains a hydrogen atom bonded to a chemical structure that is still energetically favorable after loss of H⁺.

Aqueous Arrhenius acids have characteristic properties that provide a practical description of an acid.^[2] Acids form aqueous solutions with a sour taste, can turn blue litmus red, and react with bases and certain metals (like calcium) to form salts. The word acid is derived from the Latin acidus, meaning 'sour'.^[3] An aqueous solution of an acid has a pH less than 7 and is colloquially also referred to as "acid" (as in "dissolved in acid"), while the strict definition refers only to the solute.^[1] A lower pH means a higher acidity, and thus a higher concentration of positive hydrogen ions in the solution. Chemicals or substances having the property of an acid are said to be acidic.

Common aqueous acids include hydrochloric acid (a solution of hydrogen chloride that is found in gastric acid in the stomach and activates digestive enzymes), acetic acid (vinegar is a dilute aqueous solution of this liquid), sulfuric acid (used in car batteries), and citric acid (found in citrus fruits). As these examples show, acids (in the colloquial sense) can be solutions or pure substances, and can be derived from acids (in the strict^[1] sense) that are solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.

The second category of acids are Lewis acids, which form a covalent bond with an electron pair. An example is boron trifluoride (BF₃), whose boron atom has a vacant orbital that can form a covalent bond by sharing a lone pair of electrons on an atom in a base, for example the nitrogen atom in ammonia (NH₃). Lewis considered this as a generalization of the Brønsted definition, so that an acid is a chemical species that accepts electron pairs either directly or by releasing protons (H⁺) into the solution, which then accept electron pairs. Hydrogen chloride, acetic acid, and most other Brønsted–Lowry acids cannot form a covalent bond with an electron pair, however, and are therefore not Lewis acids.^[4] Conversely, many Lewis acids are not Arrhenius or Brønsted–Lowry acids. In modern terminology, an acid is implicitly a Brønsted acid and not a Lewis acid, since chemists almost always refer to a Lewis acid explicitly as such.^[4]

^ ^a ^b ^c IUPAC Gold Book - acid
^ Petrucci, R. H.; Harwood, R. S.; Herring, F. G. (2002). General Chemistry: Principles and Modern Applications (8th ed.). Prentice Hall. p. 146. ISBN 0-13-014329-4.
^ Merriam-Webster's Online Dictionary: acid
^ ^a ^b Otoxby, D. W.; Gillis, H. P.; Butler, L. J. (2015). Principles of Modern Chemistry (8th ed.). Brooks Cole. p. 617. ISBN 978-1305079113.

[IUPAC_acid-1] IUPAC Gold Book - acid

[2] Petrucci, R. H.; Harwood, R. S.; Herring, F. G. (2002). General Chemistry: Principles and Modern Applications (8th ed.). Prentice Hall. p. 146. ISBN 0-13-014329-4.

[3] Merriam-Webster's Online Dictionary: acid

[Oxtoby8th-4] Otoxby, D. W.; Gillis, H. P.; Butler, L. J. (2015). Principles of Modern Chemistry (8th ed.). Brooks Cole. p. 617. ISBN 978-1305079113.

Acid information

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Acid

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Hydrochloric acid

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Boric acid

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Nitric acid

Formic acid

Citric acid

LSD

Fatty acid

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Oxalic acid

Nucleic acid

Carboxylic acid

Lactic acid

Glycolic acid

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Tartaric acid

Bile acid

A Is for Acid

Benzoic acid

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Acids and bases

Acceptor number Acid Acid–base reaction Acid–base homeostasis Acid strength Acidity function Amphoterism Base Buffer solutions Dissociation constant Donor number Equilibrium chemistry Extraction Hammett acidity function pH Proton affinity Self-ionization of water Titration Lewis acid catalysis Frustrated Lewis pair Chiral Lewis acid
Acid types
Brønsted–Lowry Lewis Mineral Organic Oxide Strong Superacids Weak Solid
Base types
Brønsted–Lowry Lewis Organic Oxide Strong Superbases Non-nucleophilic Weak
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