Acid
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An acid is a molecule or ion capable of either donating a proton, known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.
Zinc, a typical metal, reacting with hydrochloric acid, a typical acid
Svante Arrhenius
Hydrochloric acid (in beaker) reacting with ammonia fumes to produce ammonium chloride (white smoke)
This is an ideal titration curve for alanine, a diprotic amino acid. Point 2 is the first equivalent point where the amount of NaOH added equals the amount of alanine in the original solution.
Lewis acids and bases
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A Lewis acid is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any species that has a
Acid
…of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. 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…
Diagram of some Lewis bases (left) and acids (right)