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  • Chemistry Notes – For W.B.C.S. Examination – Theory Of Acids And Bases.
    Posted on July 16th, 2019 in Chemistry
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    Chemistry Notes – For W.B.C.S. Examination – Theory Of Acids And Bases.

    There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the Brönsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in 1923. Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions.

    The Arrhenius Theory

    When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate , into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H +, and a base as a compound that can dissociate in water to yield hydroxide ions, OH  . For example, hydrochloric acid, HCl, dissociates in water to yield the required hydrogen ions, H +, and also chloride ions, Cl  . The base sodium hydroxide, NaOH, dissociates in water to yield the required hydroxide ions, OH , and also sodium ions, Na +.

    The Brönsted-Lowry Theory

    Some substances act as acids or bases when they are dissolved in solvents other than water, such as liquid ammonia. The Brönsted-Lowry theory, named for the Danish chemist Johannes Brönsted and the British chemist Thomas Lowry, provides a more general definition of acids and bases that can be used to deal both with solutions that contain no water and solutions that contain water. It defines an acid as a proton donor and a base as a proton acceptor. In the Brönsted-Lowry theory, water, H 2O, can be considered an acid or a base since it can lose a proton to form a hydroxide ion, OH , or accept a proton to form a hydronium ion, H 3+ . When an acid loses a proton, the remaining species can be a proton acceptor and is called the conjugate base of the acid. Similarly when a base accepts a proton, the resulting species can be a proton donor and is called the conjugate acid of that base. For example, when a water molecule loses a proton to form a hydroxide ion, the hydroxide ion can be considered the conjugate base of the acid, water. When a water molecule accepts a proton to form a hydronium ion, the hydronium ion can be considered the conjugate acid of the base, water.

    The Lewis Theory

    Another theory that provides a very broad definition of acids and bases has been put forth by the American chemist Gilbert Lewis. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF 3, can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.

     

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