Acids _ Bases

							       Characteristics of
        Acids & Bases

             Characteristics of acidic and
             basic solutions, and identify
            the ions responsible for these
                      properties.
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            Acids and Bases

   Acids are generally a            Bases, or alkaline
    class of substances               substances, are
    that taste sour, such             characterized by
    as vinegar, which is              their bitter taste and
    a dilute solution of              slippery feel.
    acetic acid.




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        The Proton in Water:
         Arrhenius Theory

 First precise                    Swedish chemist
  definition of an an               Arrhenius defined an
  acid and base was                 acid as a substance
  given by Swedish                  that ionizes in water
                                    to give hydrogen
  Chemist Svante                    ions, and a base as
  Arrhenius.                        a substance that
 Referred to as                    ionizes in water to
  Arrhenius Theory.                 give hydroxide ions.

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            Arrhenius Theory

   Hydrochloric acid, HCl, is a strong acid,
    and is very soluble in water. It
    dissociates into its component ions in
    the following manner:

   HCl (g)       H+ (aq) + Cl- (aq)


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            Arrhenius Theory

 Hydrogen ion interacts strongly with a
  lone pair of electrons on the oxygen of
  a water molecule.
 Resulting ion, H3O+ is called the
  hydronium ion.



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            Hydronium ion



H+ + H-O-H                                 H3O+
ACIDIC solutions are formed when an acid
 transfers a proton to water




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The reaction of HCl with water



    HCl (aq) + H2O (l)            H3O+ (aq) + Cl- (aq)



    HCl (aq)           H+ (aq) + Cl- (aq)



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  Brønsted-Lowry Concept of
       Acids and Bases

 Acids are substances that are
  capable of donating a proton, and
  bases are substances capable of
  accepting a proton.
 http://www.science.ubc.ca/~chem/
  tutorials/pH/section0/hcl.html

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Brønsted-Lowry Concept


   Proton transfer
                              http://www.science.ub
    from Chloride
                              c.ca/~chem/tutorials/p
     Ion to Water             H/index.html
       Molecule




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Brønsted-Lowry Concept


 Water     can act as an acid or a
  base.
 NH3(aq) + H2O(l)
        NH4+(aq) + OH-(aq)


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Brønsted-Lowry Concept

 H2O    acts as a Brønsted acid by
    donating a proton to NH3 which
    acts as a Brønsted base.
   All Arrhenius acids are also Brønsted
    acids.
    All Arrhenius bases are also Brønsted
    bases.

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Conjugate Acid-Base Pairs

     Reaction of NH3 and H2O :
     (1) NH3 + H2O      NH4+ + OH-

     Reverse of this reaction:
     (2) NH4 + OH-       NH3 + H2O

      NH4+ acts as an acid which donates a proton
     to OH-. OH- acts as a base
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Conjugate Acid-Base Pairs
    An acid and a base that are related by
     the gain and loss of a proton are called:


  conjugate      acid-base pair



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     Conjugate Acid-Base


 NH4+   is the             NH3   is the
  conjugate acid             conjugate base
  of NH3                     of NH4+.
 Every acid has            Every base has
  associated with            associated with
  it a conjugate             it a conjugate
  base.                      acid
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     Conjugate Acid-Base

 base       acid                 conj         conj
                                  acid         base

   NH3     + H2O                  NH4+ + OH-




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     Conjugate Acid-Base

   Acids and bases can be either neutral or
    charged species:
       H2O (acid or base, neutral),

            O2- (base, charged), C2H3O2- (acid,
            charged) .

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     Conjugate Acid-Base
   For any reaction:
       HA + H2O                     H3O+ + A-

   If HA is a strong acid because it gives up its
    proton readily, then A- is a weak base
    because it has little affinity for the proton.



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     Conjugate Acid-Base

      HA + H2O         H3O+ + A-

   If HA is a weak acid because it donates
    very few protons to the water, then A-
    has a high affinity for a proton, and A-
    is a stronger base than water.

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    Neutralization Reactions

   In a Neutralization reaction a strong
    acid will react a strong base to yield a
    salt and water:

   Ca(OH)2 + HNO3                  Ca(NO3)2 + H2O



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 Lewis acid and Lewis base

   Neither the Arrhenius or the Brønsted-
    Lowry theory of acids and bases
    accounts for all the existing acids and
    bases. A new definition of acid-base
    behavior, called Lewis theory, is more
    comprehensive.


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 Lewis acid and Lewis base

   A Lewis acid is an electron-pair
    acceptor.

   A Lewis base is an electron-pair
    donor.



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 Lewis acid and Lewis base

 Everything that is a Brønsted-Lowry
  base (a proton acceptor) is also a Lewis
  base.
 Lewis bases can donate their electrons
  to substances other than H.



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 Lewis acid and Lewis base

                 H                                  H     +
H+          +   :N- H                             H-N-H
                 H                                  H

   NH3 is a Brønsted-Lowry base and a
    Lewis base

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 Lewis acid and Lewis base

  H         F                      H F
H-N: +      B-F                  H-N-B-F
  H         F                      H F
  Base      Acid

   Here, NH3 is behaving as a Lewis base
    only (and BF3 is a Lewis acid).
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