ACIDS AND BASES AND SOLVENT SYSTEMS

							ACIDS AND BASES AND SOLVENT SYSTEMS

    Utility of a solvent depends on :
1.    Its auto-dissociation properties
2.    Its acidity or bascity
     electron or proton donor/acceptor properties
3.   Its dielectric constant
4.   Temperature over which it is a liquid

1.  Auto-dissociation – A proton is transferred from one solvent
    molecule to the next, generating the protonated cation of the
    solvent and the deprotonated anion of the solvent.
These solvents contain ionisable protons and are called Protic Solvents
 Examples:
2H2O = H3O+ + OH-
2HCl = H2Cl+ + Cl-

Class exercise: Write the autodissociation of HF, H2SO4 and NH3
 ACIDS AND BASES AND SOLVENT SYSTEMS

-Many definitions are known for acids and bases
   Brönsted-Lowry Definition
  Solvent System Definition
  Lewis Definition
Brönsted-Lowry Definition:
An acid is a proton donor and a base is a proton acceptor.
Note: In water any substance that increases [H3O+] is acidic and
  any substance which lowers it is basic.
      Any substance that increases [OH-] is basic and any
      substance which lowers it is acidic.
- Consider HCl is acidic and B(OH)3 (boric acid) is also acidic
- Similarly both NaOH and NH3 are basic.
       Explain.
         Solvent System Definition
   Acid base properties are closely connected with the properties of
    solvents
   This definition applies in all cases where the solvent is capable of
    autoionization whether protons are involved or not.

   Solvents are defined as protic – contains ionizable protons (H2O,
    HCl, HF, H2SO4) (write autionisation for each)

   or aprotic – no ionizable protons (CCl4, DMF, DMSO THF, BrF3)

   In this definition a solute that produces +ve solvent ions is an acid
    and a base is a substance that produces –vely charged solvent ions.

   Thus in liquid ammonia a substance which produces NH4+ ions is an
    acid an a substance which produces amide ions, NH2- would be a base
        Solvent System Definition
   Note: This is similar to Brönsted-Lowry Definition when applied to
    aprotonic solvent

   For some substances acidic or basic character can only be specified
    in relation to a particular solvent

    So, acetic acid (CH3COOH) is an acid in water but a base in sulfuric
    acid: (write relevant equations on board)
       Lewis Definition – Most General
   G. N. Lewis defined an acid as an electron pair acceptor and a base
    is an electron pair donor. (see examples pg. 227 Basic Inorganic)

   This definition covers many systems where protons are not
    involved. Note that this definition includes the Brönsted-Lowry
    and Solvent System definitions as special cases.

   In the Lewis sense all ligands are Lewis bases and all metal ions are
    Lewis acids.

   Lewis acidity or basicity is affected by the nature of substituents
    (electronic and steric effects)
Class Exercise: Arrange (CH3)3N, F3N and H3N in order of increasing
                base strength. Explain.
Lewis Definition

   Metal ions are placed into 2 groups depending on their
    preference for various ligands

   Type A – Alkali, alkaline-earth and lighter more highly
    charged metal ions such as
    Ti4+ , Fe3+, Co3+ and Al3+
    These ions (acids) are small compact and not very polarizable
    so they prefer ligands (bases that are also small and not
    very polarizable. – HARD ACIDS AND BASES

   Type B – Heavier transition metal ions such as
    Hg2+ , Pt2+, Ag+ and Cu+
    These ions (acids) are large and highly polarizable so they
    prefer ligands (bases) that are also large and highly
    polarizable. – SOFT ACIDS AND BASES
    SUPER ACIDS

These are very strong acids, up to 1010 times as
 strong as conc H2SO4.
They only exist in non-aqueous media

The Hammett scale, which extends beyond the
 normal pH scale is used to measure
 superacidity

Examples HSO3F/SbF3 and SbF5/HF