ACIDS AND BASES AND SOLVENT SYSTEMS

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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
• 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

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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.

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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

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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)

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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.

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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

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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