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ACIDS AND BASES AND SOLVENT SYSTEMS

ACIDS AND BASES AND SOLVENT SYSTEMS. -Many definitions are known for acids and bases Br

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ACIDS AND BASES AND SOLVENT SYSTEMS

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    1. 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

    3. 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

    4. 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)

    5. 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.

    6. 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

    7. 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

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