CHEM --> Lecture 2

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The Bronsted-Lowry concept

For a H atom to be acidic (i.e. be donated as a H+ ion), it must be bound to an atom that has a high electronegativity (usually from Group 16 or 17) e.g. HCl, H2SO4 etc.

Bases generally contain an atom of high electronegativity with a lone pair of electrons & are commonly metal oxides, hydroxides, or carbonates (e.g. NaOH, MgO, Na2CO3) or molecular compounds containing Group 15 or 16 elements

H2O CAN ACT AS BOTH!!! its Amphiprotic

Equilibrium constant

If K>>1 then [products] >> [reactants]

i.e. reaction goes nearly to completion (‘strong’ acid)

If K<<1 then [products] << [reactants] i.e. reaction hardly proceeds at all (‘weak’ acid)

Autoionisation of water: H2O(l) + H2O(l) ⇌ H3O+(aq) + OH–(aq)

Pure water is neutral because it contains equal concentrations of H 3 O+ and OH – H 3 O+ and OH – exist in ALL aqueous solutions, but • in acidic solutions [H 3 O +] > [OH –] • in basic solutions [H 3 O +] < [OH –]

A strong acid (e.g. HCl) reacts completely with water to give quantitative formation of H 3 O+ (& A - ions)

• A weak acid (e.g. CH 3COOH) reacts incompletely (partially) with water to form less than stoichiometric amounts of H 3 O+ (& A - ions)

• A strong base (e.g. NaOH) dissolves completely in water to give quantitative amounts of OH – (& B + ions)

• A weak base (e.g. NH 3) reacts incompletely with water to form less than stoichiometric amounts of OH – (& B + ions

A strong acid has a REALLY WEAK conjugate base - so weak it is ineffective at proton accepting

• A weak acid has a conjugate base partner that can accept protons ‘reasonably’ well…

• A strong base has a REALLY WEAK conjugate acid partner;

• A weak base has a stronger conjugate acid partner that can donate protons ‘reasonably’ well…

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