Buffers
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what is a buffer solution?
a system that minimises pH changes on addition of small amounts of an acid or a base
what are the two methods of preparing a buffer solution?
• a weak acid and a salt of the weak acid (e.g.CH3COOH/CH3COONa) • excess of a weak acid and a strong alkali (preparation by partial neutralisation of a weak acid) (e.g.excess CH3COOH/NaOH)
how do you prepare a buffer solutions from a weak acid and its salt?
• CH3COOH would partially dissociate: CH3COOH(aq) <–> H+(aq) + CH3COO-(aq) • the salt completely dissolves: CH3COONa(s) + aq –> CH3COO-(aq) + Na+(aq)
how do you prepare a buffer solution from excess weak acid and a strong alkali?
• weak acid is partially neutralised by the alkali to form its conjugate base (salt) • some of the weak acid is left over unreacted
what are the 2 components of a buffer solution?
• a weak acid • its conjugate base
what does the weak acid do?
HA - removes added alkali
what does the conjugate base do?
A- - removes added acid
how do conjugate base in an acid buffer minimise changes in pH?
conjugate base removes added acid - on addition to acid [H+], it increases - H+ ions react with the conjugate base (A-) - the equilibrium position shifts left, removing most of the H+ ions
how do weak acid in an acid buffer minimise changes in pH?
weak acid removes added alkali - on addition to the alkali, OH- - [OH-] increases - H+ ions reacts with the OH- ions: H+ +OH- –> H2O - HA dissociates shifting equilibrium position to shift to the right to restore most of the H+ ions
what is an acidic buffer?
weak acid + its conjugate base
what is the general equations for buffers?
HA <–> H+ + A-