Ch.21 buffers and nutralisation

minimises PH change when small amounts of acid and base are added

a weak acid to remove alkalis a conjugate base to remove acid

mix an acid solution with a solution of one of its salts or by adding an alkali solution to an excess of the weak acid the weak acid is partialy nutralised by the alkali forming a conjugate base, some acid is left over resulting in a mixture of the weak acid and the salt

when [HA] = [A-] the PH of the buffer solution is the same as the pKa of HA operating range is over 2 PH units centred at the PH of the pKa

[H+] = Ka x [HA] / [A-] (when [HA] and [A-] are different) use normal products over reactants if they are the same

acid - shifts left alkali - shifts right

should be 7.4 bellow 7.35 - acidosis - fatigue, shortness of breath above 7.45 - alkalosis - muscle spasms, light headedness, nausea

carbonic acid - hydrogen-carbonate buffer system H2CO3 <==> H+ + HCO3-

increase in [H+] H+ react with conjugate base HCO3- eq shifts left, removing H+ ions

increase in [OH-] reacts with small amount of H+ ions H2CO3 dissociates eq shifts right to replenish most H+ ions

point in the centre of the vertical section of a PH titration curve when the vol of both solutions have reacted exactly

x axis is vol of base or acid added y axis is PH as base (acid) is added the PH slowly increases (decreases) as the acid (base) is in excess then the PH rapidly increases (decreases) in the vertical section only a drop or 2 is needed for the whole vertical section it then levels of and the PH continues to increase (decrease) slowly as the base (acid) is now in excess

weak acid that has a very different colour to its conjugate base

equal concs of HA and A- colour will be in between the 2 extremes Ka=[H+] pKa=PH

acid - red end point - orange alkali - yellow

it must have a colour change that coincides with the vertical section of the PH titration curve

phenolphthalein methyl orange

phenolphthalein

methyl orange

no suitable indicator