Lecture 9 Acid-Base Equilibria -I

Strong acids: HCl HBr HI HNO 3 H 2 SO 4 HClO 4 (HO) n E=O Weak acid (HO) n EO 2 Strong acid We titrate with strong acids ONLY!

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Concentrations below M do not affect the pH value require special treatment (in practice, this will be still affected mostly by CO 2 and NH 3 from air) Only concentrations above M may change the pH value – and only in absence of other acids or bases Conclusion: we cannot titrate low concentrations of acids or bases in water

Weak acids and bases: K a is a small number (<< 1)

acid base acid HA  A + H + A + H 2 O  HA + OH - HA  A + H + A + H 2 O  HA + OH - + H 2 O  H + + OH - K a  K b = = [H + ][OH - ]

pH value of weak acid solution (C> M) The main source of H + is HA, not H 2 O HA  A + H + [A] = [H + ][HA] = C - [H + ] A small number [H + ] 2 = K a C Check the resulting [H + ] and correct if necessary

Any acid is “strong” at very low concentration – but it does not change the pH!

pH value of weak base solution (C> M) The main source of OH - is A, not H 2 O [HA] = [OH - ][A] = C - [OH - ] A small number [OH - ] 2 = K b C Check the resulting [OH - ] and correct if necessary A + H 2 O  HA + OH - K a  K b =[H + ][OH - ]=10 -14

pH value of mixture of a weak base and weak acid (C> M) HA  A + H + At high concentrations, [A]=C A and [HA]=C HA

pH value of mixture of a weak base and weak acid (C> M) buffer - pH does not change when you vary concentration a little In practice, one buffer pair (acid+ conjugate base) determines the pH value of the whole multicomponent mixture

A complex mixture: 0.1 M Harris, p. AP12 pK a =9.24 pH= 9.24 All other species in the system (indicator, any admixtures, …) will follow the pH determined by the buffer

Enzyme activity as a function of pH

Effect of [A]/[HA] on pH or Effect of pH on [A]/[HA]

pK=5

pH calculations: pH = -log[H + ] K a  K b =[H + ][OH - ]= K>1 Strong acid or strong base [H + ]=C acid [OH - ]=C base K<<1 Weak acid or weak base A mixture of a weak base and weak acid