Section13_WeakAcidsAndBases

Section13_WeakAcidsAndBases - Weak Acids and Bases By the...

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Page 1 of 22 Weak Acids and Bases By the end of this section, you will be able to: (1) Define weak acids and bases (2) Calculate K a and p K a for a weak acid, and K b and p K b for a weak base (3) Calculate the pH of a solution containing a weak acid or base (4) Understand polyprotic acids (5) Calculate % ionization of a weak acid (6) Understand the acid/base properties of salt solutions
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Page 2 of 22 Recall: Bronsted definitions of acids and bases An acid is a proton donor A base is a proton acceptor In a reaction between an acid (AH) and a base (B), a proton is transferred from the acid to the base: AH + B [BH] + + A - The words “strong” and “weak” (acid or base) are relative descriptors: we choose water as the benchmark. S trong acids ( e.g. , HCl) are fully deprotonated by water: AH + H 2 O [H 3 O] + + A -
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Page 3 of 22 Strong bases ( e.g. , NaNH 2 ) are fully protonated by water: B + H 2 O [BH] + + [OH] - WEAK ACIDS Acids that are only partially protonated by water are known as weak acids . AH + H 2 O [H 3 O] + + A - Here, an equilibrium is established between AH and H 2 O. The magnitude of the equilibrium constant, K , depends on how good a proton donor AH is relative to [H 3 O] + . Many organic acids, like acetic acid (CH 3 CO 2 H), are weak acids: CH 3 CO 2 H + H 2 O [H 3 O] + + CH 3 CO 2 -
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Page 4 of 22 K is given by: K = [ H 3 O + ][ CH 3 CO 2 " ] [ H 2 O ][ CH 3 CO 2 H ] However, [H 2 O] is large (55.5 M) and therefore approximately constant for dilute solutions. So, instead of K, we use K a : K a = K " [ H 2 O ] = [ H 3 O + ][ CH 3 CO 2 # ] [ CH 3 CO 2 H ] In general, for an acid HA, K a is given by: K a = [ H 3 O + ][ A " ] [ AH ]
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Page 5 of 22 The larger the K a , the stronger the acid. See the following table. K a for acetic acid = 1.74 10 -5 . Very small numbers like this are typical for weak acids, so it ʼ s more convenient for us to use pK a instead.
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Page 6 of 22 pK a = –log 10 K a For acetic acid, pK a = 4.76. An acid
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Section13_WeakAcidsAndBases - Weak Acids and Bases By the...

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