Polyprotics

Polyprotics - HSO 4 ( aq ) + H 2 O( l ) !" !...

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1 Polyprotics Slide 1 Polyprotic Acid/Base Equilibria Chem 6BH Polyprotics Slide 2 Polyprotic Acids and Bases An acid or base that can sequentially accept or donate more than one H is a polyprotic acid or base. Examples: H 2 SO 4 H 2 CO 3 H 3 Cit H 3 PO 4 CO 3 –2 HPO 4 –2 H 2 PO 4 C 2 O 4 –2 ( oxalate )
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2 Polyprotics Slide 3 Carbonic Acid Consider a common polyprotic, carbonic acid: Because of this, for almost all polyprotics, one needs only consider the first K a ; the others will be too small to be significant EXCEPTIONS: i) if K a ’s are close; ii) sulfuric acid: H 2 CO 3 ( aq ) + H 2 O( l ) !" ! # ! ! H 3 O + ( aq ) + HCO 3 ( aq ) K a 1 = 4.3 $ 10 –7 HCO 3 ( aq ) + H 2 O( l ) !" ! # ! ! H 3 O + ( aq ) + CO 3 2– ( aq ) K a 2 = 5.6 $ 10 –11 Notice K a 1 ! K a 2 (a general trend found in almost all polyprots) H 2 SO 4 ( aq ) + H 2 O( l ) !" ! H 3 O + ( aq ) + HSO 4 ( aq ) K a 1 ( H 2 SO 4 ) > 1000 HSO 4 ( aq ) + H 2 O( l ) !" ! #! ! H 3 O + ( aq ) + SO 4 2– ( aq ) K a 2 ( H 2 SO 4 ) = 1.2 $ 10 –2 So step1 is essentially 100% but step 2 is still significant! Polyprotics Slide 4 pH of a 0.02 M H 2 SO 4 Solution Assume that all of the H 2 SO 4 ionized; then [H 2 SO 4 ] = 0, [HSO 4 ] = 0.02 M and [H 3 O + ] = 0.02 M: So total [H 3 O + ] = 0.02 M + 6.3 x 10 –3 ; this gives a pH = 1.58
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Unformatted text preview: HSO 4 ( aq ) + H 2 O( l ) !" ! #! ! SO 4 2 ( aq ) + H 3 O + ( aq ) Init . 0.02 M 0.02 M $ x + x + x Eq . (0.02 x ) M x M (0.02+ x ) M K a 2 = ( x )(0.02 + x ) (0.02 x ) = 1.20 ! 10 2 ; x = 6.3 ! 10 3 M 3 Polyprotics Slide 5 Salts of Polyprotics These are amphiprotic: HS ( aq ) + H 2 O( l ) !" ! # ! ! H 3 O + ( aq ) + S 2 ( aq ) K a 2 = 7.1 $ 10 15 HS + H 2 O( l ) !" ! # ! ! H 2 S( aq ) + OH ( aq ) K b 1 = K W K a 1 = 7.7 $ 10 8 pH calculations are complicated but simplification allows us to make the following deductions: H 2 S + H + ! " "" HS H + " # "" S 2 [ H 3 O + ] ! K a 1 K a 2 and pH ! 1 2 pK a 1 + pK a 2 ( ) Polyprotics Slide 6 Species Concentrations in Carbonic Acid 4 Polyprotics Slide 7 Species Concentrations in Phosphoric Acid...
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Polyprotics - HSO 4 ( aq ) + H 2 O( l ) !" !...

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