CHAPTER 08: MONOPROTIC ACID-BASE EQUILIBRIA

Transcription

1 Hrris: Quntittive Chemicl Anlysis, Eight Edition CHAPTER 08: MONOPROTIC ACIDBASE EQUILIBRIA

2 CHAPTER 08: Opener A

3 CHAPTER 08: Opener B

4 CHAPTER 08: Opener C

5 CHAPTER 08: Opener D

6 CHAPTER 08: Opener E

7 Chpter 81. Strong Acids nd Bses Wter lmost never produces 10 7 M [H ] nd 10 7 M [OH ] Exmple) In 10 4 M solution of HBr, [OH ] 14 K w 4 [H 10 ] M Wter dissocition produces only M [OH ] nd M [H ]

8 Box 81 Concentrted HNO 3 is only slightly Dissocited Strong cid in dilute solution re essentilly completely dissocited. As concentrtion increses, dissocition decreses becuse there is not enough solvent to stbilize the free ions.

9 Box 81 Concentrted HNO 3 is only slightly Dissocited 1,049 cm 1 dissocited NO 3 1 * undissocited HNO 3

10 Box 81 Concentrted HNO 3 is only slightly Dissocited

11 Temperture dependence of K w In this tble, concentrtions in K w [H ][OH ] re expressed in mollity.

12 Chpter 82. Wek Acids nd Bses HA H A, K [H ][A [HA] ] B H 2 O BH OH, K b [BH ][OH [B] ] K K b K w The conjugte bse of wek cid is wek bse. The conjugte cid of wek bse is wek cid. Exmple) HA (K ) A (K b ) As HA becomes weker cid, A becomes stronger bse. (but never strong bse. Recll tht K K b K w )

13 Chpter 83. WekAcid Equilibri In solution of ny respectble wek cid, the concentrtion of H due to cid dissocition will be much greter thn the concentrtion due to wter dissocition (le chtelier principle). HA H 1 A [H 1 ] >> [H 2 ] H 2 O H 2 OH [A ] >> [OH ]

14 Frction of Dissocition (Ioniztion Frction) Consider Acid (H 3 A) α [H 3 A] [H 2 [H A] A ] [HA ] [A ] [H3A] F (forml conc. of H 3 A) α 1 [H A 2 F ] [HA 2 ], α 2, F α 3 [A 3 F ] α cn be determined from the mss blnce nd equilibrium equtions.

15 Exmple HOCl H OCl K [H ][OCl [HOCl] ] i) F HOCl [HOCl] [OCl ] ii) K [H ][OCl [HOCl] ] K [HOCl] F HOCl [HOCl] K [HOCl](1 ) [H ] [H ] [HOCl] F [H ] [H ] HOCl K α [OCl F HOCl ] [H K ] K α 1 [HOCl] is stronger disinfectnt thn [OCl ] All wek electrolytes dissocite more s they re diluted (From the bove eqution for α 1, when the solution is diluted, [H ] will decrese nd thus α 1 increses) Fig. 92

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17 Chpter 85. Buffers Buffer solution is one tht resists chnges in ph when cids or bses re dded or when dilution occurs. The proper functioning of ny biologicl system is criticlly dependent on ph. Fig. 83. ph dependence of the rte of clevge of n mid bond by the enzyme chymotrypsin.

18 HendersonHsselbch Eqution The centrl eqution for buffers. It is merely rerrnged form of the K equilibrium eqution. If solution is prepred from HA & MA HA H A MA M A A H2O HA OH {H }{A } γ [H ] γ [A ] H A K {HA} γ HA[HA] γ HA[HA] γ [H ] K H γ [A ] A γ HA[HA] logγ [H ] logk log H γ [A ] ph pk γ log γ A HA [A ] [HA] A continued

19 HendersonHsselbch Eqution If ctivities re neglected, γ s 1 ph pk [A ] log [HA] If solution is prepred from the wek bse B nd its conjugte cid (BH ) BH H B [B] ph pk log [BH ] pk pplies to this cid * If [HA] [A ] or [BH ] [B], ph pk

20 HendersonHsselbch Eqution * For every power of 10 chnge in the rtio [A ]/[HA], the ph chnges by one unit (See Tble 91) ph pk [A ] log [HA]

21 HendersonHsselbch Eqution [B] ph pk log [BH ] [A ] ph pk log [HA] pk pplies to this cid pk pplies to this cid If there re 10 different cids nd bses in the solution, the 10 forms of HendersonHsselbch eqution must ll give the sme ph!, becuse there cn be only one concentrtion of H in solution.

22 Buffer ph The most common method of prepring buffered solution : Add mixture of wek cid (HA) nd its conjugte bse (MA) Wek cid will rect with ny strong bse dded. Conjugte bse will rect with ny strong cid dded. pk of the cid should be very close to the desired ph of the buffer solution for mximum buffer intensity.

23 Buffer ph Let s exmine buffer solution (C HA moles/l wek cid C NA moles/l conjugte bse) Equilibri ; K [H ][A [HA] ] K w [H ][OH ] Mteril Blnce ; C T,A C HA C NA [HA] [A ] C T,N [N ] C NA continued

24 Buffer ph Chrge Blnce ; [N ] [H ] [A ] [OH ] From & [A ] C NA [H ] [OH ] From & [HA] C HA [OH ] [H ] continued

25 From HendersonHsselbch Eqution. ph pk [A ] log [HA] Putting eqns. & C NA [H ] [OH ] ph pk log C [OH ] [H ] HA This is originl eqution nd hs no pproximtion except ctivities. continued

26 But for most buffer solutions, nd lmost invribly for prepred buffers C NA >> [H ] [OH ] C HA >> [OH ] [H ] ph pk log C C NA HA (slt) (cid) Approximtion; C NA [A ] C HA [HA]

27 Remrk; i) When you mix wek cid (HA) with conjugte bse (NA), you cn get wht you mix, tht is, C HA [HA] nd C NA [A ]. Pure HA dissocite very little, nd dding extr A to the solution will mke the HA dissocite even less. Similrly, A does not rect very much with wter, nd dding extr HA mkes A rect even less. HA H A A H 2 O HA OH ii) This pproximtion breks down for dilute solutions or t extremes of ph. If C HA or C NA is smll, or if [H ] or [OH ] is lrge, the pproximtion C HA [HA], nd C NA [A ] re not good. iii) In cidic solutions, [H] >> [OH], so [OH] cn be ignored in eqns. & nd in bsic solution [H] cn be ignored.

28 Exmple Wht will be the ph? mol of HA (pk 2.00) nd mol of A in 1.00 L of solution Solution i) ph pk log C C A HA 2.00 log (Wrong!!!)

29 Exmple Solution ii) C [H ] [OH ] A ph pk log C [H ] [OH ] HA pk log C C A HA [H [H ] ] The solution is cidic [H ] >> [OH ] HA H A 0.01x x 0.01x [H ][A ] x(0.01 x) K 10 [HA] 0.01 x 2.00 x [H ] [OH ] (neglected) continued

30 Exmple The conc. of HA nd A re not wht we mxied [HA] C HA [H ] M [A ] C A [H ] M ph 2.00 log In this exmple, HA is too strong nd conc. re too slow for HA nd A to be equl to their forml concentrtions (C HA nd C A, respectively)

31 Box 83 Strong Plus Wek Rects Completely A strong cid rects with wek bse essentilly completely becuse the equilibrium constnt is lrge. 1 B H BH K (for BH ) Wek bse Strong cid If B is tris(hydroxymethyl)minomethne, then the equilibrium constnt for rection with HCl is K K 1 K

32 Box 83 Strong Plus Wek Rects Completely A strong bse rects completely with wek cid becuse the equilibrium constnt is, gin, very lrge. 1 OH HA A H 2 O K (for A ) Strong bse Wek cid If HA is cetic cid, then the equilibrium constnt for rection with NOH is K b K 1 K b K for HA K w

33 Box 83 Strong Plus Wek Rects Completely The rection of strong cid with strong bse is even more complete thn strong wek rection: 1 H OH H 2 O K (for A ) Strong cid Strong bse * If you mix strong cid, strong bse, wek cid, nd wek bse, the strong cid nd bse will neutrlized ech other until one is used up. The remining strong cid or bse will then rect with the wek bse or wek cid. K b

34 Prepring Buffer in Rel Life Tris Buffer [tris(hydroxymethyl)minomethne] BH (Tris hydrochloride) B (Tris) pk If you wnt to get tris buffer of 7.80, you cn clculte the conc. of BH nd B with Henderson Eq. ph pk log B (tris) BH

35 Prepring Buffer in Rel Life! If you relly wnted to prepre tris buffer of ph 7.60, you would not do it by clculting wht to mix. Suppose tht you wish to prepre 1.00 L of buffer contining M tris t ph of You hve vilble solid tris hydro chloride nd pproximtely 1 M NOH. Here s how to do it: 1. Weigh out mol of tris hydrochloride nd dissolve it in beker contining bout 800 ml of wter. 2. Plce ph electrode in the solution nd monitor the ph. 3. Add NOH until the ph is exctly Trnsfer the solution to volumetric flsk nd wsh the beker few times. Add the wshings to the volumetric flsk. 5 Dilute to the mrk nd mix. You do not mix clculted quntities, though quick clcultion is helpful so tht you hve some ide of how much will be needed.

36 Resons why clcultion would be wrong: 1. You might hve ignored ctivity coefficients. 2. The temperture might not be just right. 3. The pproximtions tht [HA] F HA nd [A ] F A could be in error. 4. The pk reported for tris in your fvorite tble is probbly not wht you would mesure in your lb. 5. You will probbly mke n rithmetic error nywy.

37 Buffer Cpcity The buffer cpcity, β, lso clled buffer intensity, is defined s β dc b dph dc dph Figure 84. ) C b vs. ph for solution contining 0.100F HA with pk 5.00 b) Buffer Cpcity vs. ph