Acidic nd Bsic Solutes Acids lwys increse the concentrtion of 3 O nd decrese tht of O. Some cids relese ions directly when they dissocite: Acidic nd Bsic Solutes Other cids increse concentrtion by cusing wter molecule to split (hydrolyze) nd combining with the O (in tht order or the reverse): Acidic nd Bsic Solutes Acidic nd Bsic Solutes Bses lwys decrese the concentrtion of 3 O nd increse tht of O. Some bses relese O ions directly when they dissocite, while others extrct n from wter molecule: NO N O 1
Acidic nd Bsic Solutes Some cids (or bses) re ble to induce the relese of multiple (or O ) ions: Some Acid/Bse Jrgon The species remining fter n cid undergoes cid dissocition is bse, which cn combine with in the reverse rection. The cid nd bse re conjugtes of one nother nd form conjugte cid/bse pir. Some Acid/Bse Jrgon Equilibrium constnt for relese of one to n O is clled the cidity constnt or cid dissocition constnt. Acid dissocition equilibrium constnts re represented s : Acid O Conjugte Bse 3 O { Conjugte Bse{ O 3 { O { Conjugte Bse{ Some Acid/Bse Jrgon Equilibrium constnt for relese of one O is clled the bsicity constnt, b. Bse ( O) Conjugte Acid O b { Conjugte Acid { Bse { O For multi-protic cid, 1,, 3 re used for the sequence from most to lest protonted
Acid/Bse Chemistry The summtion of n cid dissocition rection nd the bsicity rection for the conjugte bse lwys yields the dissocition rection of wter: Acid/Bse Chemistry Since ll s hve the following form, is direct indictor of the tendency to donte n ; i.e., the strength of the cid. PO PO 4 4 PO O PO O 4 4 O O { { PO { PO 4 { PO { O { PO4 4 6.80 b 10 { { O { O 4 7.0 10 b w Acid O Conjugte Bse 3 O { Conjugte Bse { In given solution (given { ) contining severl cid/bse pirs, lrger corresponds to greter cid dissocition. Acid/Bse Chemistry Since p increses when decreses, stronger cids hve lower p vlues. Since b equls w, the lrger, the smller b for the conjugte bse, nd vice vers. Therefore, strong cids hve wek conjugte bses, nd wek cids hve strong conjugte bses. Put nother wy, if the cid is likely to dissocite, the bse is unlikely to re-ssocite. 3
Mthemtics of Acid/Bse Specition Every cid/bse group hs core tht is unltered s or O groups ttch nd detch. In closed system in which the core prticiptes only in cid/bse rections, its totl concentrtion (TOTc i ) in solution remins constnt, even when its distribution mong cid/bse species chnges. Consider core A tht cn combine with or O to form A,A,A,orAO. Then the mss blnce on A in solution is: TOTA A A A Mthemtics of Acid/Bse Specition At equilibrium, ech conjugte cid/bse pir in system must stisfy the equilibrium constnt ( ) for the corresponding rection; i.e., in the exmple system: A A A A A O AO { A{ { A 1 3 { A { { A { AO { { A { O Mthemtics of Acid/Bse Specition The equtions chrcterizing the equilibrium specition of A in terms of the concentrtions of the A-contining species (nd ssuming tht the stndrd stte conc n for solutes is 1.0 mol/l) re: [ A] A 1 A A 3 AO A ( 1.0) A [ ] A A A [ A] TOTA A A A 4
Mthemtics of Acid/Bse Specition If TOTA, the s, nd p re known, nd the ctivity coefficients cn be estimted (e.g., from ionic strength), then we hve four equtions in four unknowns, which cn be solved to determine the specition. In the simplified cse where the ctivity coefficients re ll 1.0: ( A)( ) ( A ) 1 3 ( AO )( ) ( A ) ( A )( ) ( A) ( ) ( ) ( ) ( ) TOTA A A A AO Mthemtics of Acid/Bse Specition At equilibrium, ll the expressions nd the mss blnce on A cn nd must be stisfied independently; the fct tht some of them contin identicl terms (e.g., tht the sme species is n cid in one rection nd conjugte bse in nother) does not lter this. The reltionships re mthemticl, not chemicl. The results cn be interpreted chemiclly, but the clcultions cn be crried out just like for ny other set of lgebric equtions. Mthemtics of Acid/Bse Specition { Bse { log { The expressions require tht: p Bse Whenever the ctivity increses by fctor of x, the ctivity rtio of the cid to the conjugte bse increses by this sme fctor When ctivity equls (pp ), the cid nd bse hve equl ctivities p Grphicl Representtion of Acid/Bse Specition log {Acid / {Bse 10 8 6 4 0 - -4-6 -8-10 log { p Bse p 1.5 p 6.0 8.8 0 1 3 4 5 6 7 8 9 10 11 1 13 14 p 5