Acid-Base Equilibria Acid-Base Definitions

Transcription

1 AcidBse Equiliri Acids shrp, sour tste; Bses sopy, itter tste Neutrliztion (proton trnsfer) rections cid se slt wter (or other products) Proton (H ) strongly hydrted in wter H(H O) n Hydronium ion H O 18.1 AcidBse Definitions Arrhenius definition defines cids nd ses in terms of their ehvior in wter Acids contin H nd relese H in wter Bses contin OH nd relese OH in wter Arrhenius cidse rection comintion of the H from the cid with the OH from the se H OH H O The Arrhenius definition hs severe limittions Limited to wter solutions Some ses do not contin OH (NH, mines, N S, N O, ) BrønstedLowry definition defines cids nd ses in reltion to the H ion (proton) Acids proton donors (must contin H) Bses proton cceptors (must contin lone pir to ind the H ) All Arrhenius ses re lso BL ses, ut not ll BL ses re Arrhenius ses BL cidse rection proton trnsfer from the cid (HA) to the se (B) H HA B A HB The dissolution of cids nd ses in wter is lso BL cidse rection H HA H O A H O deprotontion of the cid HA Strong cids completely deprotonted (dissocited, ionized) in H O Hl, HBr, HI, HNO, HlO, HlO 4, H SO 4 Emple: HBr(g) H O(l) Br H O Wek cids prtilly deprotonted in H O HF, HN, H S, HOOH, H OOH, Emple: HF(q) H O(l) F H O Only smll frction (~.5%) of the HF molecules re dissocited to ions H B H O HB OH protontion of the se B Strong ses completely protonted (dissocited, ionized) in H O Group I nd II oides nd hydroides Emple: O(s) H O(l) OH Wek Bses prtilly protonted in H O NH, mines (RNH, R NH, R N), Emple: NH (q) H O(l) NH 4 OH Only smll frction (~0.5%) of the NH molecules re dissocited to ions H O is strong cid in wter H H O H O H O H O OH is strong se in wter H OH H O H O OH onjugte cidse pirs H H HA B A HB The reverse rection is viewed s BL cidse rection too HA/A nd HB /B re clled conjugte cidse pirs (cid/se cid component is written first) Differ from ech other y proton (H ) the cid component of the pir hs one more H Emples: HF/F, H O /H O, NH 4 /NH, H O/OH BL cidse rections occur when n cid nd se rect to form their conjugte se nd cid Acid 1 Bse Bse 1 Acid

2 Emple: Identify the conjugte cidse pirs in the rection HSO S SO HS HSO /SO HS /S Quntifying Acid/Bse Strengths Acid ioniztion constnt ( ) For generl wek cid, HA, in wter HA H O H O A c ][A ] [HA][H O] ][A ] [HA] [H O] cn e ssumed constnt nd comined with c c [H O] [H O ] nd [A ] % dissocition Higher Stronger cid Bse ioniztion constnt ( ) For generl wek se, B, in wter B H O HB OH c [HB ][OH ] [B][H O] [HB ][OH ] [B] [H O] cn e ssumed constnt nd comined with c c [H O] [HB ] nd [OH ] % dissocition Higher Stronger se A key structurl feture of ll BL ses is lone pir of e s needed to ind the proton 18. Autoioniztion of Wter nd ph Wter cn ct s oth proton donor (cid) nd proton cceptor (se) mphoteric H H H O H O H O OH Autoioniztion (utoprotolysis) Ionproduct constnt of wter ( w ) c [H O [H O] w ][OH ] [H O ][OH ] [H O] cn e ssumed constnt nd comined with c w c [H O] w ionproduct (utoprotolysis) constnt of wter

3 At 5, w In pure wter [H O ] [OH ] w w [H O ] [OH ] M In wter solutions [H O ] nd [OH ] re inversely proportionl to ech other w [H O ][OH ] [H O ] [OH ] H O nd OH re present in different proportions in oth cid nd se solutions Neutrl solutions [H O ] [OH ] Acidic solution [H O ] > [OH ] Bsic solutions [H O ] < [OH ] Emple: lculte the concentrtions of H O nd OH in 0. M Hl nd 0. M B(OH) solutions. Hl is strong cid completely dissocited in wter solutions; Hl H O H O l 0. M Hl [H O ] 0. M [OH ] w / [H O ] /0. [OH ] M [H O ] >> [OH ] B(OH) is strong se completely dissocited in wter solutions; B(OH) B OH 0. M B(OH) [OH ] 0.0 M [H O ] w / [OH ] /0.0 [H O ] M [H O ] << [OH ] The ph Scle The ph scle is logrithmic scle ph log[h O ] log[h O ] ph [H O ] ph [H O ] ph If [H O ] [OH ] 1 7 M (neutrl solution) ph log(1 7 ) neutrl solution ph 7 If [H O ] > 1 7 M cidic solution ph < 7 If [H O ] < 1 7 M sic solution ph > 7 Emples: [H O ] 1 ph log 1 0 [H O ] 14 ph log The pnottion px log X poh log[oh ] log[oh ] poh [OH ] poh p w log w [H O ][OH ] w (tke log) log[h O ] (log[oh ]) log w ph poh p w At 5, w p w ph poh If ph 7 poh 7 neutrl solution Acidic ( ph, poh); Bsic ( ph, poh) Emple: A household mmoni solution hs ph of Wht is the poh nd the concentrtion of OH in it? poh ph [OH ] poh Note: The # of sig. figs in [OH ] (or [H O ]) equls the numer of deciml plces in poh (or ph) Mesuring ph in the lortory Indictors hve different colors t different ph (ph pper, indictor solutions) ph meters mesure the ph y mesuring the potentil of ph sensitive electrode immersed in the solution (more ccurte)

4 nd of onjugte AcidBse Pir In solution of the wek cid HA, oth HA nd its conjugte se, A, rect with wter HA H O A H O (of HA) A H O HA OH (of A ) H O H O H O OH w [H O ][A ] [HA][OH ] ][OH ] [HA] [A ] For conjugte cidse pir w log (log ) log w p p p w At 5, p p w The stronger the cid, the weker its conjugte se nd vice vers p p Emple: The of HF is Wht is the of F? w / / Reltive AcidBse Strength nd the Net Direction of Rection An cidse rection proceeds to greter etent from the stronger cid nd stronger se towrds the weker cid nd weker se HA B A HB stronger cid stronger se weker se weker cid The rection is shifted towrds A nd HB ( c > 1) Emples: HF NH F NH 4 stronger cid stronger se weker se weker cid HF H O F H O weker cid weker se stronger se stronger cid Emple: Given (HF) nd (HN) 6., determine the preferred direction of the rection F HN HF N ompre the strengths of the cids on oth sides HF is stronger thn HN (HF hs higher vlue) ompre the strengths of the ses on oth sides Since HF is stronger cid thn HN, the conjugte se of HF, F, is weker se thn the conjugte se of HN, N N is stronger thn F (N hs higher vlue) F HN HF N is shifted to the left ( c <1) 18.4 Solving Prolems Involving Wek Acid Equiliri In wter solution of the wek cid, HA, there re two sources of H O : 1. HA H O H O A (dissocition of HA). H O H O H O OH (utoioniztion) [H O ] 1 [A ] nd [H O ] [OH ] [H O ] [H O ] 1 [H O ] [A ] [OH ] If HA is not very dilute or very wek, the utoioniztion cn e neglected nd [OH ] << [A ] [H O ] [A ] In wter solution, the wek cid HA eists in two forms undissocited (HA) nd dissocited (A ) HA H O H O A HA totl concentrtion of HA HA [HA] [A ] [HA] HA [A ] HA So [H O ], [A ], [HA] HA [H O ][A ] [HA] HA

5 Using equilirium tles If he utoioniztion of wter is neglected i c e [ ] i c e HA H O H O A HA HA 0 0 ][A ] [HA] The qudrtic eqution cn e solved for in order to determine [H O ] nd ph If is less thn 5% of HA, cn e neglected in the denomintor (5% rule) works if HA is reltively lrge nd is smll ( HA / > 400) / HA ( HA ) ½ [H O ] HA Finding Given oncentrtions or ph Emple: If the ph of 0.0 M HN solution is 4.95, clculte the of HN. [H O ] ph HN 0.0 << 5% of HN [ ] HN H O H O N i 0.0 c e 0.0 [HN] HN ][N ] / HN (1.1 5 ) / [Note: HA / 0.0/6.. 8 >> 400] HN Finding oncentrtions nd ph Given Emple: Wht is the ph of 0.0 M HF solution? ( for HF) [ ] HF H O H O F i 0.0 c e 0.0 ][F ] [HF] HF 0.0 ssume < 5% of 0.0 ( HF ) ½ ( ) ½ 1.4 [H O ] ph log[h O ] log(1.4 ) 1.85 [heck ssumption: (1.4 /0.0) 0 4.8% < 5%] [Note: HF / 0.0/ > 400] HF HF Etent of Acid Dissocition Percent dissocition HA H O H O A HA [HA] [A ] [A ] [H O ] [A ] (dissocited form of the cid) [A ] [H O ] dissocited 0 0 % HA HA HA 0 For given cid, % dissocited decreses with incresing the totl concentrtion of the cid, HA HA % dissocition Emple: lculte the % dissocition for two HF solutions with concentrtions 0.0 nd.0 M. ( for HF) For the 0.0 M HF from the previous emple: ( HF ) ½ ( ) ½ 1.4 [H O ] % dissocited (1.4 /0.0) 0 4.8% For the.0 M HF similrly: ( HF ) ½ ( ) ½ 4.5 [H O ] % dissocited (4.5 /.0) 0 1.5% Incresing the concentrtion from 0.0 to.0 M decreses the % dissocited from 4.8 to 1.5% The Behvior of Polyprotic Acids Polyprotic Acids cn donte more thn one proton (H SO 4, H PO 4, ) For generl diprotic cid, H A H A H O H O HA 1 HA H O H O A ][HA ] 1 [H A] ][A [HA ] Almost ll polyprotic cids (ecept H SO 4 ) re wek in ll stges of dissocition nd ecome weker with ech successive dissocition 1 > > ]

6 The second nd third dissocitions re less pronounced ecuse it s hrder to remove n H from negtively chrged ions Equilirium clcultions re gretly simplified y neglecting susequent dissocitions since they contriute negligile mounts of H O Emple: lculte the ph nd the concentrtions of ll ionized forms for 0. M H PO 4 solution. ( 1 7., 6. 8, 4. 1 ) H PO 4 H O H O H PO 4 1 H PO 4 H O H O HPO 4 HPO 4 H O H O PO 4 onsider only the first dissocition to get the ph H PO 4 H O H O H PO 4 i 0. c e ][HPO4] [H PO ] Assume < 5% of 0. ( 1 0.) ½ (7. 0.) ½.7 heck ssumption: (.7 /0.) 0 7% >5% The ssumption is not justified must solve the qudrtic eqution 1 (0. ) (7. ) [H O ] [H PO 4 ]. M ph log[h O ] log(. ) 1.6 To clculte the concentrtions of the other species, use the [H O ] nd [H PO 4 ] from the 1 st ioniztion ][HPO4 ] ][PO4 ] [HPO4] [HPO4 ] 8 [HPO4] 6.. [HPO4 ] 6. ]. 1 8 [HPO4 ] [PO4 ] 1.1 [H O ] Solving Prolems Involving Wek Bse Equiliri In wter solution of the wek se, B, there re two sources of OH : 1. B H O HB OH (ioniztion of B). H O H O H O OH (utoioniztion) [OH ] 1 [HB ] nd [OH ] [H O ] [OH ] [OH ] 1 [OH ] [HB ] [H O ] If B is not very dilute or very wek, the utoioniztion cn e neglected nd [H O ]<<[HB ] [OH ] [HB ] In wter solution, the wek se B eists in two forms unionized (B) nd ionized (HB ) B H O HB OH B totl concentrtion of B B [B] [HB ] [B] B [HB ] B So [OH ], [HB ], [B] B [HB ][OH ] [B] The eqution is equivlent to tht for wek cids B

7 Using equilirium tles If he utoioniztion of wter is neglected i c e [ ] i c e B H O HB OH B B 0 0 [HB ][OH ] [B] The qudrtic eqution cn e solved for in order to determine [OH ], poh nd ph If is less thn 5% of B, cn e neglected in the denomintor (5% rule) works if B is reltively lrge nd is smll ( B / > 400) / B ( B ) ½ [OH ] B Finding ph Given Emple: Wht is the ph of 0. M NH solution? ( for NH ) NH H O NH 4 OH [ ] [NH4 ][OH ] i 0. [NH] c e 0. NH NH NH 0. ssume < 5% of 0. ( NH ) ½ ( ) ½ 1. [OH ] poh log[oh ] log(1. ).87 ph poh [heck ssumption: (1. /0.) 0 1.% < 5%] Etent of Bse Ioniztion Percent ioniztion B H O HB OH B [B] [HB ] [HB ] [OH ] [HB ] (ionized form of the se) [HB ] [OH ] % ionized 0 0 B B B 0 For given se, % ionized decreses with incresing the totl concentrtion of the se, B B % dissocition Emple: lculte the % ioniztion for two NH solutions with concentrtions 0. nd 1.0 M. ( for NH ) For the 0. M NH from the previous emple: ( NH ) ½ ( ) ½ 1. [OH ] % ionized (1. /0.) 0 1.% For the 1.0 M NH similrly: ( NH ) ½ ( ) ½ 4. [OH ] % ionized (4. /1.0) 0 0.4% Incresing the concentrtion from 0. to 1.0 M decreses the % ionized from 1. to 0.4% Anions of Wek Acids s Wek Bses The nion of the wek cid (HA) is its conjugte se (A ) A rects s wek se in wter: [HA][OH ] A H O HA OH [A ] A cn e produced in solution y mens of the solule slt, MA, which dissocites completely: MA(s) M A (M is specttor ion) Emple: For n queous solution of F F(s) F F H O HF OH HA nd A re present in oth, solutions of the wek cid HA, nd solutions of its nion A Solutions of HA: HA H O H O A Solutions of A : A H O HA OH Both equiliri re shifted to the left so Solutions of HA re cidic (H O ) nd [HA] >> [A ] Solutions of A re sic (OH ) nd [A ] >> [HA] HA nd A re conjugte cid se pir so (HA) (A ) w (A ) w / (HA) Equilirium clcultions for A re crried out using the sme method s for the neutrl se B [HA][OH ] [A ] A

8 Emple: Wht is the ph of 0. M F solution? ( for HF) F(s) F F 0. M [ ] F H O HF OH i 0. c e 0. [HF][OH ] [F ] (F ) w / (HF) / ( 0.) ½ ( ) ½ 1. 6 [OH ] poh log[oh ] log(1. 6 ) 5.9 ph poh (sic) [heck ssumption: (1. 6 /0.) % < 5%] tions of Wek Bses s Wek Acids The ction of the wek se (B) is its conjugte cid (HB ) HB rects s wek cid in wter: [H O ][B] HB H O H O B [HB ] HB nd B re conjugte cidse pir so (HB ) (B) w (HB ) w / (B) Equilirium clcultions for HB re crried out using the sme method s for the neutrl cid HA ][B] [HB ] HB Emple: Wht is the ph of 0. M NH 4 I solution? ( for NH ) NH 4 I(s) NH 4 I NH4 0. M [ ] NH 4 H O H O NH i 0. c e 0. [H O ][NH ] [NH (NH 4 ) w / (NH ) / ( 0.) ½ (5.6 0.) ½ [H O ] M ph log[h O ] log(7.5 6 ) 5.1 (cidic) [heck ssumption: (7.5 6 /0.) % < 5%] ] 18.6 Moleculr Properties nd Acid Strength Binry cids (H n X) consist of H nd second element, X (HF, Hl, H O, H S, ) Acid strength increses cross period (the EN of X increses the H X ond ecomes more polr greter δ chrge on the H greter ttrction of the H to the O tom of H O) δ X H δ :OH H O X Emple: NH << H O < HF Acid strength increses down group (X ecomes lrger the H X ond ecomes longer nd weker the H comes off more esily) Emple: HF << Hl < HBr < HI Oocids (H n XO m ) consist of H, O nd third element, X (HlO, H SO 4, H PO 4, ) The cidic H toms re ttched to O toms For oocids with the sme # of O toms, cid strength increses with incresing the EN of X (X withdrws electron density from the O H ond nd mkes it more polr greter δ chrge on the H the H comes off more esily) Emple: HOI < HOBr < HOl For oocids hving the sme X, cid strength increses with incresing the # of O toms (the high EN of O drws electron density from the O H ond nd mkes it more polr greter δ chrge on the H the H comes off more esily) Emple: HlO < HlO << HlO < HlO 4 Adding more EN toms to the molecule increses the cidity further (E: H OOH < F OOH)

9 Acidity of Hydrted Metl Ions Metl ions re hydrted in wter solutions If the ion is smll nd highly chrged (M or M ), it drws electron density from the ound wter molecules H cn e relesed cidic Emple: All (s) 6H O(l) Al(H O) 6 l Al(H O) 6 H O(l) Al(H O) 5 OH H O 18.7 AcidBse Properties of Slt Solutions The cidity (sicity) of slt solutions depends on the cidse properties of their ions Acidic ctions ct s wek cids in wter The ctions (conjugte cids) of wek ses (NH 4, H NH, ) ct s wek cids Smll, highly chrged metl ctions (Al, Fe, r, u, ) ct s wek cids Neutrl ctions do not influence the ph The ctions of strong ses (Group I,, Sr, B ) nd metl ctions with 1 chrge (Ag, u, ) re etremely wek cids (weker thn H O) do not influence the ph Bsic nions ct s wek ses in wter The nions (conjugte ses) of wek cids (F, N, S, PO 4 ) ct s wek ses Neutrl nions do not influence the ph The nions (conjugte ses) of strong cids (l, Br, I, NO, lo 4 ) re etremely wek ses (weker thn H O) do not influence the ph Amphoteric nions of polyprotic cids cn ct s wek cids or ses in wter Anions with ionizle protons (H PO 4, HPO 4, HS, HSO, HSO 4 ) ct s either wek cids or wek ses depending on the reltive vlues of their nd constnts) Slts of neutrl ctions nd neutrl nions yield neutrl solutions Emple: Nl(s) N l (neutrl solution) N neutrl ction (ction of strong se, NOH) l neutrl nion (nion of strong cid, Hl) Slts of cidic ctions nd neutrl nions yield cidic solutions Emple: NH 4 l(s) NH 4 l (cidic solution) NH 4 cidic ction (ction of wek se, NH ) l neutrl nion (nion of strong cid, Hl) NH 4 H O H O NH Emple: Fel (s) Fe l (cidic solution) Fe cidic ction (highly chrged, smll ction) l neutrl nion (nion of strong cid, Hl) Fe(H O) 6 H O H O Fe(H O) 5 OH Slts of neutrl ctions nd sic nions yield sic solutions Emple: N S(s) N S (sic solution) N neutrl ction (ction of strong se, NOH) S sic nion (nion of wek cid, H S) S H O HS OH Emple: F(s) F (sic solution) neutrl ction F sic nion Slts of cidic ctions nd sic nions yield either cidic or sic solutions If of the ction is lrger thn of the nion, the solution is cidic (ction is stronger cid) If of the ction is smller thn of the nion, the solution is sic (nion is stronger se) Emple: NH 4 F(s) NH 4 F NH 4 cidic ction (ction of wek se, NH ) F sic nion (nion of wek cid, HF) NH 4 H O H O NH (NH 4 ) 5.7 F H O HF OH (F ) (NH 4 ) > (F ) NH 4 is stronger cid thn F is se the solution is slightly cidic

10 Slts of neutrl ctions nd mphoteric nions yield either cidic or sic solutions If of the nion is lrger thn its, the solution is cidic (the nion is stronger cid) If of the nion is smller thn its, the solution is sic (the nion is stronger se) Emple: Predict whether solutions of H PO 4 nd HPO 4 re cidic, sic or neutrl. H PO 4 H PO 4 neutrl ction (ction of strong se, OH) H PO 4 mphoteric nion??? H PO 4 H O H O HPO 4 (H PO 4 ) H PO 4 H O H PO 4 OH (H PO 4 ) (H PO 4 ) (H PO 4 ) 6. 8 (H PO 4 ) w / 1 (H PO 4 ) 14 / (H PO 4 ) >> (H PO 4 ) H PO 4 is stronger cid thn it is se the solution is cidic HPO 4 HPO 4 HPO 4 mphoteric nion??? HPO 4 H O H O PO 4 (HPO 4 ) HPO 4 H O H PO 4 OH (HPO 4 ) (HPO 4 ) (H PO 4 ) 4. 1 (HPO 4 ) w / (H PO 4 ) 14 / (HPO 4 ) << (HPO 4 ) HPO 4 is stronger se thn it is n cid the solution is sic 18.8 The Lewis AcidBse Definition Acids electron pir cceptors Bses electron pir donors The Lewis cidse definition does not require the echnge of proton (Lewis cids don t hve to hve H in their formuls) Epnds the scope of possile cids H itself is Lewis cid since it ccepts n e pir from se (H :B H B ) All BL cids donte Lewis cid (H ) Lewis ses must contin n e pir to donte Lewis cids must hve vcnt oritl in order to ccept the e pir from the se A Lewis cidse rection results in the formtion of coordinte covlent ond etween the cid nd the se A :B A B A B is clled n dduct or Lewis cidse comple Lewis cids with electron deficient toms hve incomplete octets (B, Be, ) se cid dduct Lewis cids with polr multiple onds (O, SO, ) se cid dduct Metl ctions s Lewis cids metl ctions hve vcnt oritls in their vlence shells (Al, Fe, Ni, u, Ag, ) u 4:NH u[nh ] 4 cid se dduct Mny metls ct s Lewis cids in iomolecules (Fe in hemogloin, Mg in chlorophyll, ) The Lewis definition hs the widest scope of the three cidse definitions, while the Arrhenius definition hs the nrrowest scope Emples: BF is Lewis cid ut not BL or Arrhenius cid F is Lewis nd BL se ut not n Arrhenius se BF :F BF 4 Lewis cid se dduct H :F H O HF OH BL se cid cid se