Fundamentals of Analytical Chemistry

Transcription

1 Homework Fundmentls of nlyticl hemistry hpter 9 0, 1, 5, 7, 9 cids, Bses, nd hpter 9(b) Definitions cid Releses H ions in wter (rrhenius) Proton donor (Bronsted( Lowry) Electron-pir cceptor (Lewis) hrcteristic ction for the solvent (solvent-system) system) Bse Releses OH - in wter (rrhenius) Proton cceptor (Bronsted( Lowry) Electron-pir donor (Lewis) hrcteristic nion for the solvent (solvent-system) system) queous Solutions Focus in this course is wter s solvent Most common reference will be Bronsted Lowry definitions ll queous solutions hve OH - ions ll queous solutions (re considered to) hve H ions. More correct is H 3 O Even more correct H 9 O 4 We will ssume H nd H 3 O re equivlent Most common pproch queous Solutions For ny queous solution H O(l) H OH - H O(l) H 3 O OH - W [H ][OH - ] 1.0 x (t 5 O ) Note tht concentrtions rther thn ctivities Should use ctivities, but µ 1.0 x 10-7 Will ignore ctivity coefficients unless specificlly sked to use them in clcultions! Note H O(l) is not prt of the equilibrium expression ctivity of solvents by definition equl to 1 queous Solutions When [H ] [OH - ] 1.0 x 10-7 Neutrl solution If [H ] > [OH - ] cidic [H ] > 10-7 [OH - ] < 10-7 Bsic [H ] < 10-7 [OH - ] >

2 Typicl Vlues oncentrtion rnge for H nd OH - re from greter thn 10 to less thn Difference between 1 1 nd $100 trillion. ompress scle using logrithms Rnge is then log (10) [1] to log (10-15 ) [-15][ Mke scle more pltble by chnging sign Rnge is then -11 to 15 ph Working definition: ph -log [H ] ctully, ph -log H ompress scle, gives mostly positive vlues Inverse reltionship to [H ] s [H ] increses, ph decreses Since cid hs been defined s [H ] > 10-7 cidic ph hs vlues less thn 7 Bsic vlues hve ph greter thn 7 Error in mesurement of [H ] 5% Since ph log function, error is ±0.0 ph units [0.05 / ln 10] ±0.0 lcultion of ph for Solution Must first define the type of solution cid Strong Wek Bse Strong Wek Buffer cid Slt None of the bove nd the lst shll be first None of the bove Nothing in the solution to cuse the hydrogen ion concentrtion to chnge (Nl( Nl) ph 7.00 Solution will still be neutrl Note tht the chnge in ionic strength d/n chnge the ctivity from 1.0 x 10-7 w H * OH - If the solution is n cid Species is proton donor Different levels of dontion ll (strong cid) Some (wek cid) Strong cids Hl, HBr,, HI HNO 3, HlO 3, HlO 4 H SO SO 4 Specil sitution will discuss lter Strong cids donte ll protons [H ] H Wht is the ph for the following solutions of Hl ( strong cid) 1.0 x x x x 10-10

3 HOW N WE DD N ID TO SOLUTION ND ME THE SOLUTION BSI??? Remember, ph vlues greter thn 7 men tht the solution is bsic! Wht hppened? For LL cidic solutions, there re sources of protons The cid Wter When [H ] is lrge, we cn ignore contribution from wter. Neutrl [H ] 10-7 Lehtlier s Principle cuses shift wy from the production of H from the utoioniztion of wter s [H ] pproches 10-7 Equilibrium shift decreses [H ] from wter becomes more significnt. Wht is the ph for solution tht is 1.0 x 10-7 M in Hl? w But lso [ H ] [ H ][ OH ] [ H ] [ H ] wter [ H ], [ H ] wter [ OH ] x Then,1.0x10 7 x 1.0x10 x 1.0x10 x 6.x x10 ± x 7 ( 1.0x10 x)( x) 0 7 ( 1.0x10 ) x10 0.6x10 1.6x10 7 ph log(1.6 x10 ) cid 14 4(1)( 1.0x10 ) (1) When do we hve to ccount for ddition of hydrogen ion due to the dissocition of wter, nd when cn we sfely ignore this contribution? When [H[ ] clc < 3 x 10-7, must ccount for wter dissocition Trnsltes to ph > 6.5 Most cses we cn ignore! Wek cids NNOT ssume complete dissocition! For the wek cid H H - Equilibrium constnt for this dissocition is Subscript indictes n cid dissocition constnt [H ][ - ] / [H] Unfortuntely, we know none of the equilibrium concentrtions Must relte nlyticl concentrtion to equilibrium concentrtion 3

4 Wht do we know? H hrge blnce eqution Since solution must be electriclly neutrl, the concentrtion or ll of the positive chrges must equl the concentrtion of ll of the negtive chrges Wht ions do we hve in solution? tion H nions -, OH - lwys hve hydroxide ions in n queous solution Using the chrge blnce eqution [H ] [ - ] [OH - ] Remember, tht s [H ] increses, [OH - ] decreses We cn ssume tht [ - ] >> [OH - ] Therefore, [ - ] [OH - ] [ - ] Substituting, [H ] [ - ] (1) True when [H ] > 3 x 10-7 Will be true for ll wek cids we work with! Substituting (1) into the equilibrium constnt expression [H ] [H ] / [H] [H ] / [H] Reduced our expression from 3 vribles to! Mss blnce eqution The nlyticl concentrtion for species tht dissocites is equl to the sum of the concentrtion of it s s identifible pieces For H, H [H] [ - ] Rerrnging, [H] H [ - ] Substituting (1), [H] H [H ] () Substituting (1) nd () into the equilibrium expression [H ] / H [H ] One eqution, one vrible [ H ] [ H H ] *( [ H ]) [ H ] rerrnging; [ H ] H [ H ] [ H ] [ H ] H Qudrtic in [ H ] [ H ] [ H ] H ± 4* 0 4 ( ) H H Works for ll situtions we will encounter, but Wht if H >> [H ] Then H - [H ] H [H ] / H Rerrnging [H ] H H [ ] H 4

5 Limittions for short eqution H >> [H ] Method 1 Do the problem the short wy, test the ssumption, nd redo the problem if ssumption is wrong. Method LWYS use the qudrtic bsed eqution (sfest) Method 3 ssumption will be vlid if reltive error < 5% If 100* < H, the ssumption will introduce less thn 5% error First test, is 100* < H YES, then you cn use the short eqution NO, then use the qudric bsed eqution NOT SURE, then use the qudrtic bsed eqution Will lwys work ph for Bses Wht chnges for bses? [OH - ] > [H ] lculte [OH - ] Must convert to ph somehow Method 1 [H ] * [OH - ] 1.0 x [H ] 1.0 x / [OH - ] ph -log [H ] ph for Bses Method [H ] * [OH - ] 1.0 x log ([H ] * [OH - ]) log (1.0 x ) -log ([H ] * [OH - ]) -log (1.0 x log [H ] -log [OH - ] ph poh poh ph 14 ) Strong bses Soluble hydroxides ph for Bses Group I hydroxides B, Sr Soluble oxides Group I oxides M O M O - O - H O OH - [OH - ] * SLT Others which re of no nlyticl importnce ph for Bses Strong bse we know [OH - ] lculte ph using Method 1 or Ignore contribution due to dissocition of wter unless: [OH - ] clc < 3 x 10-7 ph clc < 7.5 Mirrors strong cid rules. 5

6 ph for Bses Wek bses B H O HB OH - b [HB ][OH - ] / [B] [OH - ] for wek bses ph for Bses We cn use the second eqution only when B > 100 b Similr to sitution with wek cids When in doubt, use the qudrtic form [ OH ] OR b [ OH ] bb 4 b b B onjugte cid/bse Pirs conjugte cid/bse pir is pir of chemicl species tht differ by the presence/bsence of one proton (H.) The conjugte cid hs one more proton thn the conjugte bse. H / - HB / B Do not confuse conjugte bse (or cid) with bse (or cid). onjugte cid/bse Pirs onjugte bse of strong cid IS NOT bsic! Dontes protons completely doesn t wnt ny bck onjugte bse of wek cid IS bsic Will tke protons from wter Not relly, but esier to picture onjugte cid of wek bse IS cidic Will donte protons ph for Wek Bses ppendix 3 vlues only cetic cid (H H 3 O ) mmonium ion (NH 4 ) How do we get b for wek bse? Wht is the reltionship between nd b for conjugte cid/bse pir? [ H ][ ] For H H ; [ H] [ H][ OH ] For H O H OH ; b [ ] [ H ][ ] [ H][ OH ] * b * [ H] [ ] 14 * [ H ][ OH ] 1.0x10 b for conjugte cid / bse pirs : w b 1.0x10 14 w 6

7 Misc. Wter is leveling solvent Strongest cid in wter is H 3 O cids stronger thn H 3 O trnsfer protons completely to wter to form the hydronium ion. Reltive to cidic strength, ll strong cids re the sme strength Strongest bse in wter is OH - Bses stronger thn OH - rect completely with wter to form the hydroxide ion (O - ) Two properties Resistnt to ph upon the ddition of smll mount of strong cid/bse Mintin ph upon dilution Mixture of (significnt mount of ) wek conjugte cid/bse pir H H 3 O / H 3 O - NH 4 /NH 3 Resistnt to ph Strong cid rects with the bse Strong bse rects with the cid Mintins ph upon dilution ph of buffer is controlled by the rtio of the wek cid to the wek bse [ H ][ ] [ H] Then [ H ] ssume [ H] ; [ ] H H log log [ H ] H log log[ H ] log H rerrnging : log[ H ] log log H This is the Henderson Hsselblch eqution Wht if H [H] Strong wek cid Dissocition of cid cuses H > [H] Use when 10-3 nlgous eqution for strong wek bses Will not derive (or use) From the Henderson Hsselblch eqution : [ H ] [ H ]*( [ H ] [ H ] rerrnging : [ H ] [ H ] [ H ] [ H ] ( ( [ H ] H [ H ] [ H ] [ H ]) *( )[ H ] ) ( H [ H ] H [ H ]) H 0 0 ) 4 H H Wht is significnt concentrtion For buffer, the rtio of bse to cid must be between 0.1 nd 10 Using these limits Mximum ph p log (10) p 1 Minimum ph p log (0.1) p 1 For buffer, ph p ± 1 7

8 Buffer cpcity (β)( Moles of strong cid or strong bse tht cuses 1.00 L of the buffer to undergo 1.00 unit chnge in ph Function of concentrtion of components nd lso the rtio of the components Best buffer when rtio of bse to cid is 1 ph p log (1) ph p gives the idel buffer Prepring Mix wek cid nd soluble slt of the conjugte bse H H 3 O N H 3 O Mix wek bse nd soluble slt of the conjugte cid NH 3 NH 4 l Prtil neutrliztion H H 3 O NOH when NOH is the limiting rectnt NH 3 Hl when Hl is the limiting rectnt 8