ACIDS, BASES AND SALTS Water (H2O) is made of two ions H + (aq) hydrogen ion OH - (aq) hydroxide ion

Transcription

1 ACIDS, BASES AND SALTS Wter () is mde of two ions + (q) hydrogen ion - (q) hydroxide ion 1 Acids substnces tht increse + (q) conc. - definition of Arrhenius cid Strong cids - strong electrolytes, i. e., completely dissocite Cl (q) = + (q) + Cl - (q) - memorize list Cl, Br, I, N3, S4, Cl3, Cl4 Wek cids - wek electrolytes F (q) + (q) + F - (q) F (q) F (q) + + (q) + F - (q) lot little little 3P4 (q) + (q) + P4 - (q) Bses substnces tht increse - concentrtion - definition of Arrhenius bse Strong bses - strong electrolytes - ll lkli metl hydroxides Li, N,, Rb, Cs - some lkline erth metl hydroxides C(), Sr(), B() Wek bses - wek electrolytes - usully increse - (q) conc. indirectly by decresing + (q) conc. N3 (q) + (l) N4 + (q) + - (q) C3 - (q) + (l) C3 (q) + - (q) Neutrliztion rection Acid + Bse Wter + Slt Slt - ionic compound - ctions nd nions remining fter wter is mde N3 (q) + (q) (l) + N3 (q) cid bse wter slt

2 BRØNSTED LWRY ACID/BASE TERY Acid/bse rections re simply trnsfer of proton (hydronium ion) from cid to bse. Brønsted Lowry Acid - proton donor - note Cl, S4, C3 ll donte protons Brønsted Lowry Bse - proton cceptor - note - nd N3 ccept protons In Brønsted Lowry theory, wter is n cid or bse depending on the circumstnces. Cl (q) + (l) 3 + (q) + Cl - (q) - wter ccepts proton; therefore, it is bse. N3 (q) + (l) N4 + (q) + - (q) - wter dontes proton; therefore, it is cid. Conjugte Acids nd Bses Consider typicl cid/bse rection. N3 (q) + (l) N4 + (q) + - (q) - hs donted proton, it is n cid. - N3 ccepted proton, it is bse. Consider the reverse rection. N4 + (q) + - (q) N3 (q) + (l) - N4 + hs donted proton, it is n cid. - - ccepted proton, it is bse. - An cid is chnged into bse nd bse is chnged into n cid. - These pirs of cids nd bses re clled conjugte cid-bse pirs. - A conjugte cid-bse pir differ from ech other only by proton. N3 (q) + (l) N4 + (q) + - (q) bse cid conjugte conjugte cid bse Another Exmple: S4 (q) + (l) S4 - (q) (q) cid bse conjugte conjugte bse cid

3 Another Exmple: 3 C3 + N + NC3 cid bse conjugte conjugte cid bse Reltive strengths of cids nd bses The strength of conjugte bse is relted to the strength of its cid. The strength of conjugte cid is relted to the strength of its bse. Consider nitric cid: N3 (q) cid N3 - (q) conjugte bse N3 is strong cid, mening tht it fully dissocites. N3 (q) + (q) + N3 - (q) - This mens tht the N3 - ion hs no desire to ccept proton. - Therefore the nitrte ion is very wek bse. Consider queous sodium hydroxide: - bse cid N (q) N + (q) + - (q) - - ion rects fully (lmost) with + ions. - If ny dditionl + is dded, - will grb it to form wter. - - is very strong bse. - The conjugte cid, is very wek cid. As the strength of cid increses, the strength of its conjugte bse decreses. As the strength of bse increses, the strength of its conjugte cid decreses. Exmple: C3 is weker cid thn F. Which ion will more redily ccept protons in queous solution, C3 - or F -? C3 is weker cid; therefore, C3 - is stronger bse. Thus C3 - will ccept proton more redily. Exmple: Pyridine, C55N, is weker bse thn mmoni, N3. Which ion is the stronger cid, N4 +, C55N +? C55N is weker bse; therefore, C55N + is stronger cid. Therefore, C55N + will donte its proton esier tht N4 +. Pyridine is bse used in methods tht determine the moisture content of foods.

4 TE AUTINIZATIN F WATER - Wter is not nonelectrolyte, ctully it is very wek electrolyte. - ne out of every 10 million wter molecules dissocites into n + ion nd - ion. 4 (l) + (q) + - (q) Ion product of wter Write equilibrium expression c - Since is solvent, we will neglect it in equilibrium expression. No mtter wht conditions exist in n queous solution, the following expression is true. (t 5 C) Ion product is lso known s w w Aside: ther solvents utoionize s well. N3 + N3 N4 + + N - C3 + C3 C3 + + C3-14 Since is lwys true - if we know [ + ], we cn clculte [ - ] - if we know [ - ], we cn clculte [ + ] Exmple: If solution hs [ + ] = 1.8 x 10-5 M, wht is [ - ]? M 5 Exmple: If solution hs [ - ] = 9.4 x 10-1 M, wht is [ + ]? M

5 5 Exmple: Wht is the [ + ] nd [ - ] for pure wter? (l) + (q) + - (q) w [ + ] [ - ] Initil 0 0 Chnge x x Equil. x x 14 x x x M M Note: When [ + ] = [ - ], the mount of cid nd bse re in equl mounts. The solution is clled neutrl. TE YDRNIUM IN + is simply proton. - A proton will hydrogen bond with wter is hydronium ion. Current theories of wter tret proton s surrounded by 4 or 6 wter molecules. Therefore, + my be truly 94 + or is concept, not n ctul structure. Reconsider utoioniztion of wter s (l) + (l) 3 + (q) + - (q)

6 6 p SCALE Definition: p = - log [ + ] = -log[ 3 + ] p scle is used strictly for convenience - convenient to void scientific nottion. [ + ] for pure wter equls 10-7 ; therefore, p = - log(10-7 ) = - (-7) = 7 All neutrl solutions hve p = 7. If solution is cidic, p < 7. If solution is bsic, p > 7. p scle of common substnces very cidic slightly cidic neutrl gstric cid lemon juice coke coffee blood slightly bsic very bsic milk of mgnesi household mmoni household blech **In number tht is logrithm such p, only the digits to the right of deciml point re significnt.** [ + ] = p = 1.69 [ + ] = p = 4.69 [ + ] = p = 7.69 [ + ] = p = ther definitions: p = - log [ - ] pw = - log w = Autoioniztion of wter in terms of p - log([ + ][ - ]) = -log w p + p = pw = Exmple: The p of milk of mgnesi is 10.0, wht is the p? p = pw p = = 4.0 Exmple: An sodium cynide solution used to extrct gold from low-grde ore hs p of 10.67, wht is the hydronium ion concentrtion of the solution? p log 10 p M 4 Au (s) + 8 CN - (q) + (q) + (l) 4 Au(CN) (q)

7 Exmple: A bsic solution of trisodium phosphte (TSP), N3P4, from the hrdwre store hs p of.14, wht is its hydroxide ion concentrtion? 7 p M You know tht the proper nme for TSP is sodium phosphte! STRNG ACIDS AND BASES Remember lists Strong Acids: Cl, Br, I, N3, S4, Cl3, Cl4 Strong Bses: Alkli metl hydroxides, B(), Sr(), C(), Mg() Clculting p of strong cid nd strong bse solutions limited solubility Strong cids nd bses completely dissocite; therefore, [ + ] (or [ - ]) equls the concentrtion of the solute. Exmple: Wht is the p of M solution of Br? Since Br is strong cid, [ + ] = M p = - log (0.014) = - (-1.848) = Exmple: Wht the p of 1.5 M Sr() solution? Since Sr() is strong bse, 1.5molSr() mol 3.0M L 1mol Sr() M p = - log ( ) = R p = - log (3.0) = p = p = (-0.48) = 14.48

8 WEA ACIDS AND BASES For wek cids nd bses, the concentrtion of cid (bse) is not the concentrtion of [ + ] ([ - ]). 8 Wek Acids Dissocited ions re in equilibrium with undissocited molecule. C3 (q) + (q) + C3 - (q) Equilibrium expression is C3 C 3 cid-dissocition constnt - only difference from c is the lbel. Ech wek cid hs its own which does not chnge (t 5 C). Exmple: Wht is the p of.0 M scorbic cid (C676), lso known s vitmin C? (C676) = C676 C [ + ] [C676 - ] [C676] Initil M Chnge x x x Equil. x x.0 x x 0. x Assume tht x is smll. x x = p = 1.90 x 5 4 Vitmin C is necessry for the production of collgen, fibrous protein tht is found in skin, tendon, blood vessels, orgns, bone. Citrus fruit contins substntil mount of vitmin C.

9 Exmple: Wht is the p of 0.15 M iodic cid (I3)? (I3) = I3 I [ + ] [I3 - ] [I3] Initil M Chnge x x x Equil. x x 0.15 x x 015. x 017. Assume tht x is smll. x x x = 0.16 mmm! Looks like x is not smll. We will need the qudrtic eqution. x x x x x x 017. x b b 4c x x = [ + ] = p = 1.01 Iodic cid is used in the production of sodium iodte nd potssium iodte. Both iodte compounds re dded to tble slt to iodize the slt. Iodine is necessry for helthy thyroid function.

10 Exmple: Clculte the p of n unknown cid solution with concentrtion of M nd p of [ + ] [A - ] [A] Initil M Chnge x x x Equil. x x x Equil A A p log log Polyprotic Acids - Mny cids hve more thn one proton to donte. Consider S4 S4 is strong cid. S4 (q) + (q) + S4 - (q) Bisulfte ion lso hs proton to donte, though it is less willing to do so. S4 - (q) + (q) + S4 - (q) 8 S4 S nd proton disssocited Consider 3P4 Phosphoric cid hs 3 protons to donte - before second proton leves, lmost ll of the first protons hve left. P P 3 4 P P 4 3 P P

11 When we dd 3P4 to wter, the disssocition is 3P4 (q) + (q) + P4 - (q) 11 ow cn we get the second proton to leve? - Remove + ; i.e., dd bse. P4 - (q) + (q) + P4 - (q) Wek Bses Conc. of - is not concentrtion of wek bse. Consider mmoni Equilibrium expression is N3 (q) + (l) N4 + (q) + - (q) N 4 N 3 b b bse-dissocition constnt - only difference from c is the lbel. Consider generl bse B (q) + (l) B + (q) + - (q) Equilibrium expression is b B B Exmple: Clculte the - concentrtion of 0.01 M solution of pyridine, C55N. b(c55n) is C C N C C C C C + N C C C C5 5N CN 5 5 b [C55N] [C55N + ] [ - ] Initil Chnge x x x Equil x x x x 9 b x

12 Assume tht x is smll. 1 x x x = [ - ] = M p = 5.35 p = 8.65 Exmple: Clculte the p of 3.4 M solution of sod sh, otherwise known s sodium crbonte, NC3. b(c3 - ) = When NC3 is dissolved in wter, it is fully disssocited. (All sodium slts re soluble.) NC3 (q) = N + (q) + C3 - (q) Consider tht C3 - is the conjugte bse of C3 -. C3 - (q) + (q) C3 - (q) + - (q) C3 C3 b [C3 - ] [C3 - ] [ - ] Initil Chnge x x x Equil. 3.4 x x x b x 34. x Assume tht x is smll. x x x = [ - ] = M p = 1.60 p = 1.40 Check ssumption: % 0.74% % 5% Also = Assumption is good. Sod sh is combined with clcium crbonte nd snd (Si ) to mke sod lime glss, the type of glss most commonly used in bottles nd windows.

13 Percent Ioniztion (Degree of Ioniztion) 13 While the dissocition constnt gives mesure of the strength of wek cid or bse, it doesn t give ny indiction of how much electrolyte ctully dissocites. By definition, the percent ioniztion is the percentge of wek electrolyte molecules tht hve dissocited in solution For wek cid, the percent ioniztion is electrolyte dissocited A electrolyte A A % ioniztion 100% 100% or 100%. initil 0 0 Note the mbiguity in the definition. - For pure wek cid, either definition is vlid. - If extr A - is dded, use [ + ] definition. - If extr + is dded, use [A - ] definition. - If in doubt, smller number is correct. For wek bse, the percent ioniztion is electrolyte B dissocited electrolyte B B % ioniztion 100% 100% or 100%. initil For pure wek bse, either definition is vlid. - If extr B + is dded, use [ - ] definition. - If extr - is dded, use [B + ] definition. - If in doubt, smller number is correct. Exmple: Clculte the percent ioniztion of hydrofluoric cid in 1.0 M, 0.10 M nd M solutions. (F) = To clculte percent ioniztion, we need [ + ], therefore, we need n ICE tble. For vrible initil concentrtion c F F [ + ] [F - ] [F] Initil 0 0 c M Chnge x x x Equil. x x c x x c x 4 4

14 Assume tht x is smll. x c x c x c [ ] x % ioniztion 100% c x For c = 1.0 M, 0.06 % ioniztion 100%.6% 1.0 x For c = 0.10 M, % 8.% 0.10 ops! Now x is not smll! x x cx x xc0 c x x 41 c 1 b b 4c c = [F]0 [ + ] % ionized p Note tht s the concentrtion of wek cid decreses, more of it dissocites! Artists use hydrofluoric cid to etch glss.

15 Reltionship between nd b 15 Consider b of mmoni. b(n3) = N3 (q) + (l) N4 + (q) + - (q) b Now consider of conjugte cid, N4 + Now multiply nd b. N 4 N 3 N4 + (q) + (l) N3 (q) (q) N 3 3 N 4 3 N3 N 4 N 4 N 3 b 3 Thus for conjugte cid-bse pir: b = w Exmple: If b(n3) = , wht is for N4 +? w b Exmple: If (F) = , wht is b for F -? b w b = w lso implies p + pb = 14.00

16 Exmple: Identify which picture represents strong cid nd which represents wek cid. Approximte the p of wek cid. 16 The representtion on the right, with no -A bonds is the strong cid. The representtion on the left hs 5 + (ornge), 5 A - (kentucky blue) nd 5 A totl. Therefore the is [ + ][A - ]/[A]0 = 55/5 = 1. p = - log[ + ] = - log(1) = 0 YDRLYSIS Mny slts when dissolved in wter will chnge its p. - Ctions my ttrct the hydroxide ion. - Anions my ttrct the hydrogen ion. The process where ions cuse wter to split prt is clled hydrolysis. The cidic or bsic properties of slt solution depend on the reltive bilities of the ction nd the nion to cuse hydrolysis. Ctions re cidic (usully) 1. Ctions from strong bses hve negligible cidity. - Li +, N +, +, Rb +, Cs +, C +, Sr +, B +. igher ctionic chrge cuses greter hydrolysis. - igher chrges ttrct - more effectively. - p of TlCl soln. higher thn PbCl soln. Pb()6 + Pb()5() Smller ctions re more cidic thn lrger ctions. - igher chrge density ttrcts - more effectively. - p of G(N3)3 sol. higher thn Al(N3)3 sol. 4. Trnsition metl ions re more cidic thn lkli or lkline erth metl ions. - Incomplete d subshell ttrcts - nd. - Co(Cl3) more cidic thn Mg(Cl3). Co()6 + Co()5( - ) Conjugte cids of nitrogen bses re cidic. - N4 +, C55N +, (C3)3N +, etc N4I (q) + (l) N3 (q) (q) + I - (q)

17 Anions re bsic 1. Anions from strong cids re not bsic. - Cl -, Br -, I -, N3 - S4 -, Cl3 -, Cl4-17. ther soluble montomic nions re bsic. - S, Cs, Be solutions re bsic. S - + S Conjugte bses of wek cids re bsic. - Al(C3)3, SrF, LiC4 re bsic slts. F - + F Note: This is the dissocition of wek bse. Exmples: Predict whether the following slt solutions re cidic, bsic or neutrl. ) Cl The + comes from strong bse; therefore, it should not ffect the p (it doesn t ttrct - to itself). Cl - comes from wek cid; therefore, it should rise the p (it ttrcts + little). Thus Cl solution should be bsic. b) Ni(N3) The Ni + will ttrct - ; therefore, it hs the tendency to mke the solution cidic. The N3 - comes from strong cid; therefore, it won t ffect the p. verll Ni(N 3 ) solution is expected to be cidic. c) SrBr Sr + neutrl Br - neutrl verll SrBr is neutrl.

18 Slts derived from wek cid nd wek bse my be cidic or bsic 18 - Both ction nd nion hydrolyze wter. - p depends on which ion hs stronger cid/bse properties. - if bse is stronger; i.e., b > then p > 7. - if cid is stronger; i.e., > b then p < 7. Exmple: Is N4N solution cidic, bsic or neutrl? w N b N b N w N > b, thus p < 7. N4N solution is cidic Sodium nitrite is used in some mets s preservtive nd to prevent the color of met chnging from red to brown. CEMICAL STRUCTURE F ACIDS AND BASES Bsic Principles of Acidity 1. Acidity is relted to bond strength. - Se is more cidic thn S becuse the Se bond is weker thn the S bond (Se is lrger thn S; therefore the bond overlp for the Se bond is weker thn for the S bond).. Acidic hydrogens ionize from polr bond. 3. Polrity of the cidic hydrogen bond cn be ffected to lesser extent by other toms. 4. Strong ttrction of the wter (solvent) molecules is the driving force. Binry Acids - Bonds between wter molecules nd ions re stronger thn bonds between cid molecules. 11 Cl Cl These bonds re stronger thn these bonds. Cl

19 ydrofluoric cid s cidity is peculir. - Bonds between F molecules re very strong. 19 F (q) + F (q) [F F] - (q) + + (q) + These bonds re stronger thn these bonds. F - - Even though F is very polr, it is wek cid since it forms strong intermoleculr bonds with itself. F etches glss (nd bone!). It cnnot be stored in glss bottle. xycids - xycids re essentilly moleculr compounds. - Bonds holding + must be strongly polr to llow + to disssocite. - Must be considerble difference in electronegtivity to crete polr bond. Cl polr bond - Secondry effects re very importnt. - Electron withdrwing of other oxygen toms is the difference between Cl4 being strong cid rther thn wek cid. Compre to Cl. Cl Exmple: Which is the stronger cid, cetic cid or trichlorocetic cid? Chlorine toms re much more electronegtive tht hydrogen tom C C Cl C C Cl cetic cid trichlorocetic cid Chlorine toms re much more electronegtive tht hydrogen toms; therefore, the chlorine toms re mking the bond more polr thus incresing the cidity. Trichlorocetic cid is the stronger cid. bond is less polr since fewer oxygen toms re drwing wy electron density Trichlorocetic cid is used in some tttoo removl processes. Cl

20 LEWIS ACIDS AND BASES Definitions Lewis cid electron pir cceptor Lewis bse electron pir donor 0 Lewis cid/bse theory is more generl thn Brønsted Lowry cid/bse theory. - Brønsted Lowry cids re Lewis cids, though not vice vers. - Brønsted Lowry bses re Lewis bses, though not vice vers. Consider 3 + (q) + - (q) (l) + is electron pir cceptor - Lewis cid - is electron pir donor - Lewis bse Consider N3 (q) + (l) N4 + (q) + - (q) N N - N3 is electron pir donor - is electron pir cceptor Not ll Lewis cid/bse rections re Brønsted-Lowry cid/bse rections. Lewis cid ccepts e - pir from Cl - Lewis bse dontes e - pir vi Cl - ion AlCl3 + C3Cl AlCl4 - + C3 + other products This is the first step in the Friedel-Crfts lkyltion tht dds methyl group to n orgnic molecule. More next yer!