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Ionic Equilibria

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Ionic Equilibria

  1. 1. IONIC EQUILIBRIA Summary www. wewwchemistry. com
  2. 2. 7- --I-‘In-:1‘! -5.‘: at r‘ ,7 ‘II . .'. .. : C. I-3 I “I I PM '~- "‘ C ~'m -3‘: ’IIaA + bB : cC + dDII Buffers c d A I ] [B] »’ Weak acids Weak Bases IHA+H2O~‘—*A'+H3O*I B+H2O~“—*BH* +OH' . A" H Ot I Ht OH" I Sparrngly soluble salts Ka = [—[II#] Kb = E. g. MX2 IV| X2(s) = M2+(aq) + 2X'(aq) Ksp : IMZIIIIIX-I2 Self-ionisation of water Indicators 2H2O-‘—’ H3O+ + oH- Hlnd + H20: Ind‘ + H3O+ K = — W + [Ind ] K = O Ind [H3 IX [Hlnd] For conjugate acid-base pair, Kw = Ka x Kb Colour change is detected ' > when [Ind']/ [Hind] = 1/10 orl0 wwwwewwc hem Isl ry. com
  3. 3. wwwwewwc hem is! ry. com I he p—IIo‘ta. ‘tIon : As applied to: Ht concentration I pH = —| og,0[H+] Ka I pKa : _t0gl0Ka Kb PKb = ‘I09ioKb Kw pKw Z -tOgl0Kw
  4. 4. Weak / ixcids / Weak i3ases : Bronsted-Lowry definition Acid — proton donor must have a hydrogen atom that can be lost = Base — proton acceptor must have alone pair of electrons which can form a coordinate bond with a hydrogen ion : Weak acids / bases dissociate partially in solution : An acid always works in tandem with a base — one to donate proton, the other to accept. wwwwewwc hem is! ry. com
  5. 5. Weak Acids / Weak Bases » Conjugate acid-base pairs HA + H20 7-‘ A‘ + H3O+ acid base conjugate conjugate I I base acid » This equation may be simplified as: HA«——‘ A" + H+ www. wewwchemistry. com
  6. 6. Weak / ixcids / Weak Bases : Conjugate acid-base pairs B + H20 : BH+ + : -:~ base acid conjugate conjugate acid base wwwwewwc hem ist ry. com
  7. 7. Weak / ixcicls / Weak Bases : Acid Dissociation Constant HA + H20~——‘A- + H30+ Ka= iA‘][H30*] A [HA1 2 W In dilute aqueous solutions, amount of H20 that reacts with HA is insignificant compared to the total amount of water present. ~ [H20] remains relatively constant, and is left out of the K2 expression. wwwwewwc hem ist ry. com
  8. 8. Weak / ixcicls / Weak Bases : To calculate pH of weak acid, HA Initial conc / mol dm'3 Change in conc / mol dm"3 Eqm conc / mol dm'3 i where [HA], is the initial concentration of HA, before x of it dissociates to form A‘ and H30+ ions wwwwewwc hem ist ry. com
  9. 9. Weak / ixcicls / Weak Bases : To calculate pH of weak acid Assuming degree of acid dissociation is small such that [HA], >> x, and [HA], — x 2 [HA], = [A‘][H3O*] x2 x2 [HA1 ' [HA]. — x ” [HA]: x = [H‘“] = 2/Ka x {[HA]i — x} z K, x [HA], equals to [HA] _| g 0 K8 at equilibrium pH= r pH =5 —lg -‘/ Ka x [HA]i wwwwewwc hem ist ry. com
  10. 10. Weak / ixcicls / Weak Bases : Base Dissociation Constant B + H20: BH+ + 0H" : [BH“][0H"] K” [B] i Kb is derived similarly. wwwwewwc hem ist ry. com
  11. 11. Weak Acids / Weak Bases » To calculate pH of weak base, B Initial conc / mol dm‘3 Change in conc / mol dm'3 Eqm conc / mol dm'3 0 where [B], is the initial concentration of B, before x - ' dissociates to form BH+ and OH’ ions ‘ www. wewwchemistry. com
  12. 12. Weak Acids / Weak Bases » To calculate p0H of weak base 0 Assuming degree of base dissociation is small such that [3], >> x, and [B], — x z [3], Kb = [BH“][OH'] = x2 ~ x2 [B] [B]i — x " [B1- x = [OH'] 7/i<bx-. ~. Kb x [B], equals to [B] pOH = —lg Kb X [B] r at equilibrium o POH ~ -l9VKb X [Bli WWW. W€WWCh€fTllSIFy. COfTl
  13. 13. Weak Acids / Weak Bases » Ionic Product of Water, Kw 2H2O : H30+ + OH‘
  14. 14. Weak Acids / Weak Bases » Temperature dependence of Kw Kw [Ht] [0H‘] Temperature / X 1044 DKW / ]0_7 / ]0_7 pH / C moi? dm‘5 mol dm‘3 mol dm‘3 0 0.114 14.9 0.338 0.338 7.47 25 1.01 14.0 1.00 1.00 7.00 50 5.48 13.3 2.34 2.34 6.63 0 pH + p0H = 14 is only applicable at 25 °C! 0 pH of pure water decreases with temperature as [Ht] increases. /5 water becoming more acidic? www. wewwchemistry. com
  15. 15. Weak Acids / Weak Bases > Relationship between Kw, Kb and Kb 0 Consider a weak acid HA, HA + H20 2:‘ A‘ + H30+ acid conjugate base K =1A‘11H30*1 a [HA] 0 Its conjugate base, A‘, undergoes salt hydrolysis as follows: A‘ + H20: HA + 0H‘ K iHA11oH‘1 b = T ~ [A 1
  16. 16. Weak Acids / Weak Bases > Relationship between Kw, Ka and Kb Ka X Kb =4A71IH30*1 x1H»er11<_>H‘1 LHKI ILA/ ]’ Kb x Kb = [H30‘“][0H‘] = Kw
  17. 17. Weak Acids / Weak Bases D p H rat O n C U rVe S Source: catalog. flatworldknowledge. com 14.00 12.00 10.00 8.00 6.00 pH of solution 4.00 2.00 4°-’ No-° 0°-’ Volume of NaOH added (mL) (a) Weak acids titrated with strong base °" 14.00 12.00 10.00 8.00 6.00 pH of solution 4.00 2.00 0 qf? 59 Volume of HCl added (mL) (h) Weak bases titrated with strong acid ’°-’ o-° . ;»°-’ {:9
  18. 18. Weak Acids / Weak Bases » Choice of Indicator 0 pH transition range of chosen indicator should fall within the region where a sharp change in pH is observed on the titration curve. 0 E. g. pH 14 Source: www. chemguide. co. uk - - - - - — — - - - - — - - - - - - . - - - equivalence point volume 01 add added ((1113)
  19. 19. Weak Acids / Weak Bases i. e. salt of weak acid HA » Buffers o Acidic buffer: weak acid HA HA: HT + A‘ 01:; MA -. M+ + A‘ i. e. salt of strong mineral acid, e. g. HCI [fee 0 Alkaline buffer: weak base B B+H2O: BH++0 ‘ BH+Cl‘ —- BH+ +Cl' www. wewwchem1stry. com
  20. 20. Weak Acids / Weak Bases » How do buffers work? 0 Consider an acidic buffer, an equimolar mixture of CH3CO2H and CH3C02‘Na+,
  21. 21. +0H‘ pH of buffer increases only slightly pH of buffer decreases only slightly wwwwewwc 11 e 111 1st ry . c o 111
  22. 22. Weak Acids / Weak Bases » To calculate pH of an acidic buffer Initial conc / mol dm‘3 Change in conc / mol dm‘3 Eqm conc / mol dm‘3 o where [HA], and [A‘], are initial concentrations of HA and A‘ respectively, before x of HA dissociates to - I ions. www. wewwchem1stry. com
  23. 23. Weak Acids / Weak Bases 1 To calculate pH of an acidic buffer Ka = [A'][H30+] = [H3O+] X [A’] [HA] [HA] = + E | gKa |9iH1+|9[HA] _ + = _ E | g[H1 |9Ka+|9[HA] [A-1 Henderson-Hasselbalch pH= pKa Hg: [HA] Equation
  24. 24. Weak Acids / Weak Bases 1 To calculate pH of an acidic buffer 0 Assuming [HA], — X z [HA], and [A‘], + X z [A‘], {[A_]1 + X} {1HA1. — x} (where X = [H+]) pH = pKa +| g [A'] pH = pK + lg: 8‘ [HA1 E pH z pK + lg
  25. 25. Weak Acids / Weak Bases 1 Similarly, expressions to calculate p0H of alkaline buffers may be derived. 1BH*1 OH= K I ——— P P b+9 B] BH*- pOH~pKb+mL——L [B]: www. wewwchem1stry. com
  26. 26. Indicators 1 An indicator is a weak acid whose conjugate base is of a different colour. 0 or a weak base whose conjugate acid is of a different colour. Hln + H20 7-‘ In‘ + H30+ acid conjugate Co/ our A base I Colour B it- www. wewwchem1stry. com
  27. 27. indicators flnd’] Kind = [H3O+] X [Hlnd] 1 The end-point is the sudden colour change seen in a titration. 1 A colour change is typically detected when [lnd'] =1 Org [Hlnd] 10 1
  28. 28. indicators 1 Difference between end point and equivalence point 0 The equivalence point of the titration is reached when moles of acid exactly react with moles of base. o If indicator is chosen correctly, the end point will be very close to the equivalence point. l ‘ WWW. W€WWCh€fYllSITV. COfT‘|
  29. 29. Sparingly Soluble Salts 1 Solubility Product, Kb, 0 Consider a sparingly soluble salt, MX2 MX2(s) 7-‘ M2+(aq) + 2X‘(aq) 0 Since MX2 is a solid, its concentration is taken to be its density, and is a constant. 0 Thus Ksb does not contain [MX2(s)] in its expression. K5,, = [lVl2+][X']2
  30. 30. Sparingly Soluble Salts 1 To predict occurrence of precipitation 0 Calculate ionic product (IP), and compare with Ksb. 0 IP = Kbb 2 saturated solution - 0n the brink of precipitation - Precipitate forms immediately if seed crystal is added. 0 IP < Ksb = ~ unsaturated solution - No precipitation 0 IP > Kbb 2 over—saturated solution - Precipitate forms. www. wewwchem1stry. com
  31. 31. Sparingly Soluble Salts i Solubility Product, Ksb ionic Product (IP) r IP expression is exactly the same as Ksb expression Only used with ionic Applicable to all types of concentrations in a solutions (unsaturated or saturated solution saturated) of any concentration At constant temperature, IP value keeps changing Ksb value is constant with ionic concentration, even at constant temperature www. wewwchem1stry. com
  32. 32. Sparingly Soluble Salts 1 Common ion Effect 0 Consider a sparingly soluble salt, MX2, MX2(s) 7-‘ M2+(aq) + 2X‘(aq) <1 When a common ion (i. e. M“ or X‘) is added to a solution of MX2, by Le Chate| ier’s Principle, - equilibrium shifts left to decrease concentration of common ion. - More MX(s) is precipitated. - Solubility of MX(s) is reduced. www. wewwchem1stry. com

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