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CHEM 163 Chapter 18

CHEM 163 Chapter 18. Spring 2009 Instructor: Alissa Agnello aagnello@sccd.ctc.edu. Acids & Bases in Water.  NaCl ( aq ) + HOH (l). HCl ( aq ) + NaOH ( aq ). Net ionic:. H + (aq) + OH - (aq).  H 2 O (l). Neutralization reaction. s trong acid + strong base  water.

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CHEM 163 Chapter 18

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  1. CHEM 163Chapter 18 Spring 2009 Instructor: AlissaAgnello aagnello@sccd.ctc.edu

  2. Acids & Bases in Water  NaCl (aq) + HOH (l) HCl (aq) + NaOH (aq) Net ionic: H+ (aq) + OH- (aq)  H2O (l) Neutralization reaction strong acid + strong base  water HA + H2O (l)  H3O+ (aq) + A- Hydronium ion

  3. Acid-Base Definitions Arrhenius: what does the substance yield when dissociated in water? • Acid: yields H3O+ • Base: yields OH- Bronsted-Lowry: Can the substance donate or accept proton(s)? • Acid: donates proton(s) • Base: accepts proton(s) Not all bases contain OH-! Lewis: • Acid: accepts an e- pair • Base: donates an e- pair

  4. Strong or Weak? HCl (g) + H2O (l)  H3O+ (aq) + Cl- (aq) • Strong acids and bases dissociate completely in water Favors products a lot tiny • Weak acids and bases dissociate only slightly in water ↔ HCN (aq) + H2O (l)  H3O+ (aq) + CN- (aq) Favors reactants, but both products and reactants are present

  5. Strong or Weak: Ka Ka: acid dissociation constant • measure of dissociation • Equals Kc without H2O term ([H2O] changes negligibly) • Stronger weak acid: high Ka (~10-2) • Weaker weak acid: low Ka (~10-10) ~10% dissociated ~0.001% dissociated

  6. Strong or Weak? Strong bases: Group 1A(1) or 2A(2) ion + OH- or O2- Strong acids: HCl, HBr, HI HNO3, H2SO4, HClO4 LiOH NaOH KOH Li2O Na2O K2O Oxoacid with most oxygen atoms Group 1A(1) Ca(OH)2 Sr(OH)2 Ba(OH)2 CaO SrO BaO Group 2A(2)

  7. Water in Solution? 2 H3O+ (aq) + OH- (aq) Autoionization H2O (l) ↔ at 25 °C If… ↑ [OH-]… [H3O+]? ↓ Kw? no change ↑ [H3O+]… [OH-]? ↓ Kw? no change

  8. 3- minute Practice Calculate the concentration of hydroxide ions in coffee, an aqueous solution containing 1.0 x 10-5 M H3O+ ions Calculate the concentration of hydroxide ions in milk, which contains 5.0 x 10-7 M H3O+ ions

  9. pH In any aqueous solution: [OH-] and [H3O+] [H3O+] > [OH-] [H3O+] = [OH-] [H3O+] < [OH-] acidic pH < 7 no change pH = 7 basic pH > 7

  10. 3-minute Practice Calculate the pH of a solution that has… • [H3O+] = 1.0 x 10-8 M • [OH-] = 4.0 x 10-2 M

  11. pKa (Low pKa high Ka) at any T for any (aq) solution Only at 25 °C

  12. Bronsted-Lowry Acids & Bases • Acids: donate a proton HCl + H2O  H3O+ + Cl- • Bases: accept a proton NH3 + H2O  NH4+ + OH- Conjugate acid-base pairs: one loses a proton, other gains a proton HA + B A- + BH+ H+ donor H+ acceptor H+ acceptor H+ donor A- is the conjugate base of HA BH+ is the conjugate acid of B

  13. Weak-Acid Calculations • H3O+ from two sources: HA and H2O • Assume H2O portion is negligible • All H3O+ comes from HA • Weak acid = small Ka • Assume dissociation is negligible • Make I.C.E. table HA + H2O (l)  H3O+ + A- Starting with 0.10 M HA….

  14. Polyprotic Acids • Acids with multiple protons to lose H2SO3 (aq) + H2O (l)  H3O+ (aq) + HSO3- (aq) HSO3- (aq) + H2O (l)  H3O+ (aq) + SO32- (aq) Ka1 > Ka2 > Ka3

  15. Weak Bases A (aq) + H2O (l) ↔ BH+ (aq) + OH- (aq) (Low pKb base dissociation constant (base isn’t really dissociating) high Kb  Stronger weak base) • Proton acceptors: • amines (and NH3) • anions of weak acids For conjugate acid-base pairs:

  16. Sample Calculation What is the [OH-] concentration of 2.0 M NaF (aq)? The Kb of F- = 1.5 x 10-11 • Write reaction • Set up table • OH- from water is negligible • Kb is small; base reacted is negligible • Solve for x • Plug in • Check assumptions

  17. Acid Strength Trends • Nonmetal hydrides • Across a period, strength ↑ • electronegativity • Down a group, strength ↑ • Size, bond strength • Oxoacids • More O atoms, strength ↑ • Same # O atoms? ↑ electronegativity of nonmetal, ↑ strength • Hydrated metal ions • High charge density metal ion

  18. Acid-Base Properties of Salt Solutions • Salts dissolve in water • Which salts form… • Neutral Solutions? • Basic Solutions? • Acidic Solutions? cation from strong base and anion from strong acid NaNO3 NaNO3 (s)  Na+ (aq) + NO3 - (aq)

  19. cation from strong base and anion from weak acid • Basic solutions form from salts that contain: • Acidic solutions form from salts that contain: NaF NaF (s) Na+ (aq) + F - (aq) cation from weak base and anion from strong acid NH4Cl NH4Cl (s) NH4+ (aq) + Cl - (aq)

  20. cation from weak base and anion from weak acid • Weak acid anions + weak base cations? NH4F NH4F (s) NH4+(aq) + F - (aq) NH4+ (aq) + H2O (l) NH3(aq) + H3O+ (aq) F- (aq) + H2O (l) HF (aq) + OH- (aq) Which reaction goes further to the right? If Kb > Ka : basic If Ka > Kb : acidic

  21. Acid-Base Definitions Arrhenius: • Acid: yields H3O+ in water • Base: yields OH- in water Bronsted-Lowry: • Acid: donates proton(s) • Base: accepts proton(s) Not all bases contain OH-! Lewis: • Acid: accepts an e- pair • Base: donates an e- pair

  22. Lewis Acids & Bases • Lewis Bases • Contain lone pair(s) of e- • Lewis Acids • Do not need to contain H A + B ↔ H+ + B ↔ A—B adduct H—B

  23. Examples of Lewis Acids • Molecules with e- deficient central atom • Compounds of B or Al • Molecules with polar multiple bonds • Metal cations • When they dissolve in water

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