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Intersection 11

Intersection 11. 11/12/05 Reading: 16.8 (p794-800) 17.2 (p 828-836). Equilibrium Representations. Water Projects…Now What?. Outline. Strong vs. Weak Acids and Bases Which H’s are acidic? Which groups are basic? Periodic Trends and Acid Strength What affects the pH of a solution?

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Intersection 11

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  1. Intersection 11 11/12/05 Reading: 16.8 (p794-800) 17.2 (p 828-836)

  2. Equilibrium Representations

  3. Water Projects…Now What?

  4. Outline • Strong vs. Weak Acids and Bases • Which H’s are acidic? Which groups are basic? • Periodic Trends and Acid Strength • What affects the pH of a solution? • Polyprotic acids • Concept Questions • Acid/Base Titration: A Closer Look

  5. A Strong vs. Weak

  6. A Vocabulary Strong (16 definitions): Weak (10 definitions): Favorable reaction.. ionizing freely in solution ionizing only slightly in solution Strong Reaction goes to completion spontaneous exothermic product favored

  7. A Strong Acids An acid that dissociates completely (the equilibrium is shifted all of the way to its conjugate base and hydronium ion) is said to be a strong acid. HCl(aq) +  H2O(l) →   H3O+(aq) + Cl-(aq) acid  conj. base An acid that does not dissociate completely (an equilibrium is established in solution between the acid, its conjugate base, and hydronium ion) is said to be a weak acid. HClO2(aq) + H2O(l)↔  H3O+(aq) + ClO2-(aq)      acid conj base Ka = ([H3O+][ClO2-]) / [HClO2]

  8. A Strong Bases A base that dissociates completely (the equilibrium is shifted all of the way to its conjugate acid and hydroxide) is said to be a strong base. NaOH(aq) +  H2O(l) →   OH-(aq) + Na+(aq) + H2O(l) baseconj. acid A base that does not dissociate completely (an equilibrium is established in solution between the base, its conjugate acid, and hydroxide) is said to be a weak base. (CH3)3N(aq)  +  H2O(l)↔ (CH3)3NH+(aq)  + OH-(aq) baseconj. acid Kb = ([(CH3)3NH+][OH-]) / [(CH3)3N]

  9. A Strong Acid Weak Acid

  10. A There are six strongly dissociating acids: HCl HNO3 HBr HClO4 HI H2SO4 There are also five bases that dissociate completely in solution (strong): LiOH Ca(OH)2 NaOH Ba(OH)2 KOH You should commit the strong acids and bases to memory. Appendix F in your text book lists Ka and Kb values for many weakly dissociating acids and bases.

  11. M Problem 1 Trimethylamine (CH3)3N has a Kb of 6.5 x10-5.   Write out its chemical reaction with water: What is the [OH-] of a 0.010 M solution of triethylamine? What is the pOH? What is the pH?

  12. M Which H’s are acidic?What groups are basic?

  13. M What kinds of hydrogen atoms (protons) are acidic? H-halogen (HF, HCl, HBr, HI) H2O H2S (Ka1 = 8.9x10-8) Oxoacids (H-polyatomic ions) (H2CO3, HNO3, etc.) HCN

  14. M Organic Acids RCOOH

  15. M Bases OH R3N

  16. A Periodic Trends and Acid Strength

  17. A Groups 7 and Period 2 Acid strength: HI > HBr > HCl> HF HF>H2O >> H3N >>> CH4

  18. A Oxoacids I HOY + H2O H3O+ + OY- Y = I, Br, Cl Since a negatively charged ion must be formed, it will be most stable when Y is an atom that is most effective at attracting electron density to itself, thereby stabilizing the negative charge. So the more electronegative atom as Y, yields a stronger acid Acid strength: HOI < HOBr < HOCl

  19. A Oxoacids II What trend do you see and why?

  20. M What affects the pH?

  21. M Question 1 • How could you change the pH of a solution of acetic acid (CH3COOH)?

  22. M Question 2 Apatite, Ca5(PO4)3OH is the mineral in teeth. Ca5(PO4)3OH(s)  5 Ca+2(aq)  + 3PO4-4(aq)  +  OH-(aq) Sour milk contains lactic acid.  Not removing sour milk from the teeth of young children can lead to tooth decay.  Use chemical principles to explain why.

  23. M Can salts affect the pH of a solution? Ca(OH)2 Ksp = 7.9x10-6 Na(CH3COO) KaCH3COOH = 1.8 x10-5 NH4Cl NH4+ + H2ONH3 + H3O+ NaNO3 Kb NH3 = 1.8 x10-5 NaH2PO4 Na2HPO4

  24. M Name ions found in salts that would not affect the pH:

  25. M Question 3 A carbonated beverage is left open to the atmosphere. Will the pH change? CO2(aq) + H2O(l) ↔ H2CO3(aq) H2CO3 + H2O(l) ↔ H3O + + HCO3¯ Ka1 = 4.2 × 10-7 HCO3¯ + H2O(l) ↔ H+ + CO32¯ Ka2 = 4.8 × 10 -11

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