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AP Chemistry

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  1. AP Chemistry Ch. 14 -Acids, Bases, and Acid-Base Equilibrium

  2. Properties of Acids • pH below 7 • Conduct electricity in solution • React with to form Products • Metals to form H2 • Carbonates to form CO2, H20, and a salt • Bicarbonates to form CO2, and H20 • A Base to form H20 and a salt

  3. Properties of Bases • pH above 7 • Turns red litmus paper to blue • “Basic Blue” • Conduct electricity in solution • React with to form Products • Metal ions to form Precipitates • Bicarbonates to form CO3, and H20 • An acid to form H20 and a salt

  4. Definitions of Acids & Bases • Arrhenius Definition of Acids and Bases: • Most elementary of ideas about acids • a) Acids produce H+ ions in solutions • b) Bases produce (OH)- ions in solutions

  5. HI – hydroiodic 7 you need HBr-hydrobromic to remember. HCl - hydrochloric HClO4 - perchloric H2SO4 - sulfuric HClO3 - chloric HNO3 - nitric Strong Acids

  6. Acid and Base Strength • Strong acids are completely dissociated in water. • Their conjugate bases are quite weak. • Weak acids only dissociate partially in water. • Their conjugate bases are weak bases.

  7. LiOH – lithium hydroxide NaOH Sodium Hydroxide KOH Potassium Hydroxide Ca(OH)2 Calcium Hydroxide Sr(OH)2 Strontium Hydroxide Ba(OH)2 Barium Hydroxide Strong Bases 6 you need to remember

  8. Problems with Arrhenius • The Arrhenius definitions of acids and bases did not properly explain why other substances are acids or bases • Example) NH3

  9. Bronsted-Lowry Definition • These two scientists devise better definitions for acids and bases which would encompass more substances • Bronsted-Lowry Acid: A proton donor • Bronsted-Lowry Base: A proton acceptor

  10. More About Bronsted-Lowry • A proton is simply a H+ ion • The hydrogen in substances is described as being ionizable • This ionizable hydrogen is attracted to a center of negative charge (lone pairs of electrons)

  11. H H O O+ H H Cl Cl- H H More About Bronsted-Lowry • HCl + H2O  H3O+ + Cl-

  12. More About Bronsted-Lowry • 1) Show the reaction between HCl and NH3 and clearly identify the acid and the base • 2) Identify the acid and the base when NH3 is placed in water

  13. Conjugate Acids and Bases • When an acid-base reactions occurs, a conjugate acid and base is formed • Conjugate acids are the original base plus a hydrogen ion (ie it is the acid on the product side of the equation)

  14. Conjugate Acids and Bases • Conjugate bases are the original acid minus a hydrogen ion (ie it is the base on the product side of the equation)

  15. Conjugate Acids and Bases: • Reactions between acids and bases always yield their conjugate bases and acids.

  16. Identify the Bronsted-Lowry Acids and Bases & their conjugates • 1) H2S + NH3 NH4+ + HS- Acid base con. Base con. base • 2) OH- + H2PO4-  H2O + HPO42 base acid con. acid con. base

  17. Significance of Conjugate Acid-Bases • The stronger an acid, the weaker is its conjugate base • The stronger a base, the weaker is its conjugate acid • Acid-Base reactions favor the direction of the stronger member to the weaker member of each pair

  18. Acids & Base Definitions Definition #3 – Lewis Lewis acid - a substance that accepts an electron pair Lewis base - a substance that donates an electron pair

  19. Lewis Acids & Bases Formation ofhydronium ion is also an excellent example. • Electron pair of the new O-H bond originates on the Lewis base.

  20. Lewis Acid/Base Reaction

  21. AP Practice • In the lab H2(g) can be produced by adding which of the following to 1.0M HCl(aq)? • I. 1 M NH3 (aq) • II. Zn (s) • III. NaHCO3 (s) A. I only C. III only E. I, II, and III B. II only D. I and II only

  22. AP Practice • In liquid ammonia, the reaction represented below occurs. 2NH3 NH4+ + NH2- In the reaction, NH4+ acts as: • A. a catalyst • B. Both an acid and a base • C. the conjugate acid of NH3 • D. The reducing agent • E. the oxidizing agent

  23. AP Practice #4 • Write the balance net ionic equation for : A 0.1M nitrous acid solution is added to the same volume of a 0.1M sodium hydroxide solution. HNO2 + OH- NO2- + H2O

  24. AP Practice #4 • Write the net ionic equation for: Hydrogen iodide gas is bubbled into a solution of lithium carbonate. 2 HI + CO3-2 2 I- + H20 + CO2

  25. AP Practice #4 • Write the balanced net ionic equation for: Concentrated hydrochloric acid is added to a solution of sodium sulfide. 2 H+ + S-2 H2S

  26. AP Practice #4 • Write the balanced net ionic equation for: A solution of ethanoic (acetic) acid is added to a solution of barium hydroxide: HC2H3O2 + OH- H2O + C2H3O2-

  27. Strength of Acids And Bases

  28. Strong and Weak Acids/Bases The strength of an acid (or base) is determined by the amount of IONIZATION.

  29. Strong and Weak Acids/Bases • Weak acids are much less than 100% ionized in water. *One of the best known is acetic acid = CH3CO2H

  30. Strong Acid Weak Acid 15.4

  31. CaO Strong and Weak Acids/Bases • Strong Base:100% dissociated in water. Have very weak conjugate acids. NaOH (aq) ---> Na+ (aq) + OH- (aq) Other common strong bases include KOH andCa(OH)2. CaO (lime) + H2O --> Ca(OH)2 (slaked lime)

  32. Strong and Weak Acids/Bases • Weak base:less than 100% ionized in water One of the best known weak bases is ammonia NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq)

  33. Weak Bases

  34. Significance of Acid/Base Strength • When equilibrium is established, the side in which there are stronger acids/bases will shift toward the weaker sides (LeChatlier Principle) • Thus the concentration of substances will favor the weaker members

  35. Factors Affecting Acid Strength • A) Binary Acids: The lower the bond dissociation energy, the easier the bond is broken. • More likely that acid will donate a H ion. • (Low BE = Stronger Acid (Weaker Conjugate Base) • ***Low BE = Stronger Acid***

  36. H+ + X- H X The stronger the bond Molecular Structure and Acid Strength • Bond strength • Polarity The weaker the acid HF << HCl < HBr < HI

  37. Factors Affecting Acid Strength • The larger the anion, the stronger that acid is (the easier the bond is broken) • Acid strength increases going across the table right to left while increasing going down the table • {the greater the distribution in charge (polarity) the more likely the substance will lose H ion}

  38. Factors Affecting Acid Strength • Nonpolar covalent acids are weaker than polar covalent acids which are weaker than ionic acids • Essentially, the greater the dipole in the acid, the more likely the acid is strong!

  39. d- d+ O- Z + H+ O Z H Molecular Structure and Acid Strength • The O-H bond will be more polar and easier to break if: • Z is very electronegative or • Z is in a high oxidation state

  40. AP Chemistry Quiz • 1. Explain how to determine between a Bronsted-Lowry Acid and Base. • 2. Why is HBr is a stronger acid than H2S?

  41. Oxoacids: • B) Oxoacids: Contain hydrogen, oxygen, and some other element (nonmetal). At least one H bonded to an O. • The other element’s tendency to attract other electrons assists in determining the strength of the acid

  42. Oxoacids: • If the other element attracts electrons very strongly, electrons are withdrawn from Oxygen – Hydrogen bond. This weakens the O-H bond and results in stronger acids.

  43. O H O Cl O O H O Br O • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Molecular Structure and Acid Strength 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z Cl is more electronegative than Br HClO3 > HBrO3

  44. Adding more oxygen atoms that are added (H2SO4 and H2SO3) is the same as adding more electronegative elements. • Since oxygen has a high electronegativity, this withdraws electrons from the O-H bond and results in stronger acids

  45. Molecular Structure and Acid Strength 2. Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO4 > HClO3 > HClO2 > HClO

  46. pH Scale and its calculations

  47. pH Scale • The pH Scale runs 0-14 • pH values < 7 are acidic • pH values > 7 are basic (alkaline) • pH values = 7, Neutral

  48. How Do We Measure pH? • For less accurate measurements, one can use • Litmus paper • “Red” paper turns blue above ~pH = 8 • “Blue” paper turns red below ~pH = 5 • An indicator

  49. How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

  50. Mathematical Determination of pH • pH = - log [H+] OR • - log [H3O+]