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Properties and Theories of Acids and Bases: Understanding Acid-Base Reactions and pH Scale

Learn about the properties of acids and bases, the Arrhenius and Bronsted-Lowry theories, and how acid-base reactions can be used to neutralize each other. Discover how pH and pOH are measured and how they are related. Understand the importance of the pH scale in determining the acidity or basicity of a solution.

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Properties and Theories of Acids and Bases: Understanding Acid-Base Reactions and pH Scale

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  1. ACIDS and BASESUnit 10, Chapter 19

  2. ACIDS Have a sour taste Change the color of many indicators Are corrosive (react with metals) Neutralize bases Conduct an electric current BASES Have a bitter taste Change the color of many indicators Have a slippery feeling Neutralize acids Conduct an electric current Properties of Acids and Bases

  3. The Arrhenius Theory of Acids and Bases

  4. Arrhenius Theory of Acids and Bases: an acid contains hydrogen and ionizes in solutions to produce H+ ions: HCl  H+(aq) + Cl-(aq)

  5. Arrhenius Theory of Acids and Bases: a base contains an OH- group and ionizes in solutions to produce OH- ions: NaOH  Na+(aq) + OH-(aq)

  6. Neutralization • Neutralization: the combination of H+ with OH- to form water. H+(aq) + OH-(aq) H2O (l) • Hydrogen ions (H+)in solution form hydronium ions (H3O+)

  7. In Reality… H+ + H2O  H3O+ Hydronium Ion (Can be used interchangeably with H+)

  8. Commentary on Arrhenius Theory… One problem with the Arrhenius theory is that it’s not comprehensive enough. Some compounds act like acids and bases that don’t fit the standard definition.

  9. Bronsted-Lowry Theory of Acids & Bases

  10. Bronsted-Lowry Theory of Acids & Bases: • An acid is a proton (H+) donor • A base is a proton (H+) acceptor

  11. for example… Proton transfer HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Base Acid

  12. Water is a proton donor, and thus an acid. another example… CONJUGATE BASE ACID NH3(aq) + H2O(l) NH4+ (aq) + OH- (aq) BASE CONJUGATE ACID Ammonia is a proton acceptor, and thus a base

  13. Amphoteric Substances A substance that can act as both an acid and a base (depending on what it is reacting with) is termed amphoteric. Water is a prime example.

  14. Conjugate acid-base pairs • Conjugate acid-base pairs differ by one proton (H+) A conjugate acid is the particle formed when a base gains a proton. A conjugate base is the particle that remains when an acid gives off a proton.

  15. Examples: In the following reactions, label the conjugate acid-base pairs: • H3PO4 + NO2- HNO2 + H2PO4- • CN- + HCO3- HCN + CO32- • HCN + SO32- HSO3- + CN- • H2O + HF  F- + H3O+ acid base c. acid c. base base acid c. acid c. base acid base c. base c. acid c. base c. acid base acid

  16. SUMMARY OF ACID-BASE THEORIES

  17. Strength of Acids and Bases • A strong acid dissociates completely in sol’n: • HCl  H+(aq) + Cl-(aq) • A weak acid dissociates only partly in sol’n: • HNO2 H+(aq) + NO2-(aq) • A strong base dissociates completely in sol’n: • NaOH  Na+(aq) + OH-(aq) • A weak base dissociates only partly in sol’n: • NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

  18. Acid-Base Reactions • Neutralization reactions: reactions between acids and metal hydroxide bases which produce a salt and water. • H+ ions and OH- ions combine to form water molecules: • H+(aq) + OH-(aq) H2O(l)

  19. Buffered Solutions A solution of a weak acid and a common ion is called a buffered solution.

  20. Thus, the solution maintains it’s pH in spite of added acid or base. Pg. 620 fig 19.27

  21. Universal Indicator Color ChartPAGE 602 fig 19.13 pH scale 0 7 14 Acid Neutral Base

  22. Why does it take more drops of acid or base to make the tap water change color than it does for the distilled water? • What is distilled water made of? What is tap water made of?

  23. pH and pOH Pg. 596 (in section 19.2)

  24. Ionization of water • Experiments have shown that pure water ionizes very slightly: • 2H2O  H3O+ + OH- • Measurements show that: [H3O+] = [OH-]=1 x 10-7 M • Pure water contains equal concentrations of H3O+ + OH-, so it is neutral.

  25. pH • pH is a measure of the concentration of hydronium ions in a solution. • pH = -log [H3O+] or • pH = -log [H+]

  26. Example: What is the pH of a solution where [H3O+] = 1 x 10-7 M? • pH = -log [H3O+] • pH = -log(1 x 10-7) • pH = 7

  27. Example: What is the pH of a solution where [H3O+] = 1 x 10-5 M? • pH = -log [H3O+] • pH = -log(1 x 10-5) • pH = 5 • When acid is added to water, the [H3O+] increases, and the pH decreases.

  28. Example: What is the pH of a solution where [H3O+] = 1 x 10-10 M? • pH = -log [H3O+] • pH = -log(1 x 10-10) • pH = 10 • When base is added to water, the [H3O+] decreases, and the pH increases.

  29. The pH Scale PAGE 598 Table 19.5 & fig 19.10 0 7 14 Acid Neutral Base

  30. pOH • pOH is a measure of the concentration of hydroxide ions in a solution. • pOH = -log [OH-]

  31. Example: What is the pOH of a solution where [OH-] = 1 x 10-5 M? • pOH = -log [OH-] • pOH = -log(1 x 10-5) • pOH = 5

  32. How are pH and pOH related? • At every pH, the following relationships hold true: • [H3O+] • [OH-] = 1 x 10-14 M • pH + pOH = 14

  33. Example 1: What is the pH of a solution where [H+] = 3.4 x 10-5 M? • pH = -log [H+] • pH = -log(3.4 x 10-5 M) • pH = 4.5

  34. Example 2: What is the pH of a solution where [H+] = 5.4 x 10-6 M? • pH = -log [H+] • pH = -log(5.4 x 10-6) • pH = 5.3

  35. Example 3: What is the [OH-] and pOH for the solution in example #2? • [H3O+][OH-]= 1 x 10-14 • (5.4 x 10-6)[OH-] = 1 x 10-14 • [OH-] = 1.9 x 10-9 M • pH + pOH = 14 • pOH = 14 – 5.3 = 8.7

  36. Example #4 • Classify each solution as acidic, basic, or neutral ***MUST SOLVE FOR pH and use the pH scale a. [H+] = 6.0 x 10-10 M b. [OH-] = 3.0 x 10-2 M c. [H+] = 2.0 x 10-7 M d. [OH-] = 1.0 x 10-7 M basic basic acidic neutral

  37. Example #5 • Which is the MOST basic from question #4? • B.

  38. Guess the pH of the following • Apple juice • Baking soda • Human blood • Saliva • Lemon juice • Milk of magnesia • Stomach acid • Sea water • Hand soap • coffee

  39. Acids and bases: Titrations • The amount of acid or base in a solution is determined by carrying out a neutralization reaction; • an appropriate acid-base indicator (changes color in specific pH range) must be used to show when the neutralization is completed.

  40. Read a buret volume to 2 decimal places This process is called a titration: the addition of a known amount of solution to determine the volume or concentration of another solution. Buret Solution with Indicator

  41. Textbook page 615 • Figure 19.22 a-c • End point: the point at which the indicator changes color

  42. (show lab in demo form…) 3 Steps to do a titration (pg. 615): • Add a measured amount of an acid of unknown concentration to a flask. • Add an appropriate indicator to the flask • Add measured amounts of a base of known concentration using a buret. Continue until the indicator shows that neutralization has occurred. This is called the end point of the titration

  43. 4 steps to a titration CALC: • 1) balanced equation • 2) calculate the number of moles of acid or base in known solution using concentration and volume • 3) calculate the number of moles in unknown solution used during the titration using the ratio in the balanced equation • 4) determine molarity of unknown solution using moles and volume

  44. Example: • In a titration, 27.4 mL of 0.0154 M Ba(OH)2 is added to a 20.0 mL sample of HCl solution of unknown concentration. What is the molarity and pH of the acid solution? • Equation: (Step 1) Ba(OH)2 + 2 HCl  BaCl2 + 2 H2O  (steps)

  45. Step 2 • Calculate the number of moles of known solution (Ba(OH)2)

  46. Calculate moles of known solution: Mol Ba(OH)2 = 4.22 x 10-4 mol Ba(OH)2

  47. Step 3 • Calculate moles of unknown solution • Use stoichiometry and the balanced equation: Ba(OH)2 + 2 HCl BaCl2 + 2 H2O

  48. Calculate moles of unknown solution: Mol HCl = 4.22 x 10-4 mol Ba(OH)2 x 2 mol HCl= 8.44 x 10-4 mol HCl 1 mol Ba(OH)2 Ba(OH)2 + 2 HCl  BaCl2 + 2 H2O Use coefficients from bal. eq to get molar ratio

  49. Step 4 • Determine molarity and pH

  50. Calculate the M and pH • Molarity = 8.44 x 10-4 mol HCl = 4.22 x 10-2 M HCl 0.0200 L • pH: HCl dissociates into H+ and Cl- ions 4.22 x 10-2 M HCl = 4.22 x 10-2 M H+ = 4.22 x 10-2 M Cl- • pH = -log[4.22 x 10-2 M H+] = 1.375 = 1.38

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