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Acidity and pH

Acidity and pH. Review. Acids are substances that: taste sour. react w/ bases to form salts and water. are electrolytes. turn blue litmus red. produce H 3 O +1 ions in aqueous solution. donate H +1 ions (protons). Review. Bases are substances that: taste bitter.

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Acidity and pH

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  1. Acidity and pH

  2. Review • Acids are substances that: • taste sour. • react w/ bases to form salts and water. • are electrolytes. • turn blue litmus red. • produce H3O+1 ions in aqueous solution. • donate H+1 ions (protons).

  3. Review • Bases are substances that: • taste bitter. • react w/ acids to form salts and water. • are electrolytes. • turn red litmus blue. • produce OH-1 ions in aqueous solutions. • accept H+1 ions (protons).

  4. Autoionization • H2O + H2O  H3O+1 + OH-1 • Extremely small Keq. • Keq = Kw = 1.0 x 10-14 • Only 0.00001% of H2O molecules autoionize. • In pure water, [H3O+1] = 1.0 x 10-7 Molar. • [OH-1] also equals 1.0 x 10-7 M. • Less than 2 H3O+1 and OH-1 ions per billion water molecules. • Since [H3O+1] = [OH-1], water is pH neutral. • Adding an acid or a base upsets this balance.

  5. Autoionization H O H O H H - +

  6. Adding an Acid to Water HA HA HA HA HA H2O H2O H2O A-1 H3O+1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H3O+1 H2O H2O H3O+1 A-1 H2O A-1 H3O+1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O A-1 H2O H3O+1 H2O H2O H2O A-1 OH-1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O

  7. Adding a Base to Water A-1 A-1 A-1 A-1 A-1 H2O H2O H2O H2O HA OH-1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O OH-1 H2O OH-1 H2O HA H2O HA OH-1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O OH-1 HA H2O H2O H2O HA H3O+1 H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O

  8. 1x10-1 M 1x10-1 M 1x10-2 M 1x10-2 M 1x10-3 M 1x10-3 M Neutral Solution 1x10-4 M 1x10-4 M 1x10-5 M 1x10-5 M 1x10-6 M 1x10-6 M Acid added to neutral solution 1x10-7 M 1x10-7 M 1x10-8 M 1x10-8 M Base added to neutral solution 1x10-9 M 1x10-9 M 1x10-10 M 1x10-10 M 1x10-11 M 1x10-11 M 1x10-12 M 1x10-12 M 1x10-13 M 1x10-13 M As [H3O+1] Increases, [OH-1] Decreases [OH-1] [H3O+1]

  9. [H3O+1] and [OH-1] in Water • In any aqueous solution: • [H3O+1] [OH-1] = 1x10-14 • As [H3O+1] goes up, [OH-1] must decrease. • As [OH-1] goes up, [H3O+1] must decrease. • In other words, adding an acid to water causes the solution to become more acidic and less basic. • Adding a base to water causes the solution to become less acidic and more basic.

  10. [H3O+1] and [OH-1] in Water • If [H3O+1] = 1x10-3 M, what is [OH-1]? • [H3O+1][OH-1] = 1x10-14 • (1x10-3 M)[OH-1] = 1x10-14 • [OH-1] = (1x10-14) / (1x10-3) • [OH-1] = 1x10-11 M • If [OH-1] = 1x10-8 M, what is [H3O+1]? • [H3O+1][OH-1] = 1x10-14 • [H3O+1](1x10-8 M) = 1x10-14 • [H3O+1] = (1x10-14) / (1x10-8 M) • [H3O+1] = 1x10-6 M

  11. Acidity • Acidity • How much H3O+1 is dissolved in a sol’n. • Remember, acids increase [H3O+1] in solutions. • Acidity = pH.

  12. pH • pH = power of Hydrogen • negative logarithmic (powers of ten) scale. • pH = -log10[H3O+1] • If [H3O+1] = 1x10-1 M, • pH = -log(1x10-1 M) = 1 • If [H3O+1] = 1x10-2 M, • pH = -log(1x10-2 M) = 2 • If [H3O+1] = 1x10-3 M, • pH = -log(1x10-3 M) = 3

  13. A Few Words About Logarithms • The logarithm of a number is the power to which you would have to raise a base to equal that number. • Unless otherwise indicated, assume the base is 10. • log(100) = 2 • because 102 = 100 • log(1000) = 3 • because 103 = 1000 • log(0.001) = -3 • because 10-3 = 0.001 • log(0.000 001) = -6 • because 10-6 = 0.000 001

  14. Why Use Logarithms At All? • The [H3O+1] and [OH-1] of an aqueous solution can vary by a very large degree. • [H3O+1] = 1 M for a very acidic soln • [H3O+1] = 1x10-7 M for a neutral soln • [H3O+1] = 1x10-14 M for a very basic soln • 1 M is ten million times greater than 1x10-7 M. • If you tried to plot both concentrations on the same graph, 1x10-7 M would barely register above zero. • If 1x10-7 M was 1 mm above the 0 mark, the axis would have to be ten kilometers (six miles) tall to show 1 M. • Logarithms allow us to compare numbers that are widely different by thinking of them as powers of ten.

  15. This graph shows pH as a function of hydrogen ion concentration. It isn’t a very useful graph because it is hard to get accurate information for [H3O+1] below 1x10-3 M.

  16. In this graph the x-axis is logarithmic. It allows a much greater range of data to be displayed in a readable format.

  17. pH • Each unit decrease in pH is a 10-fold increase in acidity. • Imagine a soln with a pH of 5. • A soln with a pH of: • 4 is 10 times more acidic. • 3 is 100 times more acidic. • 2 is 1000 times more acidic. • 1 is 10,000 times more acidic. • 0 is 100,000 times more acidic.

  18. Where does the pH scale come from? pH scale -1…0 1 2 3 4 5 6 7 8 9 10 11 12 13 14…15 Acidic Basic [H3O+] 1 0.1 0.01 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 A lot of H3O+ Not a lot of H3O+ Acidic Basic

  19. pOH Scale The pOH scale indicates the hydroxide ion concentration, [OH-] or molarity, of a solution. (In other words how many OH- ions are in the solution. If there are a lot we assume it is a base, if there are very few it is an acid.) Two chemists meet for the first time at a symposium. One is American, one is British. The British chemist asks the American chemist, "So what do you do for research?" The American responds, "Oh, I work with aerosols." The British chemist responds, "Yes, sometimes my colleagues get on my nerves also."

  20. pOH scale -1…0 1 2 3 4 5 6 7 8 9 10 11 12 13 14…15 Basic Acidic [OH-] 1 0.1 0.01 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14 A lot of OH- Not a lot of OH- Basic Acidic

  21. Example: • Lemon juice (citric acid) pH = 2, pOH = 12 • Pure water pH = 7, pOH = 7 • Milk of magnesia pH = 10, pOH = 4 • The last words of a chemist: • And now for the taste test. • 2. I wonder if this is hot? • 3. And now a little bit from this... • 4. And now shake it a bit.

  22. pH Scale

  23. pH Scale

  24. pH Scale

  25. pH Scale

  26. pH Scale

  27. [H3O+1] and [OH-1] [H3O+1] [OH-1]

  28. 4. Calculations Involving pH, pOH, [H3O+], and [OH-] of strong Acids and Bases 1st: determine which ion will be produced, either OH or H3O+ (Acids produce H3O+ and bases produce OH-). 2nd: use formula to determine pH or pOH. 3rd: check to see if answer is reasonable. pH = -log [H3O+] pOH = -log [OH-] pOH + pH = 14

  29. pH and pOH • What are the pH values of the following solutions? • 1x10-1 M H3O+1 • pH = -log(1x10-1 M) = 1 • 1x10-3 M H3O+1 • pH = -log(1x10-3 M) = 3 • 1x10-5 M H3O+1 • pH = -log(1x10-5 M) = 5 • 1x10-1 M OH-1 • [H3O+1] = (1x10-14) / (1x10-1 M) = 1x10-13 M • pH = -log(1x10-13 M) = 13

  30. [H3O+] and [OH-] • Given pH, you can calculate [H3O+1] and [OH-1]. • [H3O+1] = 10-pH • [H3O+1] [OH-1] = 1x10-14 • If pH = 2, • [H3O+1] = 1x10-2 M • [OH-1] = 1x10-12 M

  31. Titrations • Determining the pH of an unknown solution using the pH of a known solution • Titrations take a very long time and you have to have excellent lab technique • You add small amounts of known solution until a pre-determined endpoint is reached

  32. #H+a Ma Va = #OH-b Mb Vb • #H+ in acid formula • M= Molarity • V= Volume used to neutralize • #OH- in base formula

  33. Example You have 50 drops CH3COOH & it takes 5 drops 5M NaOH reach the endpoint. What is the molarity of the acetic acid? • (1 H+)(Ma)(50 drops)=(1 OH-)(5 M)(5 drops) • Ma = 0.5 M

  34. Example 2 • What is the molarity of sulfuric acid if it takes 12 mL of H2SO4 to neutralize 30 mL of 5 M NaOH. • (2 H+)(Ma)(12 drops)=(1 OH-)(5 M)(30 drops) • 6 M

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