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Chapter 16

Chapter 16. More Work with Acids and Bases. 2H 2 O    H 3 O + + OH -. 16.2 - pH. Self-Ionization of Water:. H 2 O + H 2 O    H 3 O + + OH -. -Occurs in 2 per 1 billion molecules in PURE (deionized or distilled) water. 2H 2 O    H 3 O + + OH -.

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Chapter 16

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  1. Chapter 16 More Work with Acids and Bases

  2. 2H2O    H3O+ + OH- 16.2 - pH Self-Ionization of Water: H2O + H2O    H3O+ + OH- -Occurs in 2 per 1 billion molecules in PURE (deionized or distilled) water

  3. 2H2O    H3O+ + OH- Equilibrium constant of water Kw = 1 x 10-14 = [OH-][H3O+] (will be given to Academic) [ ] = Concentration in Molarity (mol solute / L solution) Neutral (PURE) water: [OH-] = [H3O+] = 1 x 10 -7 M

  4. Non-neutral water (water that has an acid or base added to it) If [H3O+] >[OH-] Solution is ACIDIC If [OH-] > [H3O+] Solution is BASIC

  5. pH Scale pH = - log [H3O+] (will be given to Academic) Typical scale is 1 – 14, but can be outside of this Practice Problem:If [H3O+] = 2.4 x 10-5 M -We know solution is ACIDIC because [H3O+] >1 x 10-7 M pH = - log (2.4 x 10-5 M) = 4.61 pH < 7 pH > 7 pH =7 Acidic Basic Neutral

  6. pOH = - log [OH-] (given to Academic) 14 = pH + pOH (given to Academic) Be careful of diprotic acids and bases with more than one hydroxide! You need to calculate the MOLES of the ION, not the entire compound before you calculate pH or pOH.

  7. Examples: • [H+] = 2.3 x 10-8 M Find: pH, pOH, [OH-], Acid or Base?

  8. Examples: 2. 0.3 M H2SO4(I admit, the work we’re doing is SIMPLIFIED!) Find: pH, pOH, [OH-], Acid or Base?

  9. Example: 3. 0.20 M Ba(OH)2 Find: pH, pOH, [H3O+], Acid or Base?

  10. Titrations – Combining an Acid + Base Indicator – Chemical that changes color based on pH Picking the correct indicator – Pick the indicator that changes color at the correct pH

  11. Color of indicator when BELOW LOWER pH Range value Color of indicator when ABOVE HIGHER pH Range value Indicators Red Yellow Orange Example: Thymol Blue: When pH less than 1.2 When pH greater than 2.8 When pH between 1.2 – 2.8

  12. Phenolphthalein Commonly used indicator In Acid = Colorless In Base = Pink

  13. Endpoint vs. Equivalence Point Endpoint = The point at which the COLOR change happens. Equivalence Point = The point at which the MOLE Acid to MOLE Base ratio balances out. *We want the ENDPOINT to be as close to the EQUIVALENCE POINT as possible. (We’ll talk more about this soon!)

  14. Titration Calculations Examples: Base Acid + Indicator (+ Extra Water)

  15. Titration Calculations Examples:

  16. pH Meter = Instrument that measure pH numerically. -Can be used to see change in pH throughout the titration.

  17. Titration Curves A graph of pH versus volume of base added -Monitors pH over the entire titration **You will see a sharp increase in pH at the equivalence point**

  18. pH Titration Apparatus: Buret with Base pH meter Acid (Indicator optional)

  19. Strong Acid + Strong Base The Equivalence Point occurs at pH = 7.0 Start with a LOW pH – measuring ACID in beaker

  20. Strong Acid + Strong Base Why is the pH = 7.0?? Example: HCl + NaOH -> H2O + NaCl Strong Base Strong Acid Salt Water = NEUTRAL = pH = 7.0

  21. Weak Acid + Strong Base On simulator: Change [NaOH] to 0.5 M The pH at the equivalence point is GREATER THAN 7

  22. Weak Acid + Strong Base -The pH at the Equivalence Point is >7.0 (BASIC) -Why?? HCH3COO + NaOH -> H2O + NaCH3COO NaCH3COO -> Na+ + CH3COO- CH3COO- +H2O HCH3COO + OH- This is the CONJUGATE BASE OF A WEAK ACID This makes the solution at the Equivalence Point BASIC

  23. Weak Base + Strong Acid pH at Equivalence Point is < 7.0 (ACIDIC)

  24. Weak Base + Strong Acid WHY?? NH3 + HCl -> NH4+1 + Cl-1 NH4+1 + H2O NH3 + H3O+1 Makes the solution a little ACIDIC Weak Base + Weak Acid Depends on which WEAK is stronger!

  25. Titration Summary http://www.ausetute.com.au/titrcurv.html

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