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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:. -Occurs in 2 per 1 billion molecules in PURE (deionized or distilled) water. 2H 2 O    H 3 O + + OH -. Equilibrium constant of water.

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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: -Occurs in 2 per 1 billion molecules in PURE (deionized or distilled) water

  3. 2H2O    H3O+ + OH- Equilibrium constant of water Kw = [OH-][H3O+] (will be given) Kw = 1 x 10-14 [ ] = 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) 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 Neutral Basic

  6. pOH = - log [OH-] (given) 14 = pH + pOH (given) 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. Summary of equations:4 possible to use pH = -log [H3O+] pOH = -log [OH-] pH + pOH = 14 1 x 10-14 = [H3O+] [OH-]

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

  8. Examples: 2. 0.3 M HNO3 Find: pH, pOH, [OH-], Acid or Base?

  9. Example: 3. 0.20 M NaOH 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 Base Acid + Indicator (+ Extra Water)

  15. Titration Calculations Examples: NaOH +HCl NaCl + H2O If I titrate 25mL of 1.4M HCl with 35mL of NaOH…what is concetration (M) of NaOH?

  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

  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 Conjugate acid of a weak base. WHY?? NH3 + HCl -> NH4+1 + Cl-1 NH4+1 + H2O NH3 + H3O+1 Makes the solution a little ACIDIC

  25. Weak Base + Weak Acid Depends on which WEAK is stronger!

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

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