1. Chapter 16 More Work with Acids and Bases

2. 16.2 - pH Self-Ionization of Water: 2H 2 O H 3 O + + OH - -Occurs in 2 per 1 billion molecules in PURE (deionized or distilled) water

3. Equilibrium constant of water 2H 2 O H 3 O + + OH - K w = [OH - ][H 3 O + ] (will be given) K w = 1 x 10 -14 Neutral (PURE) water: [OH-] = [H 3 O+] = 1 x 10 -7 M [ ] = Concentration in Molarity (mol solute / L solution

4. Non-neutral water (water that has an acid or base added to it) If [OH-] > [H 3 O+] If [H 3 O+] >[OH-] Solution is ACIDIC Solution is BASIC

5. pH Scale pH = - log [H 3 O + ] (will be given) Typical scale is 1 – 14, but can be outside of this pH < 7 pH = 7 pH >7 Acidic Neutral Basic 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

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 [H 3 O + ] pOH = -log [OH - ] pH + pOH = 14 1 x 10 -14 = [H3O + ] [OH - ]

7. Examples: 1. [H+] = 2.3 x 10 -8 M Find: pH, pOH, [OH-], Acid or Base?

8. Examples: 2. 0.3 M HNO 3 Find: pH, pOH, [OH-], Acid or Base?

9. Example: 3. 0.20 M NaOH Find: pH, pOH, [H 3 O+], 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. Indicators Indicator pH Range AcidBase Thymol Blue 1.2-2.8redyellow Methyl yellow 2.9-4.0redyellow Methyl orange 3.1-4.4redorange Bromphenol blue 3.0-4.6yellowblue-violet Bromcresol green 4.0-5.6yellowblue Methyl red 4.4-6.2redyellow Bromcresol purple 5.2-6.8yellowpurple Bromphenol blue 6.2-7.6yellowblue Phenol red 6.4-8.0yellowred Cresol red 7.2-8.8yellowred Thymol blue 8.0-9.6yellowblue Phenolphthalein8.0-10.0colorlessred Thymolphthalein9.4-10.6colorlessblue Color of indicator when BELOW LOWER pH Range value Color of indicator when ABOVE HIGHER pH Range value Example: Thymol Blue: When pH less than 1.2 When pH greater than 2.8 When pH between 1.2 – 2.8 Red Yellow Orange

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 + H 2 O 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. Buret with Base pH meter Acid (Indicator optional) pH Titration Apparatus:

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

20. Why is the pH = 7.0?? Example: HCl + NaOH -> H 2 O + NaCl Strong Acid + Strong Base Strong Acid Strong Base Salt Water = NEUTRAL = pH = 7.0

21. Weak Acid + Strong Base

22. -The pH at the Equivalence Point is >7.0 (BASIC) -Why?? HCH 3 COO + NaOH -> H 2 O + NaCH 3 COO NaCH 3 COO -> Na + + CH 3 COO - CH 3 COO - + H 2 O HCH 3 COO + 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?? NH 3 + HCl -> NH 4 +1 + Cl -1 NH 4 +1 + H 2 O NH 3 + H 3 O +1 Makes the solution a little ACIDIC Conjugate acid of a weak base.

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

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