Hydrogen Ions and Acidity

1. Hydrogen Ions and Acidity The Ionization of Water and pH

2. Objectives • When you complete this presentation, you will be able to: • describe the autoionization of water • determine the hydrogen ion concentration in water when given the hydroxide ion concentration in water • determine the hydroxide ion concentration in water when given the hydrogen ion concentration in water • determine pH or pOH of a solution when given the hydrogen ion or hydroxide ion concentration • determine the hydrogen ion or hydroxide ion concentration when given the pH or pOH

3. Hydrogen Ions in Water • We are used to thinking of water as a pure liquid that contains only H2O molecules. • However, as a pure liquid, water ionizes into hydrogenions and hydroxide ions: • H2O(l) → H+(aq) + OH-(aq) • H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)

4. Hydrogen Ions in Water • The symbol for the concentration of hydrogen ions is [H+] and for hydroxide ions is [OH-]. • We can measure the concentration of the ions in purewater. • [H+] = 1.0×10-7 M • [OH-] = 1.0×10-7 M • By stoichiometry, H2O(l) → H+(aq) + OH-(aq), we see that [H+] = [OH-] in pure water.

5. Hydrogen Ions in Water • We also know that the product of the concentrations is a constant, Kw. • Kw = [H+][OH-] = 1.0×10-14 • Thismeansthat as we change [H+], we change [OH-]. • If [H+] goesup, [OH-]goes down. • If [H+] goes down, [OH-]goes up.

6. Hydrogen Ions in Water • The higher the value of [H+] and lower the value of [OH-], the more acidicthe solution. • The lower the value of [H+] and higher the value of [OH-], the more basicthe solution. • For example: [H+] = 1.0×10-6 is acidic. • For example: [H+] = 1.0×10-3 is moreacidic. • For example: [H+] = 1.0×10-8 is basic. • For example: [H+] = 1.0×10-11 is morebasic.

7. Hydrogen Ions in Water Example 1: Colas are slightly acidic. If a cola solution has [H+] = 1.0×10-5 M, what is the [OH-]? [H+] = 1.0×10-5 M Kw = [H+][OH-] = 1.0×10-14 Kw = [H+][OH-] 1.0×10-14 Kw [OH-] = = 1.0×10-5 [H+] [OH-] = 1.0×10-9 M

8. The pH Concept • pH is another way to measure [H+] and/or [OH-]. • pH = -log[H+] • pOH = -log[OH-] • If [H+] = 1.0×10-7 • then pH = -log(1.0×10-7) = 7.00 • If [OH-] = 1.0×10-7 • then pOH = -log(1.0×10-7) = 7.00

9. The pH Concept • To find pH on the TI-83 or TI-84 Plus … • press the “(-)” button • press the “LOG” button • enter the value for [H+] • press the “)” button • press “ENTER”

10. The pH Concept • To find pH on the TI-Nspire … • press the “(-)” button • press the “ctrl” button • press the “10x” button • enter the value for [H+] • press the “)” button • press “ENTER” • NOTE: The position of these buttons on your calculator may change with faceplate.

11. The pH Concept • If [H+] = 1.0×10-5 • then pH = -log(1.0×10-5) = 5.00 • If [OH-] = 1.0×10-8 • then pOH = -log(1.0×10-8) = 8.00 • If [H+] = 3.1×10-2 • then pH = -log(3.1×10-2) = 1.51 • If [OH-] = 4.5×10-11 • then pOH = -log(4.5×10-11) = 10.35

12. The pH Concept Find the pH of a solution that has • [H+] = 1.0×10-4 • [H+] = 1.0×10-9 • [H+] = 2.4×10-5 • [H+] = 6.6×10-8 • [H+] = 4.8×10-3 pH = 4.00 pH = 9.00 pH = 4.62 pH = 7.18 pH = 2.32

13. The pH Concept Find the pOH of a solution that has • [OH-] = 1.0×10-10 • [OH-] = 1.0×10-3 • [OH-] = 3.8×10-5 • [OH-] = 8.1×10-8 • [OH-] = 2.8×10-3 pOH = 10.00 pOH = 3.00 pOH = 4.42 pOH = 7.09 pOH = 2.55

14. The pH Concept • We can also convert from pH and pOH to [H+] and [OH-] respectively. • [H+] = 10-pH • [OH-] = 10-pOH • If pH = 7.00 • then [H+] = 10-7.00 = 1.0×10-7 • If pOH = 3.25 • then [OH-] = 10-3.25 = 5.6×10-4

15. The pH Concept • To find [H+] on the TI-83 or TI-84 Plus … • press the “2ND” button • press the “LOG” button • press the “(-)” button • enter the value for pH • press the “)” button • press “ENTER”

16. The pH Concept • To find pH on the TI-Nspire … • press the “10x” button • press the “(-)” button • enter the value for pH • press the “)” button • press “ENTER” • NOTE: The position of these buttons on your calculator may change with faceplate.

17. The pH Concept • If pH = 2.556 • then [H+] = 10-2.556 = 2.78×10-3 • If pOH = 9.27 • then [OH-] = 10-9.27 = 5.4×10-10 • If pH = 12.12 • then [H+] = 10-12.12 = 7.6×10-13 • If pOH = 6.678 • then [OH-] = 10-6.678 = 2.10×10-7

18. The pH Concept Find the [H+] of a solution with a • pH = 6.00 • pH = 13.00 • pH = 5.25 • pH = 9.42 • pH = 2.32 [H+] = 1.0×10-6 [H+] = 1.0×10-13 [H+] = 5.6×10-6 [H+] = 3.8×10-10 [H+] = 4.8×10-3

19. The pH Concept Find the [OH-] of a solution with a • pOH= 4.00 • pOH= 11.00 • pH = 3.79 • pH = 10.46 • pH = 3.77 [OH-] = 1.0×10-4 [OH-] = 1.0×10-11 [OH-] = 1.6×10-4 [OH-] = 3.5×10-11 [OH-] = 1.7×10-4

20. The pH Concept • pH + pOH = 14 • if pH = 2.0, • then pOH = 14.0 - 2.0 = 12.0 • if pH = 3.250, • then pOH = 14.000 - 3.250 = 10.750 • if pOH = 3.0, • then pH = 14.0 - 3.0 = 11.0 • if pOH = 7.451, • then pH = 14.000 - 7.451 = 6.549

21. The pH Concept • Solutions with pH = 7 (pOH = 7) are neutral. • Litmus paper remains colorless. • Solutions with pH < 7 (pOH > 7) are acidic. • Litmus paper turns red. • Solutions with pH > 7 (pOH < 7) are basic. • Litmus paper turns blue.

22. Measuring pH • Indicatorsmay be added to a solution to tell us the range of pH of the solution.

23. Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular rangeof pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7 ... we might use Bromthymol Blue (which changes color at about pH = 7).

24. Measuring pH Indicators are most often used where pH may change, such as in a titration. But they are also used to tell us where we are in a particular rangeof pH. For example, if we wanted to tell if a solution has a pH greater than or less than 7 ... we would not want to use Methyl Red(which changes color near pH = 5).

25. Measuring pH • Some metersprovide quick and accurate readings of pH. • They vary from the simple to the very complex.