Hydrogen Ions and Acidity

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

2. 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 hydrogen ions and hydroxide ions: • H2O(l) → H+(aq) + OH-(aq) • H2O(l) + H2O(l) → H3O+(aq) + OH-(aq)

3. 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.

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

5. Hydrogen Ions in Water • The higher the value of [H+] and lower the value of [OH-], the more acidic the solution. • The lower the value of [H+] and higher the value of [OH-], the more basic the 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.

6. 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 1.0×10-14 Kw = [H+][OH-]  [OH-] = = 1.0×10-5 [H+] [OH-] = 1.0×10-9

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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.

13. Measuring pH • Indicators may be added to a solution to tell us the range of pH of the solution.

14. 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 range of 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).

15. 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 range of 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).

16. Measuring pH • Some meters provide quick and accurate readings of pH. • They vary from the simple to the very complex.