Hydrogen Ions and Acidity

The Ionization of Water and pH

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)

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 pure water.

• [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.

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.

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 more acidic.

• For example: [H+] = 1.0×10-8 is basic.

• For example: [H+] = 1.0×10-11 is more basic.

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 MKw = [H+][OH-] = 1.0×10-14

Kw = [H+][OH-] [OH-] =Kw

[H+]=1.0×10-14

1.0×10-5

[OH-] = 1.0×10-9

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

The pH ConceptTo 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”

•press the “ctrl” button

The pH ConceptTo find pH on the TI-Nspire ...

•press the “10x” button

•enter the value for [H+]

•press the “)” button

•press “ENTER”

•press the “(-)” button

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

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

The pH ConceptTo find [H+] on the TI-83 or TI-84 Plus ...

•press the “2ND” button

•press the “LOG” button

•press the “(-)” button

•enter the pH value

•press the “)” button

•press “ENTER”

The pH ConceptTo find [H+] on the TI-Npire ...

•press the “10x” button

•press the “(-)” button

•enter the pH value

•press the “)” button

•press “ENTER”

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

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

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.

Measuring pH• Indicators may be added to a solution to tell

us the range of pH of the solution.

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).

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).

Measuring pH• Some meters provide quick and accurate

readings of pH.

• They vary from the simple to the very complex.