Hydrogen ions and acidity
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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 H 2 O molecules. However, as a pure liquid, water ionizes into hydrogen ions and hydroxide ions: H 2 O( l ) → H + ( aq ) + OH - ( aq )

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Hydrogen Ions and Acidity

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Hydrogen ions and acidity

Hydrogen Ions and Acidity

  • The Ionization of Water and pH


Hydrogen ions in water

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 water1

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.


  • Hydrogen ions in water2

    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 water3

    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.


    Hydrogen ions in water4

    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


    The ph concept

    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 concept1

    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 concept2

    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 concept3

    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 concept4

    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 concept5

    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

    Measuring pH

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


    Measuring ph1

    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 ph2

    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 ph3

    Measuring pH

    • Some meters provide quick and accurate readings of pH.

    • They vary from the simple to the very complex.


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