Aqueous Chemistry: Acids, Bases, and pH

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Aqueous ChemistryAcids, Bases, and pH
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Dissociation of Water
H3O hydronium ion
OH hydroxide ion
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Dissociation of Water
Dissociation constant
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Brønsted-Lowry Definition
An Acid can donate protons to another molecule
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Brønsted-Lowry Definition
A Base can accept protons from another molecule
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Strong acids dissociate completely
Example Hydrochloric acid
In 1 liter of pure water
Add 0.1 moles of HCl
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Strong acids dissociate completely
Example Hydrochloric acid
Add 10-8 moles of HCl to 1 liter of water
NO!!!
In 1 liter of pure water
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Weak acids partially dissociate
Example Acetic acid
What is the pH of a 0.1 M acetic acid solution?
Simplifications
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Bases bind to protons
Example Hydroxide
Add 0.1 moles sodium hydroxide to 1 liter of water
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Comparing pKa Values
pKa 4.5
pKa 10.5
Overall K for the reaction is
Therefore
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Summary

• Water dissociates into hydronium ions and
  hydroxide ions.
• The negative log of the hydronium ion
  concentration is pH, a measure of acidity.
• The negative log of the hydroxide ion
  concentration is pOH.
• pOH pH 14 for all aqueous solutions.

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Summary

• Adding an acid to a solution always lowers the
  pH.
• Strong acids release nearly all of their protons
  to form hydronium ions.
• Weak acids release relatively few protons to
  water, although addition of a base can remove
  more protons.
• Adding a base to a solution always raises the pH.