Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec1_25 August	© R. Paselk 2003
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Introduction

Syllabus

Lecture Schedule

Review of Aqueous Equilibria

Acid-Base Chemistry

Brønsted-Lowry Acid/Base Theory:

• Acids are proton donors.
• Bases are proton acceptors.
• Note that there is no restriction as to solvent, and many substances besides hydroxide ion can contribute basicity.
• Although I will signify protons in water as H⁺, you should realize that naked protons do not exist in water - they are always hydrated. At a minimum we see the hydronium ion, H₃O⁺. But hydronium ion is in fact also generally thought to be hydrated, so you will sometimes see hydrogen ion represented as H₅O₂⁺, H₇O₃⁺, etc.
• A consequence of the Brønsted definition is that all acids and bases are related to one or more conjugate bases or acids. That is, when an acid dissociates to give a proton, it also generates a conjugate base which can react with (accept) a proton in the reverse reaction. For example, in the case of water:

H2O Æ  H+  + OH-           acid       conj. base                        H+ +  OH-  Æ  H2O           base    conj. acid                        H3O+   ¨  H+  +  H2O  Æ  OH- + H+    conj. acid         acid base   conj. base

Strong vs. Weak Acids & Bases

Remember, these terms refer to the degree of dissociation of an acid or base, not its concentration:

• A Strong Acid is 100% dissociated at all concentrations up to 1M. Common strong acids include:
  • Nitric acid (HNO₃)
  • Hydrochloric acid (HCl)
  • Sulfuric acid (H₂SO₄) for the first dissociation only: H₂SO₄ ´ HSO₄^- + H⁺. The second dissociation is weak, that is it hardly dissociates at 1M.
• A Weak Acid is only partly dissociated at 1M. The degree of dissociation varies widely, from a few percent to an infinitesimal degree. Common weak acids include:
  • Acetic acid (HC₂H₃O₂ or CH₃CO₂H, etc.)
  • Formic acid (HCO₂H)
  • Hydrofluoric acid (HF)
  • Most acids of biological origin such as amino acids, fatty acids, metabolites, nucleic acids etc.
• A Strong Base is 100% dissociated at all concentrations up to 1M. Common strong bases include:
  • Sodium hydroxide (NaOH)
  • Potassium hydroxide (KOH)
• A Weak Base only partly reacts at 1M. The degree of dissociation varies widely, from a few percent to an infinitesimal degree. Common weak bases include:
  • Ammonia (NH₃)
  • Aluminum hydroxide (Al(OH)₃)
  • Magnesium hydroxide (Mg(OH)₂)

© R A Paselk

Last modified 25 August 2003