CHEM 122 - ACIDS BASES

CHEM 122 - ACIDS BASES - ACID BASE CONCEPTS 1. Arrhenius...

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ACID BASE CONCEPTS 1. Arrhenius Concept of Acids and Bases 2. Brønsted-Lowry Concept of Acids and Bases 3. Lewis Concept of Acids and Bases These are ways that we categorize a substance as an acid or a base. ACID-BASE STRENGTHS 4. Relative strengths of acids and bases 5. Molecular structure and acid strength SELF-IONIZATION OF WATER/pH 6. Self-Ionization of Water 7. Solutions of Strong Acids or Bases 8. The pH of a Solution The word Acid comes from the Latin word Acidus , which means sour. The Roman word for vinegar, which is ethanoic acid and has the structure Was acetum . We still refer to ethanoic acid as acetic acid. The word base comes from an Old English word meaning “lowered” or “lowering”. The word “debase” means “to lower in quality and character.” Acids have a general property in that they taste sour, bases have a general quality that they taste bitter. Bases are also slippery.
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ARRHENIUS CONCEPT OF ACIDS (By Svante Arrhenius (1880s)) The first general definition of acids and bases was given by Arrhenius. An acid is a substance that when dissolved in water increases the concentration of the hydronium ion (H 3 O + ). A hydronium ion is a water molecule that has gained an extra proton. Protons are always bound to something. They are never freely floating around separately. A base is a substance that when dissolved in water increases the concentration of the hydroxide ion (OH - ). The Arrhenius definition was the first to be universally accepted. A drawback is that it limits bases to compounds that contain the hydroxide ion. Some substances were found to have basic properties, but that did not contain a hydroxide ion. HCl(g) + H 2 O(l) H 3 O + (aq) + Cl - (aq) This is a base, not defined by Arrhenius. NaOH(aq) + H 2 O(l) OH - (aq) + Na + (aq) Sodium Hydroxide is an Arrhenius Base.
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BRØNSTED-LOWRY ACIDS AND BASES (1923) An acid is a species that donates a proton to a base. A base is a species that accepts a proton from an acid. A Brønsted acid is a substance capable of donating a proton, and a Brønsted base is a substance capable of accepting a proton. So now we are defining acids and bases by means of a proton transfer. PROTON TRANSFER REACTIONS Consider the reaction: H 3 O + + NH 3 H 2 O + NH 4 + In the forward reaction, hydronium is donating a proton to NH 3 . In the reverse reaction, NH 4 + is donating a proton to water. The above example is of a reversible proton-transfer reaction. There are irreversible proton- transfers as well. Within this definition, if there is an acid, there must be a base. If there is a base, there must be an acid. Brønsted-Lowry Acid-Base reactions are proton-transfer reactions. Substances in this kind of acid-base reaction that differ by the gain or loss of a proton are called a conjugate acid-base pair . A
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CHEM 122 - ACIDS BASES - ACID BASE CONCEPTS 1. Arrhenius...

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