Acid/Base Definitions and Concepts

Acid/Base Definitions

  • Arrhenius (Traditional):

    • Acids produce H+H^+ ions (or hydronium ions H3O+H_3O^+) when dissolved in water.

    • Bases produce OH−OH^- ions when dissolved in water.

    • Problem: Some bases don’t have hydroxide.

Arrhenius Acids

  • Monoprotic acid: Contains 1 ionizable H

    • HNO<em>3→H++NO</em>3−HNO<em>3 \rightarrow H^{+} + NO</em>3^{-}

    • HCl→H++Cl−HCl \rightarrow H^+ + Cl^{-}

    • HC<em>2H</em>3O<em>2→H++C</em>2H<em>3O</em>2−HC<em>2H</em>3O<em>2 \rightarrow H^{+} + C</em>2H<em>3O</em>2^{-}

  • Diprotic acid: Contains 2 ionizable Hs

    • H<em>2SO</em>4→2H++SO42−H<em>2SO</em>4 \rightarrow 2H^+ + SO_4^{2-}

    • H<em>2SO</em>4→H<em>2++SO</em>4−H<em>2SO</em>4 \rightarrow H<em>{2}^{+} + SO</em>4^{-}

  • Triprotic acid: Contains 3 ionizable Hs

    • H<em>3PO</em>4→H<em>3++PO</em>4−H<em>3PO</em>4 \rightarrow H<em>{3}^{+} + PO</em>4^{-}

Arrhenius Bases

  • Two common ways to create bases:

    • Elements in Group 1A react with water to make bases.

      • Ex: 2Na+2HOH→2NaOH+H22Na + 2HOH \rightarrow 2NaOH + H_2

      • Soluble in water and thus very corrosive!

    • Elements in Group 2A also react with water to make bases.

      • Ex: Ca+2HOH→Ca(OH)<em>2+H</em>2Ca + 2HOH \rightarrow Ca(OH)<em>2 + H</em>2

      • Not very soluble in water, thus not as corrosive.

Brønsted – Lowry Definition

  • Brønsted – Lowry (Most Commonly Used):

    • Acids are proton donors.

    • Bases are proton acceptors.

    • A “proton” is really just a hydrogen atom that has lost its electron (H+H^+).

Brønsted-Lowry Acids and Bases

  • A Brønsted-Lowry acid is a proton donor.

  • A Brønsted-Lowry base is a proton acceptor.

  • acid + base ⇌\rightleftharpoons conjugate base + conjugate acid

Conjugate Pairs

  • HCO<em>3(aq)+H</em>2O(l)⇌H<em>3O+(aq)+CO</em>32−(aq)HCO<em>3(aq) + H</em>2O(l) \rightleftharpoons H<em>3O^+(aq) + CO</em>3^{2-}(aq)

    • HCO<em>3(aq)HCO<em>3(aq) acts as an acid and CO</em>32−(aq)CO</em>3^{2-}(aq) is its conjugate base.

    • H<em>2O(l)H<em>2O(l) acts as a base and H</em>3O+(aq)H</em>3O^+(aq) is its conjugate acid.

Amphoteric Substances

  • When a substance can act like either an acid or a base, it is called amphoteric.

Learning Check

  • Label the acid, base, conjugate acid, and conjugate base in each reaction:

    • HCl+OH−→Cl−+H2OHCl + OH^- \rightarrow Cl^- + H_2O

    • H<em>2O+H</em>2SO<em>4→HSO</em>4−+H3O+H<em>2O + H</em>2SO<em>4 \rightarrow HSO</em>4^- + H_3O^+

Lewis Definition

  • Lewis (Rarely Used):

    • Lewis acid: a substance that accepts an electron pair.

    • Lewis base: a substance that donates an electron pair.