Arrhenius Theory of Acids and Bases

• Arrhenius acid — a substance that produces hydrogen ions, ${\text{H}}^{+}\text{(aq)}$, in water (in practice, ${\text{H}}^{+}$ immediately attaches to a water molecule to form the hydronium ion, ${\text{H}}_3{\text{O}}^{+}\text{(aq)}$).
• Arrhenius base — a substance that produces hydroxide ions, ${\text{OH}}^{-}\text{(aq)}$, in water.

Arrhenius theory makes neutralization easy to understand. Mix hydrochloric acid with sodium hydroxide and the beaker contains ${\text{H}}^{+}$, ${\text{Cl}}^{-}$, ${\text{Na}}^{+}$ and ${\text{OH}}^{-}$. The hydrogen and hydroxide ions combine to form water molecules, which are neutral, so the ions responsible for acidic and basic properties both disappear. The ${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$ are spectator ions; if the water is evaporated they are left behind as a salt. So every neutralization of a strong acid by a soluble strong base has the same net ionic equation, whatever the particular acid and base, and every neutralization follows the pattern acid + base → salt + water. (With a weak acid, or an insoluble base such as ${\text{Mg(OH)}}_2$, the un-ionized acid or the solid base also appears in the net ionic equation.) This is why an antacid tablet containing ${\text{Mg(OH)}}_2$ relieves excess stomach acid, and why lime is added to acidic lakes and soils.

Recall the distinction from earlier in this unit. Most bases are ionic hydroxide compounds, so they simply DISSOCIATE, releasing ${\text{OH}}^{-}$ ions that already existed in their solid lattice (like NaOH above). Acids, on the other hand, are usually molecular (covalent) compounds in their pure form — they don't contain ions until they chemically react with water. This reaction is IONIZATION: water pulls the hydrogen off the acid molecule, forming ${\text{H}}_3{\text{O}}^{+}$ and the leftover anion (like HCl above). Some courses loosely describe both processes as "dissociation" in casual speech, but understanding the underlying mechanism — separating existing ions vs. forming new ones through reaction — is what actually matters.

This is one of the most commonly confused ideas in this unit, so it's worth being very precise about it. When acetic acid molecules react with water, the reaction is reversible: ions form, but they also recombine into molecules, and at any moment only about 1% of the molecules in a 0.1 mol/L solution are ionized. A strong acid such as HCl ionizes in one direction only, so virtually no HCl molecules remain. You can see the difference in the lab without any calculation. Compare 0.1 mol/L HCl with 0.1 mol/L acetic acid: the HCl solution makes a conductivity-tester bulb glow brightly while the acetic acid makes it glow dimly, and a strip of magnesium fizzes vigorously in the HCl but only slowly in the acetic acid. Same concentration, very different numbers of ions.

• Strong acid/base — ionizes or dissociates essentially completely (nearly 100%) in water. Common strong acids: $\text{HCl}$, ${\text{HNO}}_3$, ${\text{H}}_2{\text{SO}}_4$. Common strong bases: Group 1 hydroxides (e.g. $\text{NaOH}$, $\text{KOH}$) and some Group 2 hydroxides (e.g. ${\text{Ba(OH)}}_2$).
• Weak acid/base — only partially ionizes or dissociates (a small percentage) in water; most of the substance remains as intact, unreacted molecules. Example: acetic acid, ${\text{CH}}_3\text{COOH}$.
• Concentrated/dilute — describes how MUCH solute is dissolved per litre (a completely separate property from strong/weak).