Characteristics of Acids and Bases

Key definitions

• Bases are oxides amd hydroxides of metals. They neutralize acids.
  • Examples: $CuO$ (copper oxide) and $NaOH$ (sodium hydroxide)
• Alkalis are soluble bases. They can dissolve in water to release hydroxide ions.
• Salts are ionic compounds formed when the hydrogen ions, $H^+$ in an acid are completely or partially replaced by a metal ion or an ammonium ion, $NH_4^+$.

The effect of acids and alkalis on litmus, thymolphthalein, and methyl orange

Chemical reactions of bases

1. base + acid $\rightarrow$ salt + water
2. base (alkali) + ammonium salt $\rightarrow salt + water + ammonia gas

Ions, pH, and universal indicator

Ions

• Aqueous solutions of acids contain hydrogen ($H^+$) ions.
• Aqueous solutions of alkalis contain hydroxide ($OH^-$) ions.

pH and universal indicator

• Universal indicator changes color gradually depending on the concentration of H+ ions in a solution:
  • pH 0 to 6 (acidic): A high H+ conc. corresponds to lower pH numbers. For universal indicators, acidic solutions turn deep red/orange, meanwhile weak acids turn yellow/light green.
  • pH 7 (neutral): Equal conc. of H+ and OH- ions. Universal indicator turns green.
  • pH 8 to 14 (alkaline): Higher OH- conc. corresponds to higher pH numbers. Turns blue/deep purple.

Ionic equation for neutralization

• The equation for the neutralization reaction between an acid and an alkali to produce water: $$\text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{H}_2\text{O(l)}$$

Proton transfer

• Acids are proton donors.
• Bases are proton acceptors.

Acid strength vs. solution concentration

• Strong/weak and concentrated/dilute describe completely different properties.
  • Strength refers strictly to the percentage of acid molecules that split into ions when dissolved into water.
• A strong acid is an acid that is completely dissociated in aqueous solution.
  • Every single acid molecule breaks apart to release H+ ions.
  • An example is hydrochloric acid. $\text{HCl(aq)} \rightarrow \text{H}^+\text{(aq)} + \text{Cl}^-\text{(aq)}$
• A weak acid is an acid that is partially dissociated in aqueous solution.
  • Only a tiny fraction of the molecules split into ions. The rest remain intact as whole molecules.
  • An example is ethanoic acid. $\text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)}$