Question

Identify each of the following as a strong acid, weak acid, strong base, or weak base. For each, write an equation that describes its reaction with water. (a) \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)\) (b) \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)\) (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}(a q)\) (d) \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}(a q)\) (e) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}(a q)\)

Short answer

(a) Strong acid, \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q) + 2\mathrm{H}_{2} \mathrm{O}(l) \rightarrow 2\mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{SO}_{4}^{2-}(a q)\). (b) Strong base, \(\mathrm{Ca}(\mathrm{OH})_{2}(a q) \rightarrow \mathrm{Ca}^{2+}(a q) + 2\mathrm{OH}^{-}(a q)\). (c) Weak base, \(\mathrm{Na}_{2} \mathrm{CO}_{3}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 2\mathrm{Na}^{+}(a q) + \mathrm{HCO}_{3}^{-}(a q) + \mathrm{OH}^{-}(a q)\). (d) Weak acid, \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{HC}_{6} \mathrm{H}_{5} \mathrm{O}_{7}^{2-}(a q)\). (e) Weak base, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+}(a q) + \mathrm{OH}^{-}(a q)\).

Step-by-step solution

Classify the substances

(a) \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)\) is a strong acid. (b) \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)\) is a strong base. (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}(a q)\) is a weak base. (d) \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}(a q)\) is a weak acid. (e) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}(a q)\) is a weak base.

Write equations for strong acids and bases in water

For \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q)\), the reaction is \(\mathrm{H}_{2} \mathrm{SO}_{4}(a q) + 2\mathrm{H}_{2} \mathrm{O}(l) \rightarrow 2\mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{SO}_{4}^{2-}(a q)\). For \(\mathrm{Ca}(\mathrm{OH})_{2}(a q)\), the reaction is \(\mathrm{Ca}(\mathrm{OH})_{2}(a q) \rightarrow \mathrm{Ca}^{2+}(a q) + 2\mathrm{OH}^{-}(a q)\).

Write equations for weak acids and bases in water

For \(\mathrm{Na}_{2} \mathrm{CO}_{3}(a q)\), the reaction is \(\mathrm{Na}_{2} \mathrm{CO}_{3}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 2\mathrm{Na}^{+}(a q) + \mathrm{HCO}_{3}^{-}(a q) + \mathrm{OH}^{-}(a q)\). For \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}(a q)\), the reaction is \(\mathrm{H}_{3} \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{O}_{7}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(a q) + \mathrm{HC}_{6} \mathrm{H}_{5} \mathrm{O}_{7}^{2-}(a q)\). For \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}(a q)\), the reaction is \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}(a q) + \mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+}(a q) + \mathrm{OH}^{-}(a q)\).

Key concepts

Strong Acids

Strong acids are substances that completely dissociate in water, releasing protons, or hydrogen ions (H^+). This means, in solution, all of the acid molecules separate into ions. Think of strong acids as very eager to give up their H^+ ions. Because of this, they are great conductors of electricity.

The most common strong acids include:

• Hydrochloric acid (HCl)
• Sulfuric acid (H_2SO_4)
• Nitric acid (HNO_3)

A well-known example is sulfuric acid (H_2SO_4), which reacts in water like this:

\[H_2SO_4(aq) + 2H_2O(l) \rightarrow 2H_3O^+(aq) + SO_4^{2-}(aq)\]
The H_2SO_4 releases its protons, and the water molecules accept them, forming hydronium ions (H_3O^+). This total dissociation is what defines a strong acid.

Weak Acids

Weak acids don't fully dissociate in water; only a small fraction of the acid molecules release their hydrogen ions (H^+). This partial dissociation means weak acids do not conduct electricity as well as strong acids.

Examples of weak acids include:

• Acetic acid (CH_3COOH)
• Citric acid (H_3C_6H_5O_7)
• Formic acid (HCOOH)

Citric acid, for example, reacts in water as follows:

\[H_3C_6H_5O_7(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + HC_6H_5O_7^{2-}(aq)\]
Notice the equilibrium arrow (\(\rightleftharpoons\)) indicating that the reaction does not go to completion. A relatively small number of H_3C_6H_5O_7 molecules donate a H^+ to water, forming hydronium ions (H_3O^+). In weak acids, there is a coexistence of the acid and its ionized form in solution.

Strong Bases

Strong bases are compounds that completely dissociate in water to release hydroxide ions (OH^-). They are the basic equivalent of strong acids, and they readily accept hydrogen ions in the solution. Strong bases are very effective at conducting electricity because they dissociate fully into charged ions.

Common strong bases include:

• Sodium hydroxide (NaOH)
• Potassium hydroxide (KOH)
• Calcium hydroxide (Ca(OH)_2)

For calcium hydroxide (Ca(OH)_2), the reaction in water is:

\[Ca(OH)_2(aq) \rightarrow Ca^{2+}(aq) + 2OH^-(aq)\]
Here, the Ca(OH)_2 molecules dissociate to release calcium ions (Ca^{2+}) and hydroxide ions (OH^-). These OH^- ions readily combine with H^+ ions to form water. This complete dissociation and availability of OH^- ions are what categorize a base as strong.

Weak Bases

Weak bases partly dissociate in water and only a small portion of the substance reacts to form hydroxide ions (OH^-). Due to this partial dissociation, weak bases are poor conductors of electricity compared to strong bases.

Some examples of weak bases are:

• Ammonia (NH_3)
• Methylamine (CH_3NH_2)
• Aniline (C_6H_5NH_2)

Aniline, a weak base, reacts with water as follows:

\[C_6H_5NH_2(aq) + H_2O(l) \rightleftharpoons C_6H_5NH_3^+(aq) + OH^-(aq)\]
The equilibrium arrow (\(\rightleftharpoons\)) signifies that the substance does not fully react to form OH^- ions. In solution, both aniline and the ions exist in equilibrium. Weak bases tend to balance between the non-ionized and ionized forms.