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) \(\operatorname{HCN}(a q)\) (b) \(\mathrm{NaCN}(a q)\) (c) \(\mathrm{LiOH}(a q)\) (d) \(\mathrm{CH}_{3} \mathrm{NH}_{2}(a q)\) (e) \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(a q)\)

Short answer

(a) HCN is a weak acid, (b) NaCN is a weak base, (c) LiOH is a strong base, (d) CH3NH2 is a weak base, and (e) H2C2O4 is a weak acid.

Step-by-step solution

Identify HCN as weak acid

\(\operatorname{HCN}(a q)\) is a weak acid. It partially ionizes in water, so the reaction is written as follows: \(HCN(aq) + H_2O(l) \leftrightarrow CN^-(aq) + H_3O^+(aq)\)

Identify NaCN as a weak base

\(\mathrm{NaCN}(a q)\) is a weak base. When it dissolves in water, it forms CN- ions which can accept protons (H+) from water to form HCN and OH- as follows: \(NaCN(aq) \rightarrow CN^-(aq) + Na^+(aq)\), \(CN^-(aq) + H_{2}O(l) \leftrightarrow HCN(aq) + OH^{-}(aq)\)

Identify LiOH as a strong base

\(\mathrm{LiOH}(a q)\) is a strong base. It dissociates completely in water to form Li+ ions and OH− ions, as follows: \(LiOH(aq) \rightarrow Li^+(aq) + OH^-(aq)\)

Identify CH3NH2 as a weak base

\(\mathrm{CH}_{3} \mathrm{NH}_{2}(a q)\) is a weak base. It accepts a proton (H+) from water, producing its conjugate acid and hydroxide ion as follows: \(CH_3NH_2(aq) + H_2O(l) \leftrightarrow CH_3NH_3^+(aq) + OH^-(aq)\)

Identify H2C2O4 as a weak acid

\(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(a q)\) is a weak acid. It partially ionizes in water to form H+ and C2O4^2− as follows: \(H2C2O4(aq) \leftrightarrow 2H^+(aq) + C2O4^{2-}(aq)\)

Key concepts

Strong and Weak Acids

Understanding the difference between strong and weak acids is fundamental to grasping acid-base reactions. Strong acids completely dissociate in water, which means they release all of their hydrogen ions (H⁺) into the solution. An example of a strong acid is hydrochloric acid (HCl). However, the problem exercise has introduced hydrocyanic acid (HCN) and oxalic acid (H₂C₂O₄), both categorized as weak acids.

**Weak Acids**
1. Partial Ionization: They do not completely dissociate in water, meaning only a fraction of their molecules release hydrogen ions.
2. Equilibrium Reaction: The reaction where they release H⁺ is reversible and reaches an equilibrium, like the equation for HCN: \[ HCN(aq) + H_2O(l) \leftrightarrow CN^-(aq) + H_3O^+(aq) \]
3. Less Conductivity: Due to partial ionization, solutions of weak acids are less conductive compared to strong acids, making them less effective conductors of electricity.

Strong and Weak Bases

In acid-base chemistry, distinguishing between strong and weak bases is crucial. Strong bases, like lithium hydroxide (LiOH), fully dissociate in water to provide hydroxide ions (OH⁻). This is different for weak bases, such as sodium cyanide (NaCN) and methylamine (CH₃NH₂), mentioned in the exercise.

**Weak Bases**
1. Incomplete Dissociation: Weak bases partially accept protons in water, leading to a weak reaction with water molecules.
2. Reversible Reactions: For instance, with NaCN, it dissociates to CN⁻, which can react with water: \[ CN^-(aq) + H_2O(l) \leftrightarrow HCN(aq) + OH^-(aq) \]
3. pH Effect: They generally result in a slightly basic solution as they do not release as much hydroxide ion as strong bases do.

Ionization in Water

Ionization in water is a key concept for understanding how acids and bases behave in aqueous solutions. This process involves the dissociation of compounds into ions when dissolved in water.

**Ionization in Acids and Bases**
1. Complete vs. Incomplete: In strong acids and bases, ionization is complete, meaning all molecules dissociate into ions. However, in weak acids and bases, like HCN or CH₃NH₂, ionization is incomplete and reversible.
2. Dynamic Equilibrium: For weak acids/bases, the forward and reverse reactions reach a state of balance, maintaining both reactants and products in solution.
3. Water Role: Water acts as a solvent enabling these processes, and also participates by donating or accepting protons, forming ions such as H₃O⁺ or OH⁻. These reactions underscore water’s critical role in facilitating the ionization of acids and bases.