Question

Write the reaction and the corresponding \(K_{\mathrm{b}}\) equilibrium expression for each of the following substances acting as bases in water. a. aniline, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}\) b. dimethylamine, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\)

Short answer

a. Aniline, C6H5NH2: Reaction: C6H5NH2 + H2O ⇌ C6H5NH3+ + OH- Equilibrium expression: \(K_b = \frac{[C_{6}H_{5}NH_{3}^+][OH^-]}{[C_{6}H_{5}NH_{2}]} \) b. Dimethylamine, (CH3)2NH: Reaction: (CH3)2NH + H2O ⇌ (CH3)2NH2+ + OH- Equilibrium expression: \(K_b = \frac{[(CH_{3})_{2}NH_{2}^{+}][OH^{-}]}{(CH_{3})_{2}NH} \)

Step-by-step solution

Write a balanced chemical equation

When aniline reacts with water, it accepts a proton from the water molecule, forming a hydroxide ion (OH−) and an anilinium ion (C6H5NH3+). The balanced chemical equation is: C6H5NH2 + H2O ⇌ C6H5NH3+ + OH-

Identify the conjugate acid and conjugate base

In the reaction, C6H5NH2 is acting as a base (proton acceptor) and its conjugate acid is C6H5NH3+. The conjugate base is the OH- ion.

Write the equilibrium expression using Kb

The equilibrium expression for the Kb of aniline can be written as: \(K_b = \frac{[C_{6}H_{5}NH_{3}^+][OH^-]}{[C_{6}H_{5}NH_{2}]} \) b. Dimethylamine, (CH3)2NH

Write a balanced chemical equation

When dimethylamine reacts with water, it accepts a proton from the water molecule, forming a hydroxide ion (OH−) and a dimethylammonium ion ((CH3)2NH2+). The balanced chemical equation is: (CH3)2NH + H2O ⇌ (CH3)2NH2+ + OH-

Identify the conjugate acid and conjugate base

In the reaction, (CH3)2NH is acting as a base (proton acceptor) and its conjugate acid is (CH3)2NH2+. The conjugate base is the OH- ion.

Write the equilibrium expression using Kb

The equilibrium expression for the Kb of dimethylamine can be written as: \(K_b = \frac{[(CH_{3})_{2}NH_{2}^{+}][OH^{-}]}{(CH_{3})_{2}NH} \)