the equilibrium expression of base dissociation of triethylamine

Question

Triethylamine (C₆H₁₅N) is an organic base which is useful in pesticide production and organic synthesis. It has a pKa of 10.75 and density of 726 kgm^-3 . It will partially dissociate in water following the chemical equilibrium below after dissolution in water.

C₆H₁₅N(aq) ⇌ OH^-/(aq) +C₆H₁₅NH⁺(aq)

(a) Give the equilibrium expression of base dissociation of triethylamine(C₆H₁₅N) (K_b).

(b) At a certain time, it is known that 3.18g triethylamine (C₆H₁₅N), 2.45g protonated triethylamine (C₆H₁₅NH⁺ ) and 1.53g hydroxide ion (OH^- ) was present in the solution. Please evaluate whether the solution has attained equilibrium. State the direction that the equilibrium is shifting if the system is not in equilibrium.

Answer 1

C₆H₁₅N(aq) ⇌ OH^-/(aq) +C₆H₁₅NH⁺(aq)

(a) Kb = [OH-][C6H15NH+] / [C6H15N]

(b) convert all grams to moles:

mols C₆H₁₅N = 3.18 g x 1 mol/101 g = 0.0315 mols

mols C₆H₁₅NH⁺ = 2.45 g x 1 mol/102 g = 0.0240 mols

mols OH^- = 1.53 g x 1 mol/17 g = 0.09 mols

(c) calculate Q (reaction quotient):

Q = [OH^-][C₆H₁₅NH⁺] / [C₆H₁₅N] = (0.09)(0.0240) / (0.0315) = 0.0686

(d) to know if this has attained equilibrium, we need to know the value of the Kb for this reaction. You did not provide that value, so I looked it up and find it to be 6.5x10^-5. Now we compare Q to Kb and find that Q is much greater than Kb indicating that the reaction is way beyond equilibrium and would have to shift left (back toward reactants) to reach equilibrium.