Acids, bases and titration — Question 5

5

a)

ethanamine

• provides ethanamine [1 mark]

b)

• provides H₂O(l) [1 mark]

• provides C₂H₅NH₃

⁺(aq)

[1 mark]

c)

The hydrochloric acid (H⁺) reacts with the OH⁻ ions and decreases their concentration. According to Le Châtelier’s principle, this will make the equilibrium position shift to the right to counteract decrease in OH⁻ (products). Therefore, the concentration of A will decrease.

• indicates that [OH⁻] will decrease [1 mark]

• indicates that the equilibrium will shift right [1 mark]

• indicates that A will decrease [1 mark]

d)

[CH₃CH₂NH₂] = 2.0 −x ≈2.0 as 2.0 ≫ KK_b

⁺] = [OH⁻]

Let x = [C₂H₅NH₃

⁺][OH⁻] [C₂H₅NH₃ KK_b = [CH₃CH₂NH₂] [x][x] [2.0] = x²

5.6 × 10⁻⁴ =

2.0

x² = 1.12 × 10⁻³

x = 0.033466 = [OH⁻]

pOH = –log [OH⁻] = 1.475 ≈ 1.5

pH = 14 −1.5 = 12.5 pH = 12.5 (to 1 decimal place)

• indicates assumption to support [CH₃CH₂NH₂] ≈2.0 [1 mark]

⁺] =

• indicates [C₂H₅NH₃ [OH⁻] [1 mark]

• shows substitution correctly performed [1 mark]

• determines [OH⁻] = 3.35 × 10⁻² [1 mark]

• determines pOH [1 mark]

• determines pH [1 mark]

Accept ICE table. Allow FT error from [OH⁻].

Do not penalise for incorrect decimal places/significant figures.

e)

Heat CH₃Br with KCN under reflux to produce CH₃CN and KBr

heat with Ni catalyst

CH₃CN(aq) + 2H₂(g)

ሱ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ሮ CH₃CH₂NH₂(aq)

• indicates CH₃Br reacts with KCN to produce KH₃CN and KBr [1 mark]

• identifies reaction is heated under reflux in ethanol [1 mark]

• indicates CH₃CN reacts with H₂(g) to produce CH₃CH₂NH₂ [1 mark]

• indicates heat and Ni/Pt/Pd catalyst required [1 mark]

• represents one of the reactions as a balanced chemical equation [1 mark]

Also accept as balanced equation: CH₃Br(aq) heat under reflux with KCN (ethanol) ሱ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ሮCH₃CN(aq) + KBr(aq)