Question

often served with a wedge of lemon, which contains citric acid. The amine and the acid react forming a product with no odor, thereby making the less-than-fresh fish more appetizing. Using data from Appendix D, calculate the equilibrium constant for the reaction of citric acid with methylamine, if only the first proton of the citric acid (Ka1) is important in the neutralization reaction.

Answer 1

`\large \boxed{1.5 * 10^(6)}`

Step-by-step explanation:

We have two equations:

1. HCit + H₂O ⇌ H₃O⁺ + Cit⁻; Kₐ₁ = 8.4 × 10⁻⁴

2. NH₄⁺ + H₂O ⇌ H₃O⁺ + NH₃; Kₐ = 5.6 × 10⁻¹⁰

From these, we must devise the target equation:

3. HCit + NH₃ ⇌ NH₄⁺ + Cit⁻; Keq = ?

The target equation has HCit on the left, so you rewrite Equation 1.

4. HCit + H₂O ⇌ H₃O⁺ + Cit⁻; K₄ = Kₐ₁

Equation 4 has H₃O⁺ on the right, and that is not in the target equation.

You need an equation with H₃O⁺ on the left, so you reverse Equation 2.

When you reverse an equation, you invert its K.

5. H₃O⁺ + NH₃ ⇌ NH₄⁺ + H₂O; K₅ = 1/Kₐ

Now, you add equations 4 and 5, cancelling species that appear on opposite sides of the reaction arrows.

When you add equations, you multiply their K values.

You get the target equation 3:

4. HCit + H₂O ⇌ H₃O⁺ + Cit⁻; K₄ = Kₐ₁

5. H₃O⁺ + NH₃ ⇌ NH₄⁺ + H₂O; K₅ = 1/Kₐ

3. HCit + NH₃ ⇌ NH₄⁺ + Cit⁻; Keq = K₄K₅ = Kₐ₁/Kₐ

Keq = (8.4 × 10⁻⁴)/(5.6× 10⁻¹⁰) = 1.5 × 10⁶

`K_{\text{eq}} \text{ for the reaction is $\large \boxed{\mathbf{1.5 * 10^(6)}}$}`