Question

Write all three hydrolysis reactions for weak acid L-cysteine (H₃Cys). Write all three acid ionization expressions (Kᴀ₁, Kᴀ₂, and Kᴀ₃) also. Include all ionic charges.

Answer 1

L-cysteine is a triprotic weak acid with three hydrolysis reactions, each associated with an equilibrium constant expression Kᵅ₁, Kᵅ₂, and Kᵅ₃, indicating the strength of each ionization step.

Step-by-step explanation:

The student asked to write all three hydrolysis reactions for the weak acid L-cysteine (H₃Cys) and its acid ionization expressions (Kᵅ₁, Kᵅ₂, and Kᵅ₃), including all ionic charges. L-cysteine is a triprotic acid meaning it can donate three protons (H⁺). The stepwise ionization processes result in the formation of different ionic species at each step, with corresponding equilibrium constants reflecting the strength of each ionization step.

The hydrolysis reactions for L-cysteine are as follows:

1. 
   
3. H₃Cys (aq) + H₂O (l) ⇌ H₂Cys⁻ (aq) + H₃O⁺ (aq) (First ionization)
4. 
   
6. H₂Cys⁻ (aq) + H₂O (l) ⇌ HCys²⁻ (aq) + H₃O⁺ (aq) (Second ionization)
7. 
   
9. HCys²⁻ (aq) + H₂O (l) ⇌ Cys³⁻ (aq) + H₃O⁺ (aq) (Third ionization)

The equilibrium constant expressions for these ionizations are as follows:

1. 
   
3. Kᵅ₁ = [H₂Cys⁻][H₃O⁺]/[H₃Cys]
4. 
   
6. Kᵅ₂ = [HCys²⁻][H₃O⁺]/[H₂Cys⁻]
7. 
   
9. Kᵅ₃ = [Cys³⁻][H₃O⁺]/[HCys²⁻]

Each expression describes the equilibrium state of each ionization step, where the square brackets represent the molar concentration of each species involved.