Draw the titration graphs for the following combinations. Provide equivalence point (approximate pH value) and label the axis on the graph.
a) HCN titrated with NaOH
b) KOH titrated with HBr
The correct answer and explanation is :
Here’s a breakdown of the titration graphs for the given combinations:
a) HCN (Weak Acid) Titrated with NaOH (Strong Base)
Graph Features:
- The pH starts low (~3) because HCN is a weak acid.
- As NaOH is added, the pH gradually increases due to the formation of the cyanide ion (CN⁻), which is a weak base.
- The equivalence point occurs at a pH > 7 (~8-9) because CN⁻ hydrolyzes to form OH⁻.
- Beyond the equivalence point, excess NaOH causes a sharp increase in pH.
Equivalence Point pH:
- Approximately 8-9 due to the weak acid-strong base reaction.
b) KOH (Strong Base) Titrated with HBr (Strong Acid)
Graph Features:
- The pH starts high (~13) because KOH is a strong base.
- As HBr is added, the pH drops steadily due to neutralization.
- The equivalence point occurs at pH 7 because a strong acid reacts with a strong base to form neutral water and salt.
- Beyond the equivalence point, excess HBr leads to a sharp pH drop.
Equivalence Point pH:
- Exactly 7, as this is a strong acid-strong base reaction.
Explanation (300 words)
Titration curves provide insight into acid-base neutralization by plotting pH against the volume of titrant added. The nature of the acid and base determines the curve’s shape and equivalence point.
(a) HCN vs. NaOH
HCN is a weak acid (Ka ≈ 6.2 × 10⁻¹⁰), meaning it only partially ionizes in water. When NaOH (a strong base) is added, OH⁻ reacts with HCN to form CN⁻ and water. Since CN⁻ is a weak base, it slightly increases the pH at equivalence. The buffering region appears before the equivalence point due to the presence of HCN/CN⁻. The final pH after equivalence rises sharply due to excess OH⁻.
(b) KOH vs. HBr
KOH is a strong base, and HBr is a strong acid. Their titration results in a nearly linear pH decrease until equivalence, where KBr and water form, leaving a neutral solution (pH = 7). Beyond this, excess HBr rapidly lowers the pH. The graph is symmetric, reflecting the complete dissociation of both reactants.
These titration curves help determine reaction endpoints, acid/base strength, and buffering capacity, crucial in chemistry and industry.