PREPARATORY PROBLEM 25 (PRACTICAL) Titration of maleic acid (cis-butenedioic acid) Introduction Male — Physical Chemistry — Kinetics Chemistry Question
Titration of maleic acid (cis-butenedioic acid)
PREPARATORY PROBLEM 25 (PRACTICAL)
Titration of maleic acid (cis-butenedioic acid)
Introduction
Maleic acid is an unusual compound since the pKa values for the first and second dissociation are very different, pKa(1) = 1.8 and pKa(2) = 6.1. In contrast, fumaric acid which is the corresponding trans-diacid, has pKa values tat differ only slightly: 3.0 and 4.4, respectively. [VISUAL] In this experiment the first and second dissociation of maleic acid will be demonstrated by titration using different pH indicators phenolphthalein and methyl orange.
List of chemicals needed:
maleic acid, 600 mg
NaOH, aqueous solution, c = 1 mol dm -3, 20 cm 3
phenolptphalein, few drops
methyl orange, few drops
Procedure 1
Maleic acid (290 mg) is dissolved in water (5 cm 3), 2 drops of phenolphthalein are added and the solution is titrated with a solution of NaOH (c = 1 mol dm -3) until a clearly visible, permanent change of colour has taken place.
Record the following data:
1. Volume (cm 3) of the NaOH solution used.
2. Calculated amount of maleic acid in mmol.
3. Colour of the solution before titration.
4. Colour of the solution after completion of the titration.
Determine on the basis of the titration results the minimum pKa value of
i) phenolphthalein
ii) methyl orange
Model Answer
i) pKa phenolphthalein pKa > 6.1
ii) pKa methylorange pKa > 1.8
The difference in the pKa values of maleic acid can be attributed to:
- intramolecular hydrogen bonding,
- steric hindrance of the –COOH group,
- large dipole moment of the cis-acid compared with the trans-acid,
- intermolecular hydrogen bonding.
Mark the correct answer.
Model Answer
The correct answer is the intramolecular hydrogen bonding.