Question

In the Week 2 lab, we determined the partition coefficients of benzoic acid between the binary solvent systems CH2Cl2 / H2O and CH2Cl2 / (10% NaHCO3). a. Explain why the observed partition coefficient for CH2Cl2 / H2O is greater than for CH2Cl2 / (10% NaHCO3). b. Compared to CH2Cl2 / H2O, what change would you expect to see in the partition coefficient for benzoic acid if 1M HCl were used as the aqueous solvent? What about 1M NaOH? Why

Answer 1

Partition coefficient is the ratio of concentrations of a solute in two immiscible liquids

Higher the concentration of solute in liquid 1 than in liquid 2 , higher would be the partition coefficient

Difference in concentration of benzoic acid in two systems is caused by the presence of intermolecular hydrogen bonding between benzoic acid and solvent.

Comparing between H2O and NaHCO3 , hydrogen bonding will be stronger in case of NaHCO3 because of presence of bicarbonate ion which will make stronger hydrogen bond with benzoic as comapred to H2O.

Therefore more benzoic acid will be present in 10% NaHCO3 solution then H2O

Hence , partition coefficient will become less in case of CH2Cl2/10%NaHCO3 than CH2Cl2/H2O

If 1M HCl was used as aqueous solvent , the partition function will decrease for CH2Cl2/HCl system . This is because Cl atom is more electronegative than oxygen and will do stronger hydrogen bonding. Hence more benzoic acid will dissolve . Therefore partition coefficient will decrease

If 1M NaOH is taken the partition coefficient will further decrease as there will be very strong hydrogen bonding between benzoic acid and hydroxide ion , increasing the concentration of benzoic acide in NaOH. Hence, partition coefficient for CH2Cl2/NaOH system will further decrease.