Question

1. Give five or four (4/5) basic requirements of a good recrystallizing solvent.

2. Explain how an increase in temperature increases the solubility of benzoic acid in water.
(Hint: consider benzoic acid structure and bond formation).

Answer 1

1.

• The recrystallization solvent should not dissolve the substance to be purified at room temperature, but it should dissolve it well at the solvent’s boiling point.
• The solvent should dissolve soluble impurities well at room temperature.
• The solvent should not dissolve insoluble impurities even at the solvent’s boiling point. These insoluble impurities can then be removed by gravity filtration.
• The solvent must not react with the substance to be purified.
• The solvent should be volatile.
• The solvent should be inexpensive.

2. Increasing in temperature increases solubility because some of the increased energy sufficiently lengthens the hydrogen-bonds, so that ionization occurs. Ions are by definition polar, so the general truism, like dissolves like, indicates the ions will then dissolve in water.

if we increase the temperature the overall gibb's energy change will be more negative and thus spontaneity of reaction increases and hence solubility increases....