4.6c solvent extraction and melting point and mixed melting point analysis

Question 1

What is solvent extraction

Answer 1

isolating a solute from a liquid mixture or solution by extraction using an immiscible solvent in which a solute is soluble

Question 2

Explain solvent extraction

Answer 2

• two immiscible solvents form two layers in the separating funnel
• the solute dissolved in both solvents and an equilibrium establishes between two layers
• the ration of solute dissolved in each layer is determined by the equilibrium constant K
• the lower layer is run off into a container and the upper layer is poured into a second container
• the process is repeated to maximise the quantity of solute extracted

Question 3

Explain the relationship between the quantity of solute extracted and the number of extractions

Answer 3

the quantity of solute extracted is greater if a number of extracting using smaller volumes of solvent are carried out than a single extraction using a large volume of solvent

Question 4

How do you decide what solvent to use in solvent extraction?

Answer 4

• immiscible with the liquid mixture or solution (usually water)
• one in which the solute is more soluble in than the liquid mixture or solution (usually water)
• volatile to allow the solute to be obtained by evaporation of the solvent
• unreactive with the solute

Question 5

What is the melting point of a substance?

Answer 5

the temperature range over which the solid first starts to melt, to when all of the solid has melted

Question 6

How can the identity of a pure compound be confirmed?

Answer 6

• by melting point analysis
• and a comparison of the experimentally determined melting point with a literature or known melting point value

Question 7

What can determination of the melting point of a compound give an indication of?

Answer 7

the purity of a compound
- the presence of Impurities in the compound lowers the melting point and broadens its melting temperature range
- due to the disruption in intermolecular bonding in the crystal lattice

Question 8

How would you determine a mixed melting point?

Answer 8

• mixing a small quantity of the product with some of the pure compound and determining its melting point
• the melting point value and the range of the melting temperature can be used to determine if the product and the pure compound are the same substance