Extractions (13.2.2)

Question 1

• In general, polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.

Answer 1

• In general, polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes.

Question 2

• An extraction is a technique used to separate a solute from a solvent using another solvent based on differences in the solubility of the solute.

Answer 2

• An extraction is a technique used to separate a solute from a solvent using another solvent based on differences in the solubility of the solute.

Question 3

Which of the following best explains why two immiscible solvents are used in extractions?

Answer 3

To extract a solute from one solvent by another solvent (C)

If the solvents were miscible, then they would not be physically separable from one another. This is necessary when using extraction to physically separate solutes.

Question 4

How does a nonpolar → nonpolar interaction primarily differ from a polar → polar interaction?

Answer 4

A nonpolar interaction is driven primarily by a tendency to maximize entropy in the system. A polar interaction is driven both by entropy and intermolecular attraction forces. (A)

This is primarily what distinguishes the two interactions.

Question 5

Which of the following mixtures would be considered the most miscible?

Answer 5

water and iodine (B)

Iodine is at least partially miscible in water.

Question 6

Extraction is ___________________

Answer 6

A technique in which a solute is separated from its solvent by the introduction of a second solvent that is immiscible with the original solvent. (B)

Extraction simply means extracting a solute from one solvent by using a second immiscible solvent. The second solvent must have polarity characteristics that more closely resemble those of the solute than does the first solvent.

Question 7

Suppose that you want to transfer some solute from one solvent to another. You decide to use an immiscible second solvent (that “grabs” the solute). Which of the following best explains why this process takes more time (and effort) than using a miscible solvent?

Answer 7

The transfer can only occur at the interface of the two solvents. Therefore, it takes time (and stirring) for this to happen. (C)

The solute is only transferred at the interface between the two immiscible solvents. Therefore, this will take time and effort.

Question 8

Look at the diagram of an extraction.

Which statement best explains why iodine transfers from the water to the hexane?

Answer 8

Both iodine and hexane are relatively nonpolar. (A)

This is the same as saying that “like dissolves like.” Iodine is a somewhat nonpolar solute and hexane is a nonpolar solvent. Water is a polar solvent.

Question 9

A mixture is a ______________

Answer 9

combination of two or more substances that can be separated by physical means. (D)

The important distinction here is that the substances can be separated by physical means.

Question 10

Entropy, a major factor in the solubility of solutes in solvents, is _______________

Answer 10

a measure of the tendency for matter to be as randomly distributed as possible. (C)

Entropy is randomness. The second law of thermodynamics says that a system will always try to maximize its disorder (i.e., entropy).

Question 11

Which of the following best explains the concept of “like dissolves like”?

Answer 11

Solutes mix well with solvents that more closely resemble them in polarity. (D)

The polarity is what qualifies the components as being “like” one another.

Question 12

Suppose you have a dilute solution of table salt, NaCl, in water. Why can’t the salt from this solution be extracted into dichloromethane, a relatively non-polar solvent that is more dense than water?

Answer 12

Table salt is not soluble in non-polar solvents. (C)

NaCl is an ionic (extremely polar) solute and strongly prefers the water layer over any nonpolar solvent.