MIDTERM - Extraction, Sublimation, and Melting Point Determination

Question 1

two methods of chemical analysis:

Answer 1

qualitative and quantitative

Question 2

• involves the determination of the
  chemical composition of a sample.
• It encompasses a set of analytical chemistry techniques to provide nonnumerical information about a specimen.
• can tell you whether an atom, ion, functional group, or compound is present or absent in a sample, but it does not provide information about its quantity.

Answer 2

qualitative analysis

Question 3

Quantification of a sample, in contrast, is called

Answer 3

quantitative analysis

Question 4

typically measures changes in, but not limited to:
- color, melting point, odor, reactivity, radioactivity, boiling point, bubble production, and precipitation.

Methods include, but not limited to:
- distillation, extraction, precipitation, chromatography, and spectroscopy.

Answer 4

qualitative analysis

Question 5

random error

Answer 5

human error

eg. a researcher misreading a weighing scale records an incorrect measurement

Question 6

systematic error

Answer 6

machine error

eg. a miscalibrated scale consistently registers weights as higher than they actually are

Question 7

gross errors

Answer 7

Gross errors are errors that are so serious (i.e. large in magnitude) that they cannot be attributed to either systematic or random errors associated with the sample, instrument, or procedure.

eg. writing down a value of 100 when the reading was actually 1.00.

Question 8

Caffeine is a member of the class of
compounds called

Answer 8

alkaloids

Question 9

nitrogen-containing basic compounds that are found in plants.

They usually taste bitter and often are
physiologically active in humans.

Answer 9

alkaloids

Question 10

Caffeine acts as a ________. It is physically addictive.

Answer 10

stimulant

Question 11

It is the most widely used “legal” stimulant, rivaled by its sister molecule, ____________, which is found in the cacao bean (and
chocolate).

Both of these molecules have the parent purine ring structure, which is also seen in guanine and adenine (found in
DNA and RNA).

Answer 11

THEOBROMINE

Question 12

The other components aside from caffeine in tea leaves include _______ and classes of compounds called

Answer 12

cellulose

tannins, flavins, and chlorophylls

Question 13

the physical process by which a compound (or mixture of compounds) is transferred from one phase to another.

Answer 13

extraction

Question 14

Water-soluble components in the tea leaves or coffee beans are being transferred from a solid phase, the leaves or beans, into a liquid phase, the hot water.

This is an example of

Answer 14

solid-liquid extraction

Question 15

In the extraction experiment, we make use of ORGANIC SOLVENT to extract the organic components, namely caffeine, from tea.

This process is known as

Answer 15

liquid-liquid extraction

Question 16

The physical process that rules liquid-liquid extraction is known as ________________, or the distribution of solutes between a pair of solvents.

Answer 16

solvent-solvent partitioning

Question 17

Organic solvents such as diethyl ether, toluene, and methylene chloride (dichloromethane, DCM) have a very limited solubility in water.

These solvents are _________ with water.

Answer 17

immiscible

Question 18

For example, if DCM is mixed with water, a two-layer system is obtained. The upper layer contains the less-dense solvent (water, density = 𝟏𝒈/𝒄𝒎𝟑) and the lower layer is the more-dense solvent (DCM, density = 𝟏.𝟑𝟑𝒈/𝒄𝒎𝟑).

The water layer along with its components is called the _____________.

The DCM layer along with its components is called the _____________.

Answer 18

aqueous phase

organic phase

Question 19

The tendency of a solute (in this case, caffeine) to dissolve preferentially in one solvent over another can be represented in a

Answer 19

partition (distribution) coefficient, 𝑲𝒅.

Question 20

If 𝑲𝑫 >𝟏, solute prefers the

Answer 20

organic phase

Question 21

If 𝑲𝑫 < 𝟏, the solute prefers the

Answer 21

aqueous phase

Question 22

Suppose solute A is added to a mixture of water and DCM. The system is shaken and attains equilibrium. Solute A will be present in both layers, but larger amounts of it will be in the solvent for which it has a higher affinity.

The distribution between both solvents is dictated by the _____________, of A between the two solvents.

Answer 22

partition coefficient 𝑲𝒅

Question 23

• measures the efficiency of the extraction.

Increasing the volume of organic solvent increases the percent recovery as more solute is extracted into the larger organic phase.

Answer 23

percentage recovery

Question 24

It is standard to perform ____ organic
extractions of caffeine.

Answer 24

THREE

Question 25

The tea is made under basic conditions to ionize mildly acidic flavonoids and tannins, causing them to be more ___________.

In the liquid-liquid (DCM-water) extraction, caffeine will be separated from the water-soluble pigments. After removing trace amounts of water, the low-boiling solvent is evaporated to yield the crude organic extracts.

Further purification by ________ results in white crystals of caffeine.

Answer 25

water-soluble

sublimation

Question 26

1. What is the role of sodium
   carbonate, 𝑵𝒂𝟐𝑪𝑶𝟑, in the
   extraction of caffeine from tea
   leaves?

Answer 26

NEUTRALIZES ACIDS
- It prevents caffeine from becoming protonated (positively charged) by any acidic components in the tea leaves. This ensures caffeine stays soluble in the organic solvent used later.

PRECIPITATES TANNINS
- It reacts with tannins, turning them into sodium salts that are insoluble in the organic solvent used for caffeine extraction. This prevents unwanted tannins from being extracted alongside caffeine.

Question 27

3. Which is more efficient, single
   extraction or multiple
   extraction? Why?

Answer 27

Multiple extraction is more efficient than single extraction for caffeine.

In a single pass, some caffeine remains trapped within the tea leaves. Multiple extractions with fresh solvent allow for more complete removal of caffeine.

Question 28

4. What is the role of anhydrous sodium sulfate in extraction?

Answer 28

Anhydrous sodium sulfate acts as a drying agent in caffeine extraction from tea leaves.

It removes residual water from the organic solvent layer containing the caffeine.

This ensures the extracted caffeine is purer and easier to handle later.

Question 29

the transition of a solid into a gas
without passing through the intermediary liquid stage.

This normally occurs when the vapor pressure of a substance is GREATER than the atmospheric pressure.

It is a process used in the purification of substances that have non-volatile impurities or substances that have a significantly lower vapor pressure than a pure compound.

Answer 29

sublimation

Question 30

the temperature at which a solid is
changed into a liquid.

happens when the solid phase of a substance and its liquid phase are in a
dynamic equilibrium.

can be used to determine its purity

melting point of a pure substance is fixed while an impure substance melts over a wide range of higher temperature and a lower temperature.

Answer 30

melting point

Question 31

a white crystalline powder and is the simplest of aromatic carboxylic acids.

It has a melting point of 𝟏𝟐𝟐 °𝑪 and a boiling point of 𝟐𝟒𝟗 °𝑪.

It is commonly used as a food preservative and a germicide or antifungal agent.

Answer 31

benzoic acid