Lab 1

Question 1

what types of structures can solids exist as?

Answer 1

Solids can exist either as crystalline structures or as amorphous structures.

Question 2

what is a crystalline solid? what are their intermolecular forces like?

Answer 2

crystalline solid has
atoms, ions or molecules arranged in an orderly array, known as a crystal lattice. The arrangement
of atoms, molecules or ions in a crystalline solid is such that the net attractive intermolecular forces
are at their maximum. O

Question 3

what is an amorphous structure?

Answer 3

olids that are not crystalline in structure, such as glass ,
are said to be amorphous solids; they lack a regular, internal three-dimensional arrangement of
atoms

Question 4

what does recrystalization mean? when is it successful?

Answer 4

Recrystallization literally means to crystallize (form crystals) again. A successful recrystallization
depends on a large difference between the solubility of a compound in a hot solvent and its
solubility in the same solvent when it is cold.

Question 5

what does recrystalization involve?

Answer 5

Most solid organic compounds can be purified by recrystallization. Generally, the technique
involves dissolving the solid in a hot solvent and then cooling the solution slowly to promote
crystallization of the purified solid.
solution - heat = solid + solvent

Question 6

where do impurities go after recrystalization?

Answer 6

Impurities either remain dissolved in the solvent or remain insoluble and are removed by filtration
and/or adsorption on decolourizing charcoal (Norit).

Question 7

why is the process called recrystalization?

Answer 7

The process is called recrystallization because crystals are grown slowly and incorporate only
crystallized product into their crystal lattice Recrystallization is usually more desirable than precipitation
because it results in a purer product. It is unusual for different compounds to lattice.crystallize in the same

Question 8

what is precipitation?

Answer 8

(precipitation is when crystals form rapidly and incorporate
impurities within the crystal lattice).

Question 9

what happens if the equilibrium is suddenly shifted? what else does rapid cooling cause?

Answer 9

if the equilibrium in the above equation is shifted rapidly by sudden cooling, some
impurities will be trapped within the crystals. Furthermore, rapid cooling causes the formation of smaller
crystals with a large combined surface area, making it difficult to wash them clean. Thus, recrystallization
solutions should always be cooled slowly to maximize the purity of the crystalline product.

Question 10

what should the soild selected be like?

Answer 10

uccessful recrystallization requires a large difference in the solubility of the solid in the solvent
as the temperature is varied. The solid to be recrystallized should be practically insoluble (or
only sparingly soluble) in the recrystallization solvent at low temperatures and highly
soluble in the solvent at elevated temperatures.

Question 11

what do functional groups tell us in single solvent recrystalization?

Answer 11

Based on the functional groups present in the compound to be recrystallized, solvents of similar
chemical nature are typically selected, remembering the “like dissolves like” rule. Salicylic acid
consists of a non-polar aromatic ring as well a polar carboxyl group and hydroxyl group.

Question 12

what can we predict about salicylic acid based on it’s functional groups? typical polar and non-polar solvents used in the lab?

Answer 12

we may predict that salicylic acid might be soluble in both polar and non-polar solvents.
Common polar solvents used in the laboratory include water, methanol, ethanol, and acetone. Non-
polar solvents are typified by hydrocarbons, such as pentane, hexane, benzene or toluene.

Question 13

what four criteria should be kept in mind for solvent selection?

Answer 13

. To ensure that the solvent is easily removed from the crystals, it should have a moderate
boiling point (typically in the range of 50-130 °C).
• The boiling point for the solvent should be below the melting point of the compound being
recrystallized (to prevent oiling out – see below).
• If possible, the solvent should have low flammability.
• The solvent should not react irreversibly with the compound being purified, and the
impurities should be either insoluble in the hot solvent or soluble in the cold solvent.

Question 14

what is oiling out?

Answer 14

Sometimes, during recrystallization, the crude compound separates as an oil rather than dissolving
in the solvent. This process is known as oiling out.

Question 15

what does oil mean? when is oiling out likely?

Answer 15

il is a term often used to refer to water-
insoluble organic liquids. Oiling out is most likely to happen when solids with low melting points
are recrystallized. If the boiling point of the solvent is higher than the melting point of the solid
being recrystallized, there is a strong possibility that the solid will melt instead of dissolving in the
solution. As an oil, the crude product acts as a solvent for impurities. The impurities distribute
themselves between the two immiscible phases, namely the oil and the solvent. As the
recrystallization mixture cools, the oil will become an amorphous impure solid.

Question 16

what can be done to prevent oiling out?

Answer 16

Where oiling out during recrystallization is a risk you should keep the temperature of the solvent
below the melting point of the solid to be purified. This typically involves avoiding heating the
solvent to its boiling point.

Question 17

what does percent recovery measure? what factors might affect it? how is it calculated?

Answer 17

The yield of any given purification is measured by the percent recovery of the pure solid relative
to the original crude mass. In a recrystallization process, molecules of impurities are separated
from the molecules of interest. The impurities remain in solution as the crystalline lattice of the
pure solid is formed.
In addition, a small amount of the compound to be purified also remains dissolved in the solution
(filtrate) and is difficult to recover. These two factors decrease the amount of the recrystallized
product that is recovered, and therefore the percent recovery for the recrystallization is always
less than 100%. Maximizing the percent recovery is always the goal of an organic chemist and
should also be the aim of a student in the chemistry laboratory.
final mass / original mass x 100%

Question 18

what are indicators of the purity of the compound?

Answer 18

The melting point of a crystalline organic compound and the boiling point of an organic liquid
provide an indication of their purity. By measuring these physical properties and comparing them
to literature data, one can assess the degree of purity of organic compounds.

Question 19

how is the melting point of a compound measured?

Answer 19

To measure the melting point of a solid, the compound is heated gradually until the energy of
molecules in the crystalline lattice is such that the lattice is destroyed and the solid converts to its
liquid phase. In practice, it is very difficult to measure the temperature associated with this phase
change. Therefore, two temperatures are usually noted (giving a melting point range): the
beginning of melting, when the first droplets of liquid form among the crystals, and the end of
melting, when the last crystal turns to liquid.
Many solids undergo unusual behaviour before melting, such as partial decomposition (gas
evolution may be observed), colour change, softening or shrinking of the solid (called sintering).- these changes do NOT indicated the beginning of melting

Question 20

how does melting point indicate the purity?

Answer 20

The melting point indicates the purity of a solid in two ways. As the purity of the compound
increases, its melting point more closely approaches the literature value. Second, the purer the
compound, the sharper the melting point (i.e. the narrower the melting point range). As a
substance becomes more and more impure its melting point falls further and further from the
literature value and the breadth of the melting point range steadily increases.A compound which melts sharply, i.e. within a narrow range (1-2 °C), is considered to be pure. In
the laboratory, a melting point range of 2-3 °C is acceptable. Therefore, a solid melting within the
range of 3 °C (for example, a melting point reported as 52-55 °C) is considered relatively pure.

Question 21

why might a melting point NOT be sharp?

Answer 21

There are several reasons why the observed melting point of a sample may not be very sharp, (i.e.
has a range > 3 °C). One obvious reason is that the compound may not be pure; however, the
melting point range of a sample can be affected by a number of other factors as well. These factors
include:
• Amount of sample used
• Size of crystals
• Type of melting point apparatus used
• Rate of heating

Question 22

why do we correct thermometers?

Answer 22

When recording a melting point range in the laboratory using a melting point apparatus and
thermometer, there are several factors that can cause the observed temperature (as read from the
thermometer) to stray from the true temperature of the sample within the apparatus. In particular,
slight manufacturing defects and inconsistencies can cause small but significant errors in temperature reported by the thermometer: you might have noticed already that the small number
of thermometers at your bench all report slightly different values for the ambient room
temperature.

Question 23

three methods for correcting thermometers. we use the third.

Answer 23

1. The thermometer may be corrected and calibrated by the manufacturer at the factory. This
   solution is convenient but can add considerable expense. As well, especially with electronic
   thermometers, the passage of time can cause changes in thermometer performance and
   thus render the factory calibration invalid.
2. The thermometer may be corrected by the user (you) by preparing a calibration graph. The
   thermometer is calibrated by using it to measure the melting point of a range of pure
   compounds with known melting point values. A graph can then be prepared in which the
   measured (experimental) values for these compounds are plotted against the known
   literature values, and this graph can later be used to correct values reported by the
   thermometer for any subsequent measurements. The preparation of a calibration graph
   can be quite time-consuming, and it requires that the user(s) keep track of which
   calibration data corresponds to which thermometer. Any time a thermometer is lost or
   broken, a new calibration curve must be prepared for the replacement device.
3. The thermometer may be corrected by including a melting point standard during the
   melting point measurement itself. For example, if a user required an accurate melting point
   measurement at a temperature near 120 °C the user would prepare a melting-point sample
   of a pure compound with a well-known melting point near 120 °C, and record an
   experimental value for this standard compound alongside the other samples. This melting
   point standard will thus reveal whether or not the thermometer is reporting values
   accurately, or if it is reporting values too high or too low. The experimental values for all
   measurements made near 120 °C can thus be corrected to account for any errors.
   The disadvantage of this third method is that it requires the user to have access to suitable
   melting point standards, and to prepare a melting point sample of a standard for each
   melting point experiment. However, the method does allow a user to use different
   thermometers for each experiment without keeping track of which thermometer is which.
   Thus, this third method is the preferred method in CHEM 26X

Question 24

what was the procedure used?

Answer 24

1. Dissolve a measured mass of salicylic acid in the minimum volume of boiling water needed to
   obtain a saturated solution.
2. If there are visible solid impurities, use hot gravity filtration to separate these solid impurities
   from the solution. Hot gravity filtration will not be necessary during Experiment 1.
3. Allow the filtrate (solution obtained after filtration) to cool slowly to room temperature, during
   which time the first crystals should appear. If no crystals form, initiate crystallization by
   scratching the walls of the container with a glass rod or by adding one or two crystals of pure
   salicylic acid (seeding).
4. Cool the solution in an ice-water bath to complete the crystallization process.
5. Collect the crystals by vacuum filtration using a Büchner funnel. Wash them with a small
   amount of the cold recrystallization solvent (water) and allow them to dry under dynamic
   vacuum.

Question 25

gravity vs vacuum filtration

Answer 25

. Cold gravity filtration – employed to separate a solid from a liquid at room temperature.
2. Hot gravity filtration – employed to separate a solid from a liquid at a temperature close to the
boiling point of the liquid. This method is used to remove undissolved solid impurities from a
solution during a recrystallization procedure. We won’t be using this method during Experiment
1, but it is still an important technique to be aware of
acuum filtration (as opposed to gravity filtration) is a method used for separating a solid from a
liquid at a pressure lower than atmospheric pressure (partial vacuum). Vacuum filtration is used
in the organic chemistry laboratory primarily to collect solids (crystals). A heterogeneous mixture
of a solid suspended in a solution (called a slurry) is poured into a Büchner funnel containing a
filter paper (see the diagram below). Application of a partial vacuum draws the liquid through the
filter paper, leaving the solid behind (on the filter paper). The apparatus for vacuum filtration is
shown in the diagram below.

Question 26

what do boiling stones do?

Answer 26

as a rule, boiling stones must be added to any solution being heated to its boiling point.
Boiling stones (boiling chips) are made of a material with microscopic pores. These pores are filled
with air, which is released in the form of small bubbles when the temperature of the liquid
increases during heating – the bubbles promote smooth boiling. The stream of air bubbles from
the boiling chips causes turbulence in the liquid and prevents the formation of large bubbles in the
liquid. If large bubbles form, they may erupt violently and cause bumping of the liquid. Fresh
boiling stones should be added each time a solution is heated (or reheated) to boiling.

Question 27

1. List at least four pieces of safety equipment located in the organic chemistry laboratory.
2. When might ‘hot gravity filtration’ be used for the during recrystallization?
3. What would you do if, after completely dissolving your salicylic acid in the hot solvent and
   then cooling it to room temperature, crystals did not appear?
4. How would the percent recovery and the melting point of the product be affected if the crystals
   were not completely dried?
5. Why is water a suitable solvent for salicylic acid recrystallization?
6. What factors would affect the melting point range during melting point determination?

Answer 27

1. eye wash station, shower, fire extinguisher, first aid kit
2. when the desired product is soluble in hot solvent but precipitates in cool solvent
3. boil off solvent
4. melting point would be lower and percent recovery would be too high
5. like dissolves like - polar properties, etc mentioned above
6. mentioned above