CHE311 Laboratory Midterm

Recrystallization

common strategy for the purification of solid materials

typical recrystallization includes

impure compound is dissolved in a hot solvent

as the solvent cools

the solubility of the compound decreases (compound begins to come out of solution- becomes solid)

common recrystallization solvents include

water, ethanol, chloroform, ethyl acetate, and hexanes

why are the crystals rinsed with ice cold solvent

to remove any residual impurities on the surface

The ideal solvent is one in which the compound being purified is

very soluble at high temperatures (fairly insoluble at lower temperatures)

melting point

the temperature at which the solid and liquid phases are in equilibrium

Most organic compounds melt over a range of

2-3 °C AND at low enough temperatures that their melting points can be conveniently determined

What are the two reasons that melting points are determined

to help identify an unknown compound & to indicate a compound's purity

pure compounds tend to have

narrow melting ranges

Adding two compounds together, at some composition there is a minimum melting mixture- this minimum is called the

eutectic point

The fact that impure solids have ___________ melting points than pure solids can be used to identify an ___________________.

lower; unknown compound

unknown mixed with known and melting point drops

known and unknown are NOT identical

unknown mixed with known and melting point does not drop

known and unknown are identical

If impurities are present the compound will exhibit a

lower and broader melting point

is melting point alone enough to identify a compound?

no

impurities disrupt

crystal lattice

these disruptions in the crystal lattice lower the amount of

energy required to melt the solid

percent recovery

(mass recovered [purified]/ initial mass[original]) x 100

Digimelt melting point apparatus

If you successfully removed impurities from your solid, the percent recovery should be less than

100%

Distillation

common technique for the purification of liquids based on differences in boiling point

Separatory funnel

evaporating dish

three-way distillation adaptor

bent distillation adaptor/receiving adaptor

round bottom flasks

magnetic stir bar

graduated cylinder

distillation condenser

fractionating column

erlenmeyer flask

heating mantle

the heating mantle should be plugged into a

voltage controller

analytical balance

balance

rotary evaporator

what is a rotary evaporator used for?

To remove the solvent (similar to vacuum distillation)

convection oven

percent yield

actual yield/theoretical yield x 100

dilution

M1V1=M2V2

conversion of mass to volume (or vise versa)

mass (g) = volume (mL) x density (g/mL)

what is the mixed melting point technique?

a small amount of the unknown compound is mixed with the authentic sample, and the melting point of the mixture is measured

if the two compounds are the same, the compound is still pure and

the melting point will stay the same

if the melting point drops

theres an impurity

boiling points of compounds are related to their

vapor pressure

vapor pressure

the pressure exerted by molecules in the gas phase when the gas phase is in equilibrium with the condensed phase

A substance with a higher vapor pressure is said to be more

volatile

vapor pressure changes with temperature, becoming higher at

higher temperatures

A substance's boiling point is the temperature at which its vapor pressure equals

atmospheric pressure

more volatile liquids have

lower boiling points

simple distillation

the sample is boiled and the resultant vapor is immediately collected through a condensing column

simple distillations typically doesn't give good separations unless there is a

large difference in boiling points (>50)

a more effective approach is to use

fractional distillation

what is the only difference between simple and fractional distillation

a vertical fractionating column is added

since the fractionating column is packed with beads, it increases the

surface area

like dissolves

like

Polar solvents will dissolve

polar or ionic molecules.

Nonpolar solvents will dissolve

nonpolar molecules

Solubility of solids increases with

temperature

solubility

a measure of how much solute will dissolve in a given amount of solvent

Quantitatively (solubility)

g/mL

Qualitatively (solubility)

soluble, sparingly soluble, insoluble

A good recrystallization solvent

High solubility at elevated temperatures; slight solubility at room temperature

Simple

for mixtures with large differences in their boiling points, usually greater than 50°C

Fractional

for mixtures with closer boiling points

Steam

for compounds that are immiscible with water

Vacuum

for high boiling mixtures or for air sensitive liquids

simple distillation is also useful for when there is a fairly small

amount of impurity

Although simple distillation seems like a simple task, it rarely

produces a pure distillate

A boiling mixture of liquids gives a mixture of gases, and this gas, when it condenses, produces an impure distillate. Therefore, the goal of simple distillation is to

minimize, rather than eliminate, impurities

Adding vacuum will effectively

lower the boiling point of the compound so as to avoid decomposition issues

To determine the boiling point of a compound under vacuum a ___________ is used

nomograph

vaccum distillation is useful when

Liquid or solid has a high boiling point - normal simple or fractional distillation leads to decomposition

Chromatography is the

separation of a mixture of two or more compounds between two phases, one of which is stationary and the other is mobile

If the compound prefers to adhere to the stationary phase

it will take longerto move through the column

If a compound prefers to stay with the mobile phase

it will move through the column quickly

which solvents mixture was used in lab for distillation?

cyclohexane/toluene