Practical Skills - Organic Preps

Question 1

How do you calculate the density of a liquid?

Answer 1

density = mass / volume

check units!

Question 2

How do you cool an organic liquid?

Answer 2

Use an ice bath

Question 3

What is reflux?

Answer 3

Reflux allows the reactants (and products) to be heated but not escape from the reaction vessel (as heating causes them to evaporate or even boil). Any escaping vapour condenses and falls back into the flask.

Question 4

Draw a diagram for heating by reflux

Answer 4

Question 5

What do antibumping granules do?

Answer 5

Any bumping granules are placed in the flask to prevent bumping, (the sudden release of a large bubble of vapour that makes the reaction mixture jump up). The anti-bumping granules produces many small bubbles rather than a large one.

Question 6

How would you purify a liquid?

Answer 6

Distill the reaction mixture collecting at the boiling point for the desired compound.

Question 7

Draw a diagram of distillation

Answer 7

Question 8

When separating out a solid from a solution why might you add ice to the solution

Answer 8

This reduces the solubility and so the solid will come out of soution and canbe collected

Question 9

How do you collect a solid from a solution/liquid

Answer 9

Use a Buchner funnel to collect the solid under reduced pressure

Question 10

Draw a diagram of a Buchner Funnel

Answer 10

Question 11

How do you purify an impure solid?

Answer 11

Solids are purified by recrystallisation.

Question 12

Describe how recrystallisation works.

Answer 12

The impure solid product is dissolved in the minimum amount of a hot solvent to create a solution saturated with the product.

Any insoluble impurities can be filtered out (but the apparatus must be heated so the solution does not cool as it is filtered).
The saturated solution is then allowed to cool slowly, and so the product crystallises out from the solution while any other impurities remain dissolved.

Question 13

In recrystallisation why is the saturated solution cooled slowly?

Answer 13

If the saturated solution is cooled too quickly many of the impurities crystallise out as well.

Question 14

In recrystallisation why is a minimum quantity of hot water was used?

Answer 14

To ensure the hot solution would be saturated and crystals would form on cooling.

Question 15

In recrystallistion the flask is left to cool before crystals are filtered off. Why?

Answer 15

Solubility of the product will be higher if warm so the yield will be lower.

Question 16

In recrystallisation the solid sample is collected and compressed into a Buchner funnel for filtering. Why is it compressed in the funnel?

Answer 16

Air will pass through the sample (rather than around it) and ensure better drying odf the sample.

Question 17

In recrystallisation, before the sample is filtered to remove water a little cold water is poured through the crystals. Why do this?

Answer 17

To wash away soluble impurities

Question 18

Why do we dissolve an impure sample in a solvent before doing recrystallisation?

Answer 18

So that any insoluble impurities can be removed

Question 19

How do you purify a liquid?

Answer 19

Put it in a separating funnel with a solvent that does not mix with the
product, but in which impurities may dissolve.

Question 20

How do you remove water from a liquid that you are trying to purify?

Answer 20

Add a chemical that absorbs water, such as anhydrous sodium sulphate or anhydrous calcium chloride.

Question 21

After purifying a liquid by distillation and then adding other solvents to remove further impurities, how could you further purify a liquid?

Answer 21

Perform another distillation and then collect the product over a very narrow temperature range (e.g. 5°C).

Question 22

How do you test the purity of a product?

Answer 22

See what temperature the product boils or melts at. The narrow the range the purer the product

Question 23

If a substance melts over a road temperature range, what can you say about its purity?

Answer 23

It is impure

Question 24

How do you measure the yield of a reaction?

Answer 24