Recrystallisation of benzoic acid

What is benzoic acid?

Benzoic acid is an aromatic carboxylic acid.

What is benzoic acid used for?

Benzoic acid and its salts are used as food preservatives.

How can impurities from a sample of benzoic acid be removed?

We can remove the impurities by putting the sample through double filtration.

What types of impurities does the benzoic acid contain?

Both soluble and insoluble impurities.

What is recrystallisation?

Recrystallisation is a technique where we begin with crystals, form a solution, remove impurities and reform the crystals.

Why is it essential that we form a saturated solution?

In order to end up with crystals by the end of the experiment.

What type and amount of water is the benzoic acid dissolved in?

The benzoic acid containing both soluble and insoluble impurities is dissolved in the minimum amount of hot water to form a saturated solution.

What does the minimum amount of hot water dissolve?

The benzoic acid and its soluble impurities.

Why do we dissolve the benzoic acid in the minimum amount of hot water?

This maximises the yield of the pure benzoic acid.

What is the hot solution then passed through?

It is passed quickly through fluted filter paper in a funnel that has been warmed and ideally has a short stem.

Why do we filter the benzoic acid in a funnel that has been warmed and has a short stem?

This allows the solution containing the benzoic acid and the soluble impurities to pass through quickly before it begins to recrystallise.

Where are the insoluble impurities left behind?

The insoluble impurities are left behind in the fluted filter paper.

Why do we use fluted filter paper?

Fluted filter paper has a larger surface area so speeds up the rate of filtration.

What do we do after filtration?

The conical flask is placed in an ice bath.

Why is the conical flask placed in an ice bath?

To help precipitate the benzoic acid out of solution. Hence, maximising the yield of crystals.

What process can we use if the crystals are slow to come out of solution?

Seeding

Describe how to carry out “seeding”

A few crystals of pure benzoic acid are added to the filtrate and the crystals in solution build up around these and so come out of solution.

What do we do after we have placed the conical flask in the ice bath?

We carry out vacuum filtration to isolate the benzoic acid.

Give two benefits of vacuum filtration.

It is faster than gravity filtration

It also helps to pull the water away from the crystals.

Where do the benzoic acid crystals end up? (after vacuum filtration)

The benzoic acid crystals remain behind in the funnel.

Where do the soluble impurities end up after vacuum filtration?

The soluble impurities pass through the water into the Buchner flask.

Why can’t the soluble impurities form a solution?

The concentration of the soluble impurities is too low for them to form a solution.

How do we dry the crystals?

The crystals are allowed to dry out overnight or can be placed in a dessicator.

Would water by an impurity?

Yes ad it would change the melting point of the crystals.

What is the melting point of a substance?

The melting point of a substance is the temperature from which it changes from a solid to a liquid.

What is a characteristic property of the benzoic acid?

Its melting point (in fact melting point may be used to identify benzoic acid)

What is the melting point of pure benzoic acid?

122 degrees

What are 2 things these impurities (like water) will do to the melting point?

It will lower the melting point.

It will cause the solid to melt over a range of temperatures (it will not melt sharply)

What will we use to test the melting point of a substance?

An aluminum melting block.

Define homologous series

A series of compounds of similar chemical properties that show gradations in physical properties and have a general formula for its members with each member differing from the previous member by 1 CH2 unit.

Define aliphatic compounds and give 2 examples of them.

Aliphatic compounds are organic compounds consisting of open chains of carbon atoms OR closed chains of carbon atoms (rings) that resemble them in chemical properties.

eg hexane, cyclohexane

What are aromatic compounds?

Compounds that contain a benzene ring structure in their molecules.

What if some crystals recrystallise whilst the insoluble impurities are being filtered off?

A dropper is used to add a small amount of hot water to dissolve the crystals.

What if crystals are observed repeatedly coming out of solution?

Heat the conical flask from time to time on the hot plate.

How do we heat the conical flask and funnel?

By placing them in a beaker of hot water.

What are the crystals washed with in the Buchner flask?

Washed with some ice, deionised water.

What do we place the crystals in when determining melting point?

A very narrow glass tube called the capillary tube.

What do we use to grind up the crystals?

A pestle and mortar