Oxidation of phenylmethanol to benzoic acid, and recrystallization of benzoic acid

CALCULATIONS - Q2 2020, Q2 2015

Explain why water is a suitable solvent for carrying out the recrystallisation of benzoic acid

Benzoic acid has a high solubility in hot water

Explain the purpose of the first (hot) filtration

[experiment to purify benzoic acid by recrystallisation]

To remove insoluble impurities

Fluted filter paper in a warm funnel, as shown in the diagram is used in the hot filtration.

Explain why this arrangement is suitable for hot filtration

Increased surface area / faster filtration / keeps solution warm / avoids benzoic acid coming out of solution

Explain why the second filtration is only carried out after a sufficient period of cooling

To maximise the yield / to allow crystallisation

What observation would the student make that shows that the recrystallised benzoic acid was pure?

Melting point

over a narrow range / sharp / close to correct value

Describe with the aid of a labelled diagram, a method that could be used to measure the melting point range of a sample a benzoic acid

Sample (crystals) on aluminium block (shown in diagram) //

heat slowly (heat source shown in diagram) //

record the melting point temperature range

How could it have been verified that the recrystallised benzoic acid crystals were purer than the original sample?

Compare melting points

Impure (original) melting point lower, broader

purer (recrystallised) melting point higher, sharper

A student practiced the technique of recrystallisation of benzoic acid containing small quantities of two solid impurities: water-soluble potassium chloride (KCl) and black, insoluble charcoal (C).

Identify the substances collected at locations X, Y and Z

X: Charcoal / carbon / C //

Y: Benzoic acid //

Z: Potassium Chloride / Water

What steps are carried out to maximise the yield of pure benzoic acid,

i) between the first and second filtrations,

ii) after the second filtration?

i) Evaporate some water / heat filtrate / put flask in ice-water / cooling of flask / allow time for crystals to form

ii) Wash with cold water / dry crystals / recover more crystals from mother liquid / rinse flask into funnel

How would you expect the melting point value of a recrystallised sample of benzoic acid to differ from that of an impure sample?

Melting point higher / sharper range

or

Melting point impure crystals lower / broader range

Describe the appearance of phenylmethanol at room temperature

Colourless

Name a suitable piece of apparatus to measure accurately 1.5cm³ of phenylmethanol

Syringe / graduated dropper / graduated micropipette / burette

How does the appearance of the reaction mixture change when the phenylmethanol is heated along with potassium manganate(VII) solution to which sodium carbonate has been added?

Purple (violet) to //

brown

Describe what is observed during stages D and E.

Explain these observations

Brown solid disappears //

white benzoic acid crystals form / white solid appears

MnO₂ reduced / soluble (colourless) Mn²⁺ formed //

crystallisation of benzoic acid

Name a suitable technique to purify the benzoic acid crystals collected at stage F.

Recrystallisation

A sample of benzoic acid, containing a small quantity of sodium chloride as the only impurity, was purified by recrystallisation. The impure crystals were dissolved in the minimum amount of boiling water. This solution was cooled thoroughly and the crystals that formed were separated by suction filtration as shown in the diagram. The crystals were then dried.

i) Why was there no need, in this recrystallisation, to carry out a filtration of the hot solution?

No insoluble impurities /

hot filtration removes insoluble impurities /

only impurity soluble /

sodium chloride soluble

ii) State one advantage of suction filtration over gravity filtration

Faster / helps dry the crystals

iii) Explain how you could verify that the recrystallised benzoic acid was purer than the original sample

Compare melting points of both samples

Impure (original) melting point lower, broader

purer (recrystallised) melting point higher, sharper

The melting-point range of benzoic acid sample F is lower and broader than that of G.

Which is the purer benzoic acid sample?

G /

F is less pure

Describe the appearance of phenylmethanol at room temperature

Colourless

Identify A and B in the reaction scheme that were added to allow separation of the benzoic acid from the other substances after the oxidation was complete

A = HCl

B = Na₂SO₃

What changes were observed in the reaction vessel on addition of HCl and Na₂SO₃ and as cooling occurred?

Brown disappears / mixture becomes colourless //

white crystals precipitate (form / appear)

State the changes in the oxidation number of manganese during the experiment

7 to 4 / Mn⁷⁺ to Mn⁴⁺ //

4 to 2 / Mn⁴⁺ to Mn²⁺

State one way of maximising the yield of the recrystallisation process

Use minimum amount of boiling solvent (water) / cool fully / place in ice / use fluted filter paper

The melting points obtained by a student measuring the melting points of two benzoic acid samples were as follows:

A = 117 - 120^oC

B = 120 - 121^oC

Which was the purer sample? Justify your answer

B

higher melting point / narrower range

Students were required to recrystalize the impure benzoic acid. What solvent should they have used for the recrystallisation? Explain why this solvent is suitable

Water

Benzoic acid very soluble in hot water but less soluble in cold

Give one important use benzoic acid or its salts

Food preservative / disinfectant