To oxides phenylmethanol to benzoic acid

Describe the appearance of phenylmethanol at room temperature

Colourless liquid

Name a suitable piece of apparatus for measure a precise small volume of the phenylmethanol

Graduated dropper

Why is a saturated solution of potassium permanganate used

It’s an oxidising agent used to ensure it’s in excess, it will oxidise all of phenylmethanol to benzoin acid

Why is sodium carbonate required?

To create alkaline conditions so it works best and faster

Identity the source of a smell of almonds during the heating phase

Benzaldehyde

What is observed in the conical flask as the reaction mixture of phenylmethanol, potassium manganate (VII) and sodium carbonate is heated ?

Purple turns to a brown precipitate

Explain colour change in conical flask

The Mn7+ ions in KMnO4 are reduced to Mn4+ ions in MnO2 in the alkaline conditions, Mn4+ ions cause a brown colour, MnO2 is not soluble in water

What intermediate organic compound is formed in the conical flask?

Sodium benzoate

Give three reasons why concentrated hydrochloric acid is added to the conical flask?

1. To convert sodium benzoate to benzoin acid
2. To neutralise any sodium carbonate and potassium hydroxide
3. To create an acidic environment for Mn4+ ions to be reduced to Mn2+

Why is sodium sulfite (Na2SO3) added to the conical flask?

its a reducing agent it reduces the insoluble mn4+ to soluble mn2+

what is observed when sodium sulfite is added to the flask

brown precipitate decolourises and white crystals become visible

why is the conical flak placed in an ice bath

benzoic acid is only slightly soluble in cold water and it helps to ensure crystallisation of benzoic acid to maximise the yield

how are the benzoic acid crystals obtained after they have been crystallised

by vacuum filtration and the conical flask is washed with filtrate and refiltered

why is vacuum filtration better than gravity filtration

it speeds up the process and dries the crystals

why are they rinsed with cold water

to remove any soluble impurities

how are benzoic crystals dried

left to air dry on filter paper in a warm dry place or they are placed in a desiccator

Why are the benzoic acid crystals dissolved in the minimum amount of boiling water?

benzoic avid crystals are very soluble in hot water and it causes the solution to be saturated causes the maximum amount of pure benzoic crystals

**Why is the hot saturated benzoic acid solution filtered?**

to remove insoluble impurities

Why is water an ideal solvent for recrystallisation of benzoic acid?

benzoic acid is only slightly soluble in cold water but its very soluble in hot water

Why is the funnel used heated?

to avoid crystallisation while filtration is taking place

Why is the filter paper fluted?

to increase the surface area

What precaution should be taken if any benzoic acid crystals form during the filtration?

add some hot water

Why is the hot saturated benzoic acid solution cooled?

to allow recrystallisation to take place

Why does this stage remove soluble impurities from the benzoic acid?

benzoic acid crystals are only slightly soluble in cold water so they recrystallise out leaving the soluble impurities dissolved

Why is the solution first cooled to room temperature before being placed in an ice bath to cool?

to ensure the recrystallisation is complete to ensure the maximum yield

What precaution can be taken if crystals are slow to form?

1. add a 'seed’ chip
2. scrape the sides with a glass rod
3. evaporate some solvent

What is the purpose of vacuum filtering the solution?

to remove soluble impurities and isolate the pure benzoic acid

at what two stages is the temperature recorded

1. when the crystals start melting
2. when the crystals stop melting

What is the melting point range of pure benzoic acid?

121-122

two differences between the melting point of pure and impure benzoic acid

1. the melting point is below 121
2. it has a wider wage of temperature