Chemistry - Benzoic Acid

Phenylmethanol

oxidised to benzoic acid

Anhydrous Sodium Carbonate

used to make the solution alkaline

KMnO4

mixed with anhydrous sodium carbonate and phenylmethanol in a conical flask

Purple to Brown

colour change due to the formation of MnO2

Water Bath

conical flask is placed in here for 20 minutes to be heated

Conical Flask Contents

(after heating)

• sodium benzoate

• manganese dioxide

• potassium hydroxide

• water

Running Tap

used to cool outside of the conical flask

Hydrochloric Acid

added until one drop of the solution turns moist blue litmus paper red

HCl Roles

• sodium benzoate converted to benzoic acid

• neutralises sodium carbonate

Sodium Sulphite

reducing agent that is added to change MnO2 to Mn²⁺

Benzoic Acid Crystals

• solution with crystals placed in beaker of ice (to encourage recrystallisation)

• dried

• mass measured

Impure Benzoic Acid

dissolved in minimum hot solvent (water)

Water

• benzoic acid has low solubility in cold ____

• benzoic acid has high solubility in hot ____

First (Hot) Filtration

removes any insoluble impurities

Cool Filtration

recrystallises benzoic acid

Filter

to remove soluble impurities

Cold Solvent

crystals are washed with this to wash away last traces of impurity

Aluminium Block

benzoic acid crystals are heated on this during melting point determination

Mineral Oil

placed in the hole with the thermometer to ensure good thermal contact

Melts Sharply

indicates pure recrystallised benzoic acid