Chemistry - Benzoic acid expeirment Flashcards

3:3

Phenylmethanol
add Excess potassium manganate
benzaldehyde (temp almond smell)
sodium benzoate (Intermediate)
Add HCl, Add Na2SO3
benzoic acid

Phenylmethanol added to conical flask.
saturated sln of Potassim manganate VII added.
Sodium Carbonate added.
Heat in boiling water bath for 20 mins
flask cooled under boiling water

colourless liquid

Graduated dropper

KMnO4 is a oxdising agent. it oxidises the phenylmethanol
a saturated solution is used to ensure it is in excess to ensure that sufficent is present to oxidise all of the pheylmethanol to benzoic acid - keeps phenylmethanol as the limiting reagent

to create alkaline/ basic conditions.
The oxidation of phenylmethanol to benzoic acid works best and faster at a alkaline pH

formation of benzaldehyde

the purple colour turns to a brown percipertate.
the mn7+ ions are reduced to Mn4+ ions in the alkaline conditions
Mn4+ ions cause a brown colour
MnO2 is not soluble

sodium benzoate

In fumehood, place concentrated HCl in conical flaskSodium sulfite solution added to conical flask using dropper Conical flask continously swirled until brown percipitate dissapears
conical flask appered in ice bath
white crystals of benzoic acid observed

1- to convert the sodium benzoate intermediate into benzoic acid
2- to neutalise any excess alkaline sodium carbonate and potassium hydroxide present
3 - to create an acidic enviorment for Mn4+ ions to be reduced to Mn 2+ ions

a glass rod is dipped into the solution and onto blue litmus paper, when the blue litmus paper turns red, enough acid has been added

sodium sulfite is a reducing agent
It is added to reduce the insoluble Mn4+ ions present to soluble Mn2+ ions in acidic conditions

the brown precipate decolourises - insoluble Mn4+ ions are reduced to soluble Mn2+ ions
the white crystals become visible - benzoic acid crystals form

+7 - +4 - +2

benzoic acid is very soluble in hot water but only slighly soluble in cold water.
Placing the comical flask in an ice bath hekps to ensure the crystalisation of the benzoic acid crystals our of solution is complete and maxmises the yeild of crystals

the benzoic acid crystals are filtered using vaccum filtration
the conical flask is rised with filtrate from the buchner flask and is re-filtered to ensure maxium yeild of crystals obtained
benzoic acid crystals washed out with COLD WATER
left to air dry on the filter paper

by vacumm filtration
The conical flask is rised with the filtrate and is re-filtered to ensure maxium yeild of benzoic acid crystals obtained

to speed up the filtration process - faster than using regular filter paper
to help dry the crystals

to remove any solid impurities present on the filter paper or on the crystals - the water must be cold to prevent any benzoic acid crystals redissolving

allowd to air dry on the filter paper in a warm place

REDISSOLVING THE BENZOIC ACID AND HOT FILTERING - REMOVING SOLUBLE IMPURTIES
a known mass of the impure benzoic acid crystals are dissolved in the minium amount of boiling water
The saturated sln of benzoic acid sln is filtered by Hot filtration to remove insoluble impurities

RECRYSTALLISATION - REMOVING SOLUBLE IMPURTIES
the saturated benzoic acid solution is allowed to cool to room temperture and then placed in ice bath causing the dissolved benzoic acid crystals to rrystallise out of sln while the soluble impurties remain in solution

the benzoic acid crystals are filtered using vacuum filtration
the conical flask is rinsed with the filtratr from the Buchner flask and is re-filtered to ensure maxium yield of benxoic acid crystals are reformed.
The crystals are washed with cold water and then left dry on the filter paper

Insoluble impurties eg charcoal
soluble impurties eg sodium chloride

Benzoic acid crystals dissolced in the minium amount of boiling water as
Benzoic acid crystals are very soluble in hot water
Using the mininium boiling water causes the benzoic acid to be staurated
the causes the maxium amount of pure benzoic acid crystals to recrystallise out of solution

to remove any insoluble impurties

benzoic acid crystals are very soluble in hot water but only slightly soluble in cold water

the surface area of fluted filter paper is greater - the filtration process will be quicker

Using a dropper, add some hot water to dissolve the crystals

If there are no insoluble impurties

To allow recrystillisation to take place

dissolving crystals in a hot solvent to form a saturated solution and cooling this saturated solution to reform crystals that are more pure

as benzoic acid crystals are only slightly soluble in cold water, they recrystalise out of solution while soluble impurties remain dissolved

to ensure recrystallisation is complete and obtain the maximim yield of benzoic acid

a seed crystal of pure benzoic acid is added to the solution
scratch the inside of the conical flask with a glass rod

prepare and isolate the benzoic acid crystals

to speed up the filtering process
to help dry the crystals

allowed to air dry n the filter paper in a warm place

the benzoic acid crystals are ground up using a pestal or mortar
the crystals are placed in a melting point capillary tube which is placed into an aluminium melting block
the aluminium melting block is slowly heated using a hot plate while observing crystals.
the melting point range of benzoic acid crystals is recorded using a thermometer

the temperture is recorded when the benzoic crystals begin to melt the benzoic crystals have finished melting

the two tempertures gives the melthing point range

the melting poit of the benzoic crystals are found are compared to the literate value (121-122)

121- 122

the melting point of impure benzoic acid will be lower than 121-122
the crystals will melt over a wider range of temperture

the % yeild of purfied crystals
the melting point of the purified crystals is dertimed and compared to orginal impure crystals

To avoid early crystallisation of benzoic acid crystals while the hot filtration is taking place

benzoic acid and salts used are food presertavies