Benzoic acid expeirment Flashcards by Sara li

Q

What is the molar ratio between phenylmethanol and benzoic acid? (NB)

To oxidise phenylmethanol to benzioc acid

A

3:3

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Q

describe the rough overall process To oxidise phenylmethanol to benzioc acid

A

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

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Q

describe step 1 of oxidising phenylmethanol to benzioc acid

A

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

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Q

describe the apperance of the phenymethanol/ benzyl alcohol at room temperture

Step 1 To oxidise phenylmethanol to benzioc acid

A

colourless liquid

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Q

Name a suitable piece of apparatus for measuring a precise small volume of the pheylmethanol

Step 1 To oxidise phenylmethanol to benzioc acid

A

Graduated dropper

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Q

why is potassium manganate used, why is a saturated solution used?

Step 1 To oxidise phenylmethanol to benzioc acid

A

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

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Q

why is sodium carbonate required

Step 1 To oxidise phenylmethanol to benzioc acid

A

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

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Q

Identify where the almond smell occours from

Step 1 To oxidise phenylmethanol to benzioc acid

A

formation of benzaldehyde

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Q

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

Step 1 To oxidise phenylmethanol to benzioc acid

A

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

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Q

What intermediate organic compound is formed in the conical flask?

step 1 To oxidise phenylmethanol to benzioc acid

A

sodium benzoate

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Q

describe the process for step 2

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

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

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

How can the solution be tested for that enough acid had been added to the conical flask

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

why is sodium sulfite Na2SO3 added to the conical flask

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

What two obervations can be made upon the addition of the sodium sulfite and the swriling of the conical flask? (NB)

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

state the changes in oxidising number of manganse during the expeirment

step 2 :To oxidise phenylmethanol to benzioc acid

A

+7 - +4 - +2

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Q

why is the conical flask placed in an ice bath

step 2 :To oxidise phenylmethanol to benzioc acid

A

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

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Q

Describe step 3

step 3 : to oxidise phenylmethanol to benzioc acid

A

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

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Q

How are the benzoiz acid crystals isolated/ obtained after they have crystallised?

step 3 : to oxidise phenylmethanol to benzioc acid

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A

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

Q

why is vaccum filtration performed in preference to gravity filtration?

step 3 : to oxidise phenylmethanol to benzioc acid

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A

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

Q

why are the crystals washed with cold water

step 3 : to oxidise phenylmethanol to benzioc acid

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A

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

Q

How are the benzoic acid crystals futher dried

step 3 : to oxidise phenylmethanol to benzioc acid

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A

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

Q

describe step 1

To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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A

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

Q

describe step 2

To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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A

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

Describe step 3 | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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 **

What imputies could be present in benzoic acid crystals | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

**Insoluble impurties** eg charcoal **soluble impurties** eg sodium chloride

why are the benzoic acid crystals dissolved in the minium amount of boiling water?**NB** | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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**

Why is the hot saturated benzoic acid solution filtered | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

to remove any insoluble impurties

**Why is water an ideal solvent for recystallisation of benzoic acid?** | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

why is the funnel used heated? | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

Why is the filter paper fluted | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

What percaution should be taken is any benzoic acid crystals form during crystilation? | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

In what situation may this hot filtering stage not be required | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

If there are no insoluble impurties

Why is the hot saturated benzoic acid solution cooled | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

To allow recrystillisation to take place

what is recrystallisation | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

Why does recrystalisation remove soluble impurties from the benzoic acid? | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

why is the solution first cooled to room temperture before being placed in an ice bath to cool | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

what percaution can be taken if the crystals are slow to form | To recrystallise a sample of ebnzoic acid/ purifying 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

What is the purpose of vacuum filtering the solution | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

prepare and isolate the benzoic acid crystals

why is vacuum filteration performed instead of gravity filtration | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

to speed up the filtering process to help dry the crystals

how are the benxoic crystals futher dried? | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

Give two procedures carried out on the pure dry benzoic acid crystals obtained | To recrystallise a sample of ebnzoic acid/ purifying benzoic acid

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

Describe the procedure | measure the meting point of pure + impure benzoic acid and compare them

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

how is the purity of benzoic acid crystals dertimed | measure the meting point of pure + impure benzoic acid and compare them

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

What is the literature value | measure the meting point of pure + impure benzoic acid and compare them

121- 122

Give two differences noted when determininh the melting point of impure benzoic acid | measure the meting point of pure + impure benzoic acid and compare them

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

give a common use of benzoic acid or of its salts | measure the meting point of pure + impure benzoic acid and compare them

benzoic acid and salts used are food presertavies