Practical Techniques Flashcards
Steam distilllation method?
Organic liquids that have very high BP or decompose upon heating and are insoluble in water —> STEAM DISTILLATION
Steam is passed into mixture and product vapour is distilled off with steam and condensed
- organic compound and water separated with separating funnel
ADVANTAGE: product distils at a lower temp which prevents decomposition of product if it has a high BP
How can melting point indicate purity?
MP of impure compound will be LOWER than MP of PURE compound/lower than value in data booklet
Melting range will be broader if impure
Can be measured in an electronic melting point machine OR use capillary tube strapped to thermometer immersed in heating oil (which has higher BP than sample’s MP)
- put some of sample in capillary tube + tube is heated slowly near MP
Error could occur if temp of thermometer is not same as temp in the capillary tube
This question is about extracting benzoic acid from a mixture of benzoic acid, C6H5COOH,
and phenol, C6H5OH.
The following steps were carried out.
A suitable mass of the mixture was placed in a separating funnel and some ether added. The funnel was shaken to dissolve the mixture.
Aqueous sodium hydrogencarbonate was added to the separating funnel and the contents shaken.
Once the reaction was complete, the two layers were allowed to separate.
The lower aqueous layer was removed and placed in a beaker.
The ether layer in the separating funnel was washed with deionised water and the washings added to the beaker.
Hydrochloric acid was added to the aqueous solution in the beaker to precipitate the benzoic acid
The impure benzoic acid was filtered under reduced pressure.
The impure benzoic acid was purified by recrystallisation.
The melting temperature of the purified benzoic acid was measured and compared with the literature value of 122 degrees
Why is the ether layer washed with deionised water?
Why does the addition of HCl result in precipitation of benzoic acid?
A. Some of the sodium benzoate is dissolved in the ether (C6H5COONa made from benzoic acid + sodium hydrogen carbonate)
B. Adding HCl protonates the benzoate ion (in sodium benzoate) forming benzoic acid
Benzoic acid is less soluble in water than the sodium salt (sodium benzoate)
Give a possible reason why the boiling tube is clamped at an angle?
(During dehydration reaction of ethanol with the catalyst, aluminium oxide)
What would happen if ethanol is heated instead of the catalyst?
Describe a safety issue if th heat source was moved to the rubber coated clamp?
I. Ethanol could ‘run’/leak down the tube to the catalyst
(Ethanol is at the bottom of boiling tube and soaks wool , and the catalyst is further up the boiling tube)
II. Ethanol would evaporate + pass over catalyst without reaction
III. Clamp may burn/rubber will melt
