Practical techniques Flashcards
how do you recrystallise impure crystals to obtain pure benzoic acid
dissolve in the minimum quantity of hot water/ solvent
cool and filter and leave to dry
safety issue for using lead oxide, and precaution
compounds may be toxic
use fume cupboard
lead is toxic
wear gloves
methane is flammable
keep away from flame
how can you make sure that all oxygen has been removed from practical reacting Pb2O3 with ch4
-heat to constant mass, ensures all lead oxide has been reacted
-use excess methane ensuring all lead oxide has been reacted
Heating under reflux
procedure used to prepare an organic liquid without boiling off the solvent, reactants or products.
Draw a reflux setup and label everything
textbook and whiteboard.
when can a water bath be used for reflux?
if the reaction can be carried out below 100 degrees
when is a heating mantle used for reflux?
when heating flammable liquids so there is no naked flame present. Adds a level of safety if apparatus cracks and leaks.
Why are anti bumping granules added to the pear shaped flask’s liquid?
Added before flask is heated so that contents boil smoothly.
If not, large bubbles form at the bottom of liquid and glassware vibrates and jumps violently.
How and why do you add grease to the ground glass joint?
-Place condenser carefully into flask and rotate condenser gently back and forth to provide a good seal
-ensures apparatus comes apart easily at the end of experiment
Why should condensers be clamped on loosely?
glass outer jacket of condenser is very fragile so is easily broken
What would happen if you placed a stopper at the top of the condenser?
you would have a closed system and pressure would build up inside as heated air expanded. apparatus could explode
why does water always enter condenser at bottom and leave at top
to ensure that outer jacket is full
What does heating under reflux allow? 4
- a liquid to be continually boiled while a reaction takes place
-prevents volatile components from distilling out and escaping, and the flask from boiling dry.
-the vapour from the mixture rises up inner tube of condenser until it meets the outer jacket containing cold water
-the vapour then condenses and drips back into flask.
(like saucepan lid over past)
What is distillation used for?
used to separate pure liquids from its impurities.
draw a distillation set up providing reasons for each piece of equipment
use textbook and whiteboard
how is the pear shaped flask clamped in distillation?
by its neck as still head is connected to the flask
why is a still head adaptor t shaped with 2 ground glass joints?
one to fit the screw tap adaptor and one to fit the condenser
what is the other side of the condenser (not connected to flask) connected to in distillation?
the receiver and conical flask
why is the 2nd clamp in distillation placed around the receiver adaptor where condenser is attached?
removed need to clamp the condenser as as its supported sufficiently at both ends
where is the lowest point on the condenser in distillation?
by receiver adaptor
why is a flask used to collect distillate?
so that the distillation apparatus is not completely airtight
how are liquids separated during distillation 4
-flask is heated and mixture in the flask will start to boil
-different liquids in mixture have different boiling points , most volatile has lowest boiling point so will boil first
-vapour moves up flask into other parts of apparatus leaving behind less volatile components of mixture
-when vapours reach cold condenser, they condense and become liquid, and drip into collecting flask
how can you tell that water has accidentally been obtained?
you will see 2 liquid layers inside collection flask
how do you separate water/ aqueous and organic layer? 7
-ensure tap of funnel is closed
-pour mixture of liquids into separating funnel
-place stopper in funnel and invert to mix contents and then allow layers to settle
-add water, whichever layer increases in volume is the water/ aqueous layer
-place a conical flask under separating funnel, remove stopper and open tap until whole of lower layer has left funnel.
-use another separate conical flask and place under separating funnel to collect other layer
-label 2 flasks so you dont muddle organic and aqueous layer
how do you remove acid impurities from impure product?
- ass ad sodium carbonate and shake mixture in separating funnel
-any acid releases co2.
-tap should be slowly opened, holding stoppered separating funnel upside down to release gas pressure buildup
-now na2co3 layer is removed and organic layer can be washed with water before running both of these layers into separate flasks
how do you remove water traces in organic product?
add drying agent:
CaSO4 or MgSO4 for general drying
CaCl2 for drying hydrocarbons
How do you dry an organic liquid?
-add organic liquid into conical flask
-add some of drying agent to liquid and gently swirl contents together with spatula
-place stopper in flask to prevent product from evaporating away. leave for 10 mins
-if solid is stuck together in lump, there is still water so add more until more solid is dispersed in solution as fine powder
-gradually pour (decant) liquid from solid into another flask. if liquid is dry its clear
drying agent
anhydrous inorganic salt that readily takes up water to become hydrated
why do we do redistillation
-as organic liquids may have boiling points relatively close together, prepared sample may have organic impurities.
-SO clean and dry distillation setup but only collect product with bp of compound you are trying to make.
the narrower the boiling range
the purer the product
