Practical techniques

Question 1

how do you recrystallise impure crystals to obtain pure benzoic acid

Answer 1

dissolve in the minimum quantity of hot water/ solvent
cool and filter and leave to dry

Question 2

safety issue for using lead oxide, and precaution

Answer 2

compounds may be toxic
use fume cupboard

lead is toxic
wear gloves

methane is flammable
keep away from flame

Question 3

how can you make sure that all oxygen has been removed from practical reacting Pb2O3 with ch4

Answer 3

-heat to constant mass, ensures all lead oxide has been reacted
-use excess methane ensuring all lead oxide has been reacted

Question 4

Heating under reflux

Answer 4

procedure used to prepare an organic liquid without boiling off the solvent, reactants or products.

Question 5

Draw a reflux setup and label everything

Answer 5

textbook and whiteboard.

Question 6

when can a water bath be used for reflux?

Answer 6

if the reaction can be carried out below 100 degrees

Question 7

when is a heating mantle used for reflux?

Answer 7

when heating flammable liquids so there is no naked flame present. Adds a level of safety if apparatus cracks and leaks.

Question 8

Why are anti bumping granules added to the pear shaped flask’s liquid?

Answer 8

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.

Question 9

How and why do you add grease to the ground glass joint?

Answer 9

-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

Question 10

Why should condensers be clamped on loosely?

Answer 10

glass outer jacket of condenser is very fragile so is easily broken

Question 11

What would happen if you placed a stopper at the top of the condenser?

Answer 11

you would have a closed system and pressure would build up inside as heated air expanded. apparatus could explode

Question 12

why does water always enter condenser at bottom and leave at top

Answer 12

to ensure that outer jacket is full

Question 13

What does heating under reflux allow? 4

Answer 13

• 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)

Question 14

What is distillation used for?

Answer 14

used to separate pure liquids from its impurities.

Question 15

draw a distillation set up providing reasons for each piece of equipment

Answer 15

use textbook and whiteboard

Question 16

how is the pear shaped flask clamped in distillation?

Answer 16

by its neck as still head is connected to the flask

Question 17

why is a still head adaptor t shaped with 2 ground glass joints?

Answer 17

one to fit the screw tap adaptor and one to fit the condenser

Question 18

what is the other side of the condenser (not connected to flask) connected to in distillation?

Answer 18

the receiver and conical flask

Question 19

why is the 2nd clamp in distillation placed around the receiver adaptor where condenser is attached?

Answer 19

removed need to clamp the condenser as as its supported sufficiently at both ends

Question 20

where is the lowest point on the condenser in distillation?

Answer 20

by receiver adaptor

Question 21

why is a flask used to collect distillate?

Answer 21

so that the distillation apparatus is not completely airtight

Question 22

how are liquids separated during distillation 4

Answer 22

-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

Question 23

how can you tell that water has accidentally been obtained?

Answer 23

you will see 2 liquid layers inside collection flask

Question 24

how do you separate water/ aqueous and organic layer? 7

Answer 24

-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

Question 25

how do you remove acid impurities from impure product?

Answer 25

• 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

Question 26

how do you remove water traces in organic product?

Answer 26

add drying agent:
CaSO4 or MgSO4 for general drying
CaCl2 for drying hydrocarbons

Question 27

How do you dry an organic liquid?

Answer 27

-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

Question 28

drying agent

Answer 28

anhydrous inorganic salt that readily takes up water to become hydrated

Question 29

why do we do redistillation

Answer 29

-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.

Question 30

the narrower the boiling range

Answer 30

the purer the product