experimental techniques

Question 1

what is reflux used for

Answer 1

allows strong heating without losing volatile reactants and products

Question 2

how does reflux work

Answer 2

• volatile compounds evaporate and condense and fall back into the flask
• Liebig condenser has cold water running through the wall
  when hot evaporating substances hit the cold condenser they turn back into a liquid and return back to the round bottom flask to react further
  heating is done with an electric heater → mantle as we are using flammable liquids

Question 3

what is distillation used for

Answer 3

used when we want to separate substances with different bpts
distillation is useful when you want to extract a chemical before it reacts any further

Question 4

how does distillation work

Answer 4

gently heating the mixture will result in the compounds separating out in order of bpt

Question 5

what happens if your compound has a lower boiling point than the starting mixture in distillation

Answer 5

• you heat to the temperature of the boiling point of your compound you want to separate
  
  • collect product in a separate vessel

Question 6

what happens if your compound has a higher bpt than your starting mixture in distillation

Answer 6

• heat to the temp of the bpt of the compound you want to separate
  
  • compound will remain in round bottom flask

Question 7

what is steam distillation used for

Answer 7

used when we want to separate substances with high bpts or decompose when heated

Question 8

when is steam distillation used

Answer 8

if the product is immiscible with water then steam distillation is used to separate compounds that couldn’t be done under standard conditions

Question 9

how does steam distillation work

Answer 9

• if the substance were trying to collect is less volatile then the substances were trying to separate from it then the desired product will evaporate out of the flask with the steam
• the product is then condensed and collected in a separate flask
  the product can be separated using a separating funnel (if product is partially miscible then you have to use solvent extraction)

Question 10

how does steam distillation work

Answer 10

• if the substance were trying to collect is less volatile then the substances were trying to separate from it then the desired product will evaporate out of the flask with the steam
• the product is then condensed and collected in a separate flask
  the product can be separated using a separating funnel (if product is partially miscible then you have to use solvent extraction)

Question 11

what are the two types of filtration

Answer 11

gravity filtration
vacuum filtration

Question 12

what is gravity filtration used for

Answer 12

• used to separate solids from liquids
• used if we want to keep liquid and dispose of solid

Question 13

how is gravity filtration carried out

Answer 13

• place fluted filter paper in the funnel and dampen to make a seal
• pour the reaction mixture into the funnel slowly to stop overflowing
• gravity will pull through the liquid component into the vessel below which can then be purified if needed

Question 14

what is vacuum filtration used for

Answer 14

• used to separate liquids from solids
• a vacuum is used to help separate the liquid and solid thoroughly

Question 15

how does vacuum filtration work

Answer 15

• place a filter paper disc in the buchner funnel and dampen slightly to make a seal
• pour the reaction mixture into the bucner funnel with the vacuum line on
• vacuum creates a reduced pressure in the flask and pulls the liquid through → solid left in buchner funnel which can be recrystallized to purify it

Question 16

what is recrystallisation used for

Answer 16

used to purify solids

Question 17

how do you carry out recrystillisation

Answer 17

1) add just enough hot solvent to allow the impure solid to dissolve → this will mean you have a saturated solution of your impure product

2) allow the solution to cool down slowly → crystals ill start to form

3) impurities will remain dissolved in solution as there is a small quantity of them → is takes a lot longer for them to crystallise

4) filter to get the slid purified crystals and then wash with very cold solvent and dry them off

Question 18

what does the solvent do in recrystallisation

Answer 18

you want impure solid to dissolve fully in hot solvent but insoluble when its cold
- if not the substance wont dissolve in hot solvent and you cant filter the purified solvent if it isn’t soluble in cold solvent

Question 19

what can boiling points and melting points be used for

Answer 19

help detect impurities and identify compounds

Question 20

how does using boiling points to identify substances work

Answer 20

• gently heat the sample → measure the temp at which it distils using the thermometer → this is the bpt
• compare this bpt to the data booklet value
• the sample contains impurities if the bpt is higher than what is recorded in the data book as the sample will oil over a range of temps

Question 21

how does identifying a compound using melting points work

Answer 21

1) add a sample of the solid product into a capillary tube and place into the heating element of the mtp apparatus
2) slowly increase the temp until the substance starts to melt
3) there is a temp range from when the solid just started to melt to when it fully melts
4) compare the mtp against data book value
5) if substance contains impurities the mtp will be lower and the temp range the substance melts at will be larger

Question 22

what is the separation technique used for

Answer 22

separation techniques are used to remove impurities that are dissolved in water

Question 23

how does separation work

Answer 23

• add the products from distillation into a separating funnel and add water to dissolve soluble impurities to create an aqueous solution
• after allowing the solution to settle→ two layers will form
• top layer→ impure
• bottom layer→ aqueous layer containing water-soluble impurities
• drain the aqueous layer off

Question 24

how does purification by washing work

Answer 24

• removal of impurities my washing
  
  • wash the product with another liquid to remove impurities

Question 25

how does purification by drying work

Answer 25

• take impure product from the separating funnel and add to a round bottom flask
• can add a dehydrating agent such as anhydrous CaCl2 → remove aqueous substances still remaining
• filter the solid drying agent to remove, using fluted filter paper (to inc surface area)

Question 26

how do you add carbon atoms

Answer 26

friedel crafts alkylation
(nitrile produced using haloalkane and KCN)
->hydrolysis of a nitrile
grignards reagents

Question 27

how do you prepare a grignard reagent

Answer 27

bromoalkane + magnesium -> R-MgBr

conditions - ethoxyethane
reflux

Question 28

what are the properties of grignard reagent

Answer 28

act as nucleophiles because alkyl chain is slightly negitive, MgBr is slightly positive so is removed

Question 29

grignard + water

Answer 29

alkane + Mg(OH)Br

Question 30

grignard + CO2

Answer 30

Question 31

grignards + carbonyl

Answer 31

Question 32

aldehyde + grignard

Answer 32

Question 33

ketone + grignard

Answer 33

Question 34

what are the 2 methods of removing carbon atoms

Answer 34

Hoffman degradation
hunsdieker reaction

Question 35

what is the general equation for Hoffman degradation

Answer 35

amide (R- CONH2) + Br2 + 4KOH —> R-NH2 (amine) + K2CO3 + 2KBr + 2H2O

Question 36

what is the general equation for Hunsdieker reaction

Answer 36

R- COO-Ag+ + Br2 –> R-Br + CO2 + AgBr
silver salt haloalkane