general methods

Question 1

how to purify an organic solid + steps

Answer 1

recrystallisation
- dissolve in minimum volume of hot solvent
- hot filtration
- cool and crystalise in an ice bath
- vaccum filter
- wash with minimum amount ice cold solvent (removes soluble impurities)
- dry between fr

Question 2

DNP crystals

Answer 2

• add equal volumes of your carbonyls to 2,4 dnp solution
• filter
• recrystallise
• determine mp, compare to data book value

Question 3

purpose of salt bridge

Answer 3

allows ions to flow
by completing the circuit

Question 4

colour change of when you add drying agent

Answer 4

cloudy to clear

Question 5

why rinse and transfer washings

Answer 5

All the solid weighed out should be transferred

Question 6

why rinse the burette with acid

Answer 6

to prevent dilution of the acid, so the burette only contains acid

Question 7

why is water added SLOWLY to the cooled colution

Answer 7

• because the reaction between ethanoul chloride and water is vigorous
• prevents sapid evolution of toxic HCl gas

Question 8

why use reflux (2)

Answer 8

• increase the rate of reaction
• prevents the escape of volatile compounds
• reaction to completion

Question 9

3 methods to improve titration

Answer 9

• add drop by drop near the end point
• swirl the conical flask
• white tile

Question 10

why not use HCl in manganate (7) titrateion

Answer 10

• cl- would be oxidised to chlorine cl2 by the manganate ions
• nitric acid would oxidise the fe2+ to fe3+

Question 11

what happens if u dont add acid to the MnO4-

Answer 11

• only partially reduced
• to MnO2 (brown precipitate)

Question 12

purpose of ethanol

Answer 12

• cosolvent
• so allows the reactants to mix together

Question 13

EDTA

Answer 13

• coordination no of 6
• -4 charge

Question 14

suggest why u need to stir continuously

Answer 14

• reactants are immiscible - to ensure enough
  contact for a reaction to take place [adequate rate]

Question 15

State why x is generated in the reaction mixture instead of being
obtained from a chemical supplier.

Answer 15

unstable

Question 16

iodoform 2 observations

Answer 16

• yellow ppt
• antiseptic smell

Question 17

for a toxic compound, why prefer a solid rather thana lqiuid

Answer 17

easier to spill a liquid

Question 18

why must hte filter paper be flat in the buchner funnel

Answer 18

• so it seals all the holes in the buchner funnel
• no mixture is lost down the sides, so no solid is lost

Question 19

vinegar is

Answer 19

ethanoic acid

Question 20

suggest why for water u usea measuring cylinder not a pipette

Answer 20

• quicker
• NO NEED FOR THE ACCURACY OF A PIPETTE

Question 21

why does adding x solvent to a solution make it crystallise

Answer 21

the compound is less soluble in x than in water

Question 22

why is there an initial stram of bubbles when usng the thiele tube?

Answer 22

air

Question 23

why heat the side arm of the thiele tube not where the compound is

Answer 23

heat is distributed more evenly

Question 24

why can thiele tube measurements vary

Answer 24

atmospheric pressure varies

Question 25

what can absorb water

Answer 25

anhydrous sodium sulfate

Question 26

what can absorb co2

Answer 26

soda lime

Question 27

even if e cell is negative, why might reacito still occur

Answer 27

- xyz reactnat is removed - equilibrium shifst to the ...

Question 28

why use steam distillation

Answer 28

x decomposes at high temperatures

Question 29

assumptions for calorimetry (q = mc delta t)

Answer 29

- densit of solution same as water - SHC solution same as water - mass of solid ignored

Question 30

Give two reasons why pure O2(g), and not air, should be used. in tecombustion product absorption expriment

Answer 30

- air contains h2o - air contains co2 - esnures compoete combustion

Question 31

why use granules of something not powder

Answer 31

allows gas to flow through

Question 32

describe what happens when you add hexene to Kbr and I2

Answer 32

- 2 layers form - top organic layer initially goes purple, then decolourises - bottom aqueous layer fades to colourless

Question 33

why is it bad to have draughts in calorimetry

Answer 33

- use a flame gaurd - to prevent heat loss

Question 34

uncertainty vs accuracy

Answer 34

uncertainty values indicate the range (over which the measured value is valid)  accuracy is an indication of the difference between the experimental value and the data book value (1) (1)

Question 35

mass spec products of uncharged substance

Answer 35

- ion - free radical

Question 36

mass spec products of charged substance

Answer 36

- ion - free radical

Question 37

basic, how to use separating funnel

Answer 37

- add your substances - shake - invert flask, open tap every so often to release pressure - run off lower aqueous layer

Question 38

why fill condenser from bottom

Answer 38

- so water fills it all, there are no gaps - for more efficient cooling

Question 39

non litmus test for ammonia

Answer 39

- ad HCl - white smoke

Question 40

why use mineral wool?

Answer 40

- holds the alcohol in place , so isnt heated directly, doesnt come into contact with the solid - it is chemically inert - so the vapours pass over the catalyst slowly enoguh to react

Question 41

why remove delibery tube immediatly

Answer 41

- prrevent suckback after heating - due to drop in pressure in tube - the cold water could cause thetube to crack

Question 42

if calorimetry anomaly, why?

Answer 42

mixture not stirred properly so temp wasnt unfirom

Question 43

why remove soluble impuritis

Answer 43

would otherwise add to the mass of crystals formed

Question 44

explain how the adiiton of sodium hydrogen carbonate in step 2 reduces the loss of so2 from the flask

Answer 44

forms co2 which forms a layer at the top preventing loss of so2

Question 45

why is ethanol a good solvent

Answer 45

- alkyl groups can london froce with non polar - polar bond can pdpd with haloalkane polar pbond

Question 46

extrapolation, why record initial temperatures for 3 mins

Answer 46

- to ensure equilibration with surroundings - to get mean initial

Question 47

why use the extrapoltaion method

Answer 47

- as reaction increases temp as exo - but there is HEAT LOSS, so it accounts for heat loss

Question 48

why is fractional distillation better

Answer 48

- longer reaction time - more effective separation

Question 49

when labelling separating funnel make sure

Answer 49

- only one organic layer - lower is squoeus

Question 50

why rough

Answer 50

- approx value for titre - so can complete others faster and then go dbd near end pont

Question 51

why not hcl in the titration

Answer 51

cl- oxidised to cl2

Question 52

why not hno3 in the titration

Answer 52

the fe2+ would reduce the no3-

Question 53

how to add water down the side of the condenser

Answer 53

slowly reaction is very exothermic prevent loss of volatile species / rapid evolution of gas

Question 54

why not use a solid dryung agent to dry a solid

Answer 54

* difficult to separate the solid drying agent from the product

Question 55

why for clock reaciton have the others in huge xs

Answer 55

so their concentrations dont change so only conc x affects the rate, can measure its effect

Question 56

fehlings what is the red ppt

Answer 56

Cu2O (cu+)

Question 57

why wash crystals with ice cold

Answer 57

- ice cold so crystals dont dissolve - wash so remove impurities and then name them

Question 58

purpose of thiosulfate

Answer 58

- reacts with iodine to form i- - when all thiosulfate runs out, i2 reacts w starch to go from colourless to blue black - so solution doesnt immediately change colour