3. synthesis, purification, determination of purity

Question 1

name the two synthesis methods

Answer 1

heating under reflux, vacuum filtration

Question 2

name the three purification methods

Answer 2

distillation, solvent extraction, recrystallisation

Question 3

name the two determination of purity methods

Answer 3

melting point analysis and thin layer chromatography

Question 4

what is heating under reflux?

Answer 4

a way of heating a reaction over an extended period of time without any vapours escaping

Question 5

what is the purpose of a condenser?

Answer 5

to condense any reactant that has evaporated and escaped the flask

Question 6

what is added to the round bottom flask and why?

Answer 6

anti-bumping granules - to ensure smooth boiling

Question 7

drawing a diagram of heating under reflux

Answer 7

:)

Question 8

what is vacuum filtration?

Answer 8

a way of separating solids and liquids that is faster than gravity filtration

Question 9

why is vacuum filtration faster than gravity filtration?

Answer 9

due to reduced pressure in the vacuum flask

Question 10

what is one way of creating a vacuum?

Answer 10

using a water pump

Question 11

draw a labelled diagram of vacuum filtration

Answer 11

:)

Question 12

describe the 4 steps of recrystallisation

Answer 12

1. add a minimal volume of hot solvent to your product
2. filter the hot mixture to get rid of any insoluble impurities
3. let the solvent cool slowly to recrystallise the desired product
4. filter off the product, leaving soluble impurities in the solvent

Question 13

what must be considered when choosing a solvent for recrystallisation?

Answer 13

• product must be soluble at high temperature, but insoluble at low temperature
• solvent must not react with products
• solubility of impurities

Question 14

how does melting point analysis help to determine the purity of a substance?

Answer 14

by determining the melting point and comparing it to a known melting point from literature

Question 15

what is the melting point range?

Answer 15

the temperature range over which a substance starts to melt to when it finishes melting

Question 16

how does the presence of impurities affect the melting point range and why?

Answer 16

it lowers and brodens it
they disrupt the intermolecular bonding in the original lattice

Question 17

how does mixed melting point analysis determine the purity of a substance?

Answer 17

our sample is mixed with a pure sample of what we synthesised
if the melting point is at the correct literature value and the range is narrow, we have succeeded
if we have not succeeded, the pure sample will act as an impurity and lower the melting point

Question 18

how does chromatography separate compounds?

Answer 18

based on their polarity

Question 19

what does TLC use as the stationary phase?

Answer 19

a film of silica or aluminium oxide

Question 20

which variables can affect how far the sample travels?

Answer 20

• solvent used
• temperature of TLC platr

Question 21

how are the spots on the chromatogram visualised?

Answer 21

using UV light or a developing agent

Question 22

how does one calculate the Rf?

Answer 22

distance travelled by sample/distance travelled by solvent

Question 23

how does TLC tell us that we have a pure compound?

Answer 23

there is only 1 spot in our sample
the Rf value matches that of the pure compound

Question 24

what is distillation?

Answer 24

the process of separating out compounds by their boiling point

Question 25

what can distillation be used for an why?

Answer 25

to identify compounds, as the b.p. of the fraction collected can be compared to literature values

Question 26

how can distillation be used to purity substances?

Answer 26

by only collecting any distillate within a certain range of the target b.p.

Question 27

what is solvent extraction used to do?

Answer 27

extract desired product from an aqueous solution, often leaving behind any impurities

Question 28

in which piece of apparatus is solvent extraction usually carried out?

Answer 28

separatory funnel

Question 29

which qualities must the second solvent in solvent extraction have?

Answer 29

1) immiscible with original solvent
2) one which desired substance is more soluble in
3) volatile to allow desired product to be easily separated from solvent
4) unreactive with substance

Question 30

what is formed while solvent extraction is being carried out?

Answer 30

two layers, with the substance being extracted dissolving in both. this causes an equilibrium to be established

Question 31

how is K worked in solvent extraction?

Answer 31

K = substance in upper/substance in lower

Question 32

the greater the value of K…

Answer 32

the more substance present in the upper layer

Question 33

what happens once the flask has been shaken and left to settle into layers?

Answer 33

the lower layer is run off into a container and the upper a second container. the process is repeated to maximise the quantity of substance extracted

Question 34

how is the process of solvent extraction improved?

Answer 34

by using smaller volumes of solvent more often