unit 3: organic practical techniques

Question 1

synthesis of compound techniques

Answer 1

heating under reflux, vacuum filtration

Question 2

purification of compound techniques

Answer 2

distillation, solvent extraction, recrystallisation

Question 3

determination of purity techniques

Answer 3

melting point analysis, mixed melting point analysis, thin layer chromatography (TLC)

Question 4

heating under reflux

Answer 4

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

Question 5

heating under reflux equipment

Answer 5

condenser, round bottom flask, heating mantle

Question 6

anti bumping granules

Answer 6

used to ensure smooth boiling

Question 7

vacuum filtration

Answer 7

separates solids and liquids and is much faster than gravity filtration

Question 8

vacuum filtration equipment

Answer 8

buchner funnel connected to a vacuum flask with an arm at the side connected to a water pump

Question 9

purifying

Answer 9

removes any impurities and side products formed during the synthesis

Question 10

recrystallisation

Answer 10

involves adding product to hot solvent to dissolve it and letting the product recrystallise as it cools down, any soluble impurities will stay dissolved in the solvent

Question 11

things to consider when choosing a solvent

Answer 11

product must be soluble in solvent at high temperature and minimally soluble at room temperature, it must not react with the products, the solubility of the impurities

Question 12

recrystallisation technique

Answer 12

add minimal volume of hot solvent to your synthesised product, filter the hot mixture to get rid of any insoluble impurities, let the solvent slowly cool down which will crystallise the desired product, filter off the product which will remove any soluble impurities left in the solvent

Question 13

melting point analysis

Answer 13

works out the purity of the sample by finding it’s melting point and comparing it to a known melting point value, impurities in the sample lower and broaden the melting point since they disrupt the intermolecular bonding in the crystal lattice

Question 14

melting point range

Answer 14

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

Question 15

mixed melting point analysis

Answer 15

mix the sample with a pure sample of what we meant to synthesis, if the melting point is at the correct value and the range is narrow, it is pure. if a different product was made the pure sample will act as an impurity and lower the melting point

Question 16

thin layer chromatography

Answer 16

separates compounds by their polarity using a film of silica or aluminium oxide as the stationary phase, sample is spotted on the base line and a solvent carries the compound up, a developing agent of UV light visualises the spots and the Rf value is calculated

Question 17

Rf

Answer 17

distance travelled by the sample / distance travelled by the solvent

Question 18

if there is only 1 spot in TLC

Answer 18

there are no impurities

Question 19

distillation

Answer 19

separates compounds by boiling point, more volatile substances with lower boiling points will pass through first. can identify compounds by comparing boiling points or purify substances by only collecting within the target boiling point, any collected outside will be impurities

Question 20

distillation equipment

Answer 20

burner, distilling flask, thermometer, condenser, receiving flask

Question 21

when to use solvent extraction

Answer 21

if the product is in an aqueous solution we can use a second solvent to extract it leaving behind impurities

Question 22

choosing a second solvent in solvent extraction

Answer 22

immiscible with original solvent, desired substance is more soluble in it, volatile to allow the desired product to be easily separated, unreactive with the substance

Question 23

solvent extraction

Answer 23

done in a separatory funnel. when the extraction is carried out two layers are formed in the funnel, the substance being extracted will dissolve in both solutions and equilibrium is established. the ratio of solute dissolved in both solvent is determined by K. once settled they are run off into speared containers and process is repeated

Question 24

K in solvent extraction

Answer 24

= [substance in upper]/[substance in lower]

Question 25

improving solvent extraction

Answer 25

using smaller volumes of solvent more often

Question 26

back titration is used to

Answer 26

work out the quantity of a substance in a solid with a low solubility (meaning a standard solution cannot be made), can also be carried out if the direct titration is too slow or there is no suitable indicator

Question 27

back titration

Answer 27

the substance being analysed (A) is reacted with an excess volume of a reactant of known concentration (B). the resulting mixture is titrated with a chemical that reacts with B (C). this allows us to calculate the number of moles of B in excess and therefore the number of moles of A which reacted with it

Question 28

volumetric analysis

Answer 28

uses a standard solution to determine the concentration of another solution, usually done using a titration

Question 29

concordant results

Answer 29

within 0.1cm^3 of each other

Question 30

complexometric titrations

Answer 30

formation of a complex between a metal ion and a complexing agent e.g. EDTA, they react in a 1:1 ratio

Question 31

indicator in EDTA titrations

Answer 31

murexide indicator which turns purple at the end point

Question 32

standard solution

Answer 32

a solution of accurately known concentration

Question 33

preparing a standard solution

Answer 33

by accurate dilution of an existing solution or from a primary standard

Question 34

preparing standard solution from primary standard

Answer 34

accurately weight out primary standard and dissolve in a small volume of solvent in a beaker, transfer with rinsing to volumetric flash and make up to graduation mark with solvent, stopper and invert to mix

Question 35

preparing standard solution by dilution

Answer 35

pipette required volume of standard solution into volumetric flask, make up to the graduation mark, stopper and invert to mix

Question 36

characteristics of primary standard

Answer 36

high purity, stable as a solid in air and in solution, soluble in water, reasonably high formula mass

Question 37

common primary standards

Answer 37

EDTA, silver nitrate, oxalic acid

Question 38

sodium hydroxide

Answer 38

is not a primary standard as it absorbs moisture from the air and is unstable as a solid and in solution

Question 39

if sodium hydroxide must be used in a titration

Answer 39

a solution of approximate concentration can be prepared and standardised by carrying out another titration first to find the accurate concentration of NaOH

Question 40

stoichiometry

Answer 40

the study of mole relationships involved in chemical reactions

Question 41

yield is reduced by

Answer 41

mechanical losses, purification of product, side reactions, equilibrium position

Question 42

experimental errors

Answer 42

human error (scale reading), methodology (side reactions), uncertainties (pipettes/burettes)

Question 43

classes of equipment

Answer 43

different classes have different levels of accuracy, class A glassware is more accurate than class B

Question 44

weight accurately approximately 1g

Answer 44

the mass should be around 1g but with an accurate reading

Question 45

outline how to prepare a standard solution

Answer 45

dissolve desired reactant in deionised water and transfer with rinsings, make up to the mark in a volumetric flask

Question 46

why would you test a filtrate with standard solution

Answer 46

to check the reaction is complete

Question 47

why would volumetric analysis rather than gravimetric analysis be more appropriate

Answer 47

titration can be used with lower concentrations

Question 48

desiccator

Answer 48

prevent any water from the air reacting with the chemical

Question 49

what analytical technique could be used to determine if a specific element is present in a substance

Answer 49

absorption/emission spectra

Question 50

in TLC what affects the Rf value for an individual component

Answer 50

the solvent used

Question 51

% by mass

Answer 51

mass in 100cm^3