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ADV CHEM UNIT 4 > experiments: practical skills > Flashcards

experiments: practical skills Flashcards

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1
Q

what does heating under reflux allow?

A

heat energy to be applied to a chemical reaction mixture over an extended period of time without volatile substances

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2
Q

describe the process of carrying out heating under reflux

A

-the reaction mixture is placed in a round-bottomed flask with anti-bumping granules

-the flask is fitted with a condenser

-the flask is then heated using an appropriate source of heat

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3
Q

what is recrystallation?

A

used to purify solids, based upon solubility

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4
Q

describe the yield of recrystallation

A

if carried out correctly the final product will be both of a high yield was well as pure

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5
Q

how must the the solvent for recrystallisation be carefully selected?

A

such that the compound is insoluble at higher temperatures

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6
Q

describe what happens to the impure compound formed in recrystallisation

A

its dissolved gently in the minimum volume of hot solvent then filtered while hot (preheat the funnel and flask) to remove any insoluble impurities

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7
Q

why is the filter paper best fluted?

A

to increase its surface area and speed up the filtration

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8
Q

describe what happens to the filtrate in recrystallisation

A

its allowed to coo slowly to crystallise the pure compound

any soluble impurities are left behind in the solvent

filtration can then take place

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9
Q

what is thin layer chromatography (tlc)?

A

a method for analysing mixtures by separating the compounds in the mixture

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10
Q

what can tlc be used to help determine?

A

the number of components in a mixture, identify the compounds, and the purity of a compound

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11
Q

what are the three tlc steps?

A

spotting, development and visualisation

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12
Q

what is the first thing that happens to the sample in tlc?

A

its dissolved in a volatile (easily evaporated) solvent to produce a very dilute (about 1%) solution

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13
Q

describe spotting

A

using a micropipette to transfer a small amount of this dilute solution to one end of a tlc plate

the spotting solvent quickly evaporates and leaves behind a small spot of the material

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14
Q

describe development

A

placing the bottom of the tlc plate into a shallow pool of a development solvent, which then travels up the plate, it moves over the original spot

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15
Q

what does the outcome of development depend on?

A

a balance among three polarities- that of the plate, the development solvent and spot material

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16
Q

what happens if the development of the solvent is polar enough?

A

the spot will move some distance from its original location

different components in the original spot, having different polarities so will move different distances from the original spot location and show up as separate spots

when the solvent has travelled almost to the top of the plate, the plate is removed, the solvent front marked with a pencil, and the solvent allowed to evapourate

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17
Q

describe one method of visulisation

A

the silica gel on the tlc plate is impregnated with a fluorescent material that glows under uv light

a spot will interfere with the fluorescence and appear as a dark spot on the glowing blackboard

while under uv light, the spots can be outlined with a pencil to mark their locations

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18
Q

describe a second method of visulisation

A

by placing the plate into iodine vapours for a few minutes

most organic compounds will form a dark-coloured complex with iodine

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19
Q

why is it good practice to use at least two visualisation techniques?

A

in case a compound does not show up with one particular method

20
Q

what is the Rf value?

A

its used to quantify the movement of the materials along the plate

21
Q

what is Rf equal to?

A

the distance travelled by the substance divided by the distance travelled by the solvent (zero to 1)

22
Q

whats melting point?

A

the temp at which a solid melts is known as the melting point of that substance

its a physical property of a solid and be used to help identify substance

23
Q

if a compound melts over a very narrow range…

A

…it can usually be assumed that the compound is relatively pure

24
Q

besides melting over a wide range, impure solids…

A

…also melt at a temperature lower that that for the pure compound

25
Q

describe the melting point determination method

A

-sample added to a small glass capillary tube, which is closed at one end

-temp of sample measured with thermometer

-sample heated slowly as temp approaches the mp while the sample is carefully

26
Q

how do you know when the temp is in melting point range?

A

the temp at which the first drop of liquid is observed is recorded as the beginning of the mp range

27
Q

what is “mixed melting point determination”

A

if the mp range is lowered and widened it means that the two are different compounds.

if the mp stays the same it means that the two compounds are likely identical

28
Q

describe solvent extraction

A

-aqueous layer is first transferred to a separating funnel

-a portion of solvent equal to abt one-third of the volume of the aqeous mixture is then added to the funnel

-the mixture is shaken throoughly and the tap opened frequently to release any pressure

-the mixture is allowed to settle and there should be a sharp dividing line between the two layers

29
Q

why are solvent extractions carried out using small volumes of solvent rather than one extraction using a large volume?

A

so that a greater amount of product can be recovered

30
Q

why might a number of extractions using smaller volumes of solvent be carried out rather than a single extraction using a large volume of solvent?

A

smaller volumes allow for more surface area contact between the two solutions

mixing two smaller volumes is faster and more thorough than with larger portions, and so a greater amount of product can be recovered

31
Q

why would concentrated phosphoric acid used to dehydrate the alcohol rather than concentrated sulfuric acid?

A

sulfuric acid can lead to unwanted oxidation reactions

32
Q

why is distillation a suitable technique in this case to separate the product from the reaction mixture

A

they have different boiling points

33
Q

why was crude cyclohexene shaken with unsaturated sodium chloride solution?

A

its polar so attracts water and cyclohexanol

34
Q

why was anhydrous calcium chloride added to the cylohexene

A

it was used to remove traces of water from cyclohexene

35
Q

what are the possible reasons for the yield not being 100%

A

unexpected reactions can occur which dont yield the desired product, not all reactants are converted to products in the reaction e.g. not all cyclohexanol is dehydrated and mechanical losses

36
Q

how does edta act as a ligand?

A

-its hexadentate ligand
-has 6 donor atoms with lone electron pairs
-these are used to bond to a central metal atom or ion

37
Q

why is murexide a suitable indicator in a complexometric titration?

A

it binds less strongly to the Ni2+ ions than it does to the edta

38
Q

identify the major factors contributing to the difference between the theoretical and experimental %s

A

-experimental innacuraices
-impurities present
-uncontrolled conditions
-difficult to see end point

39
Q

what was done to help accurately detect the end point of this titration?

A

-white tile used
-keeping the first titrated solution as a compasriosn

40
Q

EDTA is an important complexometric reagent and
can be used to…

A

determine the concentration of metal ions in solution

41
Q

what is colorimetry

A

an analytical technique used to determine the concentrations of coloured substances in solution.

42
Q

what does colorimetry rely on?

A

-that a coloured substance absorbs light of a colour complementary to its own

-the amount of light it absorbs (absorbance) is proportional to its concentration.

43
Q

how is the concentration of the solution being tested through colorimetry determined?

A

-its absorbance
-by referring to the calibration

44
Q

What can distillation be used for?

A

to purify a compound by separating it from less volatile substances in the mixture

45
Q

how do you perform a back titration?

A

-reacting it with an excess volume of a reactant of known concentration

-The resulting mixture is then titrated to work out the number of moles of the reactant in excess

-From the initial number of moles of that reactant, the number of moles used in the reaction can be determined

-The initial number of moles of the substance being analysed can then be calculated

46
Q

what is a condenser used for?

A

reduce a gas or vapour to a liquid

47
Q

why is vacuum filtration a useful technique?

A

it allows a greater rate of filtration

ADV CHEM UNIT 4 (3 decks)

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