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practical techniques Flashcards

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1
Q

TITRATION
describe the procedure for using a pipette

A
  • use a safety bulb to suck water into the pipette
  • fill the pipette so the bottom of the meniscus of the water sits on the mark
  • let solution run out naturally into flask and touch the side of the flask with the bottom of the pipette
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2
Q

QUALITATIVE ANALYSIS OF IONS
what order must qualitative ion tests be carried out in + why

A

can-snakes-actually-hum
carbonate then sulfate then ammonium then halide

this is because Ba2+ also reacts with CO3 2- to form BaCO3 and Ag+ also reacts with SO4 2- to form Ag2SO4 – both of these compounds are insoluble precipitates and would interfere with sulfate and halide tests
- essentially this prevents false positives

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3
Q

QUALITATIVE ANALYSIS OF IONS
outline the test for carbonate ions

A
  • add dilute acid (HCl) to sample in test tube
  • immediately attach a bung with a delivery tube - this should transfer gas to a different test tube containing lime water
    POSITIVE= effervescence and the limewater should turn cloudy due to formation of a white precipitate
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4
Q

QUALITATIVE ANALYSIS OF IONS
equations for carbonate ion test

A

CO3 2-(aq) + 2H+(aq)&raquo_space; CO2(g) + H2O(l)

CO2(g) + Ca(OH)2(aq)&raquo_space; CaCO3(s) + H2O(l)

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5
Q

QUALITATIVE ANALYSIS OF IONS
outline the test for sulfate ions

A
  • acidify sample with dilute acid (HCl) in a test tube
  • add drops of aq barium nitrate
    POSITIVE= formation of a white precipitate (barium sulfate)
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6
Q

QUALITATIVE ANALYSIS OF IONS
equations for sulfate ions test

A

Ba 2+(aq) + SO4 2-(aq)&raquo_space; BaSO4(s)

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7
Q

QUALITATIVE ANALYSIS OF IONS
outline the test for ammonium ions

A
  • add aq NaOH to sample in a test tube
    POSITIVE= formation of ammonia gas which has a pungent smell of rotten eggs and turns red litmus blue - the gas will not bubble
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8
Q

QUALITATIVE ANALYSIS OF IONS
equations for ammonium ions test

A

NH4 +(aq) + OH-(aq)&raquo_space; NH3(g) + H2O(l)

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9
Q

QUALITATIVE ANALYSIS OF IONS
outline the test for halide ions

A
  • add dilute nitric acid to the sample in a test tube (to prevent false results)
  • add silver nitrate
    POSITIVE= a coloured precipitate will form
    Cl - white
    Br - cream
    I - yellow

these colours can be subtle so to follow up add dilute then concentrated ammonia solution
Cl - white precipitate will dissolve in dilute NH3
Br - cream precipitate will dissolve in concentrated NH3
I - yellow precipitate is insoluble in NH3

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10
Q

QUALITATIVE ANALYSIS OF IONS
equations for halide ion test

A

Ag+(aq) + X-(aq)&raquo_space; AgX(s)
- X=halogen

AgX(s) + NH3(aq)&raquo_space; [Ag(NH3)2]+(aq) + X-(aq)
- this occurs if the precipitate is soluble, as Ag forms a complex ion with 2 coordinate bonds

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11
Q

ENTHALPY CHANGE
outline a method to find the energy within an alcohol

A
  • measure out 100cm3 of water and pour into a beaker/calorimeter
  • insert thermometer + record temp of water
  • weigh mass of spirit burner
  • light the spirit burner under the beaker and leave water to heat for a period of time or until a certain temperature change
  • when finished, extinguish the flame and record the temperature of the water
  • record the final mass of the spirit burner
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12
Q

ENTHALPY CHANGE
outline a method for measuring the enthalpy of a reaction

A
  • set up a polystyrene cup with a thermometer inside
  • add the first reactant and record the temp
  • add the second reactant + stir + immediately add a lid on the cup
  • record the maximum temperature reached
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13
Q

MOLES DETERMINATION
outline a method to determine water of crystallisation with a hydrated salt

A
  • use a 3dp balance to record mass of an empty crucible
  • add a small amount of hydrated salt and reweigh + record mass
  • gently heat for some time (e.g. 2 mins), allow to cool and reweigh
  • continue heating + reweighing until mass is constant to ensure all water of crystallisation is removed
  • use mass to calculate moles, can then be used to calculate water of crystallisation
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14
Q

MOLES DETERMINATION
outline a method to find the Mr of a substance by measuring gas volume produced (Mg+HCl method)

A
  • 3dp balance to record mass of strip of Mg ribbon
  • place Mg ribbon in clinical flask
  • in measuring cylinder measure an excess amount of dilute HCl
  • carefully add acid to flask and quickly fit bung connected ti a gas syringe - records gas mass produced
  • when syringe stops moving record final mass produced, then calculate Mr with mol=vol/24, etc
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15
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
outline the test for alkenes + equation

A
  • react with bromine water
    POSITIVE= the solution decolourises from orange-brown to colourless

alkene + Br2(aq)&raquo_space; dibromoalkane

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16
Q

OXIDIATION OF ALCOHOLS
outline the oxidation of a primary alcohol to an aldehyde + equation

A
  • heat alcohol with K2Cr2O7 + H2SO4 under distillation
  • the dichromate will change colour from orange to green

alcohol + [O]&raquo_space; aldehyde + H2O

17
Q

OXIDIATION OF ALCOHOLS
outline the oxidation of a primary alcohol to a carboxylic acid + equation

A
  • heat alcohol with K2Cr2O7 + H2SO4 under reflux
  • the dichromate will change colour from orange to green

alcohol + 2[O]&raquo_space; carboxylic acid + H2O
OR aldehyde + [O]&raquo_space; carboxylic acid

18
Q

OXIDIATION OF ALCOHOLS
outline the oxidation of a secondary alcohol to a ketone + equation

A
  • heat alcohol with K2Cr2O7 + H2SO4 under reflux
  • the dichromate will change colour from orange to green

alcohol + [O]&raquo_space; ketone + H2O

19
Q

OXIDIATION OF ALCOHOLS
outline the oxidation of a tertiary alcohol

A

this is not possible - as the OH group is attached to a C atom bonded to 3 alkyl groups, no H can be removed so oxidation cannot occur
- the dichromate will remain orange as no reaction takes place

20
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
outline a test to identify the following haloalkanes - chloroethane, bromoethane, iodoethane

A
  • add 2cm3 ethanol to 3 test tubes and heat in a water bath to around 60C
  • add a few drops of each haloalkane to separate test tubes
  • follow this with 1cm3 of AgNO3(aq) solution
  • immediately start a stop watch after this step and record observations + times
    POSITIVE= a precipitate should form in each one, white for chloroethane, cream for bromoethane, yellow for iodoethane
    the yellow precipitate will form first, then the cream one, then the white one - this is because C-I bonds break faster than C-Br bonds, which break faster than C-Cl bonds due to bond enthalpies
21
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
outline a test to identify an alcohol

A
  • react under distillation with acidified potassium dichromate
    POSITIVE= colour change from orange to green
22
Q

TECHNIQUE explain the difference between distillation and reflux

A

reflux is strong heating and continuous evaporation and condensation so volatile compounds cannot escape, whereas distillation is gentle heating and evaporation then condensation allowing the most volatile compounds to escape/distil out and be collected

23
Q

PURIFICATION
describe how an organic liquid could be purified

A
  • add CaCO3 to remove acid impurities
  • pour substance into a separating funnel and invert several times until aqueous and organic layers are separated
  • if aq layer is on the bottom, run out majority with tap ensuring no organic product goes with it, if organic layer is on the bottom run out entire organic layer, even if some aq gets in mixture
  • discard aqueous layer
  • add a drying agent e.g. anhydrous MgSO4
  • filter or decant or redistil to remove drying agent
24
Q

TECHNIQUE
how could you identify the aqueous and organic layers if you do not have densities

A

add some distilled water to the separating funnel when layers are already separated, the layer which grows is the aqueous layer

25
Q

TECHNIQUE
what is decanting

A

pouring a liquid mixed with some solid slowly into a new beaker, prevents the solid from going too and easily separates a mixture

26
Q

PURIFICATION
you are given a sample of benzoic acid contaminated with 2 compounds
- benzoic acid is soluble in hot but not cold water
- compound 1 is soluble in both hot and cold water
- compound 2 is not soluble in water at all

outline how to obtain a pure sample of benzoic acid

A
  • dissolve in minimum volume of hot solvent
  • filter while hot to remove compound 2
  • allow filtrate to cool at room temperature then in an ice bath
  • filter again under reduced pressure to remove insoluble benzoic acid
  • rinse with distilled water and let dry e.g. overnight
27
Q

PURIFICATION
give 2 ways to check that a product is pure

A
  • determine melting point and compare to database values, if pure it should be sharp + high
  • ## use TLC and compare to a pure sample, or compare Rf to database values
28
Q

TECHNIQUE
how could you track the progress of a reaction without disturbing the reaction

A

using TLC - take samples of the reaction mixture at set points, and compare these with a sample of products or reactants - comparing Rf values will indicate how far reaction has progressed

29
Q

PURIFICATION
describe how an organic solid could be purified

A
  • dissolve in minimum volume of hot solvent
  • cool at room temp then in an ice bath
  • rinse with cold distilled water/solvent and dry
30
Q

CRYSTALLISATION
how can you check crystals are forming / make crystals form

A

scratch the surface of the beaker with a glass rod, this can help crystals form

31
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds will turn litmus paper red

A

carboxylic acids and phenols

32
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds will turn litmus paper blue

A

amines

33
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
test for phenols

A
  • react with bromine water
    POSITIVE= the solution decolourises from orange-brown to colourless + white precipitate 2,4,6-tribromophenol forms
34
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds react with Na to produce effervescence

A

carboxylic acids, phenols, alcohols - effervescence is H2 gas

35
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds react with NaCO3 to produce effervescence

A

carboxylic acids - effervescence is CO2 gas

36
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds react with water to produce white fumes

A

acyl chlorides - white fumes are HCl

37
Q

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS
what compounds react with warm NaOH to produce a pungent smelling gas which turns litmus paper blue

A

amides - gas is NH3

chemistry practicals (1 decks)

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