practical techniques Flashcards by Emma Younes

TITRATION
describe the procedure for using a pipette

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

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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QUALITATIVE ANALYSIS OF IONS
outline the test for carbonate ions

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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QUALITATIVE ANALYSIS OF IONS
equations for carbonate ion test

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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QUALITATIVE ANALYSIS OF IONS
outline the test for sulfate ions

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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QUALITATIVE ANALYSIS OF IONS
equations for sulfate ions test

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

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QUALITATIVE ANALYSIS OF IONS
outline the test for ammonium ions

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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QUALITATIVE ANALYSIS OF IONS
equations for ammonium ions test

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

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QUALITATIVE ANALYSIS OF IONS
outline the test for halide ions

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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QUALITATIVE ANALYSIS OF IONS
equations for halide ion test

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

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

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

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

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

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

alkene + Br2(aq)&raquo_space; dibromoalkane

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

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

alcohol + [O]&raquo_space; aldehyde + H2O

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

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

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

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

alcohol + [O]&raquo_space; ketone + H2O

OXIDIATION OF ALCOHOLS
outline the oxidation of a tertiary alcohol

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

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

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

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

react under distillation with acidified potassium dichromate
POSITIVE= colour change from orange to green

TECHNIQUE explain the difference between distillation and reflux

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

PURIFICATION
describe how an organic liquid could be purified

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

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

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

TECHNIQUE what is decanting

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

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

- 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

PURIFICATION give 2 ways to check that a product is pure

- 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 -

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

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

PURIFICATION describe how an organic solid could be purified

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

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

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

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

carboxylic acids and phenols

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

amines

QUALITATIVE ANALYSIS OF ORGANIC COMPOUNDS test for phenols

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

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

carboxylic acids, phenols, alcohols - effervescence is H2 gas

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

carboxylic acids - effervescence is CO2 gas

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

acyl chlorides - white fumes are HCl

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

amides - gas is NH3