chap 11: qualitative analysis Flashcards
what are the similarities between NaOH and NH3?
-both are alkalis
-both dissociates in water to give OH- ions
what is the difference between NaOH and NH3?
NaOH: strong alkali (dissociates fully in water to give a high concentration of OH- ions)
NH3: weak alkali (dissociates partially in water to give a low concentration of OH- ions)
what are the observations when NaOH and NH3 is added to and added in an excess to NH4^+ (ammonium)?
NaOH: no precipitate formed. on warming, effervescence observed and gas evolved turns moist red litmus paper blue. ammonia gas is produced
what are the observations when NaOH and NH3 is added to and added in excess to Cu^2+ (copper)?
NaOH: light blue precipitate Cu(OH)2 is formed
NaOH in excess: precipitate is insoluble in excess NaOH
NH3: light blue precipitate of Cu(OH)2 is formed
NH3 in excess: precipitate is soluble in excess NH3, forming a dark blue solution
what are the observations when NaOH and NH3 is added to and added in excess to Fe^2+ (iron ii)?
NaOH: green precipitate of Fe(OH)2 is formed
NaOH in excess: precipitate is insoluble in excess NaOH
NH3: green precipitate of Fe(OH)2 is formed
NH3 in excess: precipitate is insoluble in excess NH3
what are the observations when NaOH and NH3 is added to and added in excess to Fe^3+ (iron iii)?
NaOH: red-brown precipitate of Fe(OH)3 is formed
NaOH in excess: precipitate is insoluble in excess NaOH
NH3: red-brown precipitate of Fe(OH)3 is formed
NH3 in excess: precipitate is insoluble in excess NH3
what are the observations when NaOH and NH3 is added to and added in excess to Ca^2+ (calcium)?
NaOH: white precipitate of Ca(OH)2 is formed
NaOH in excess: precipitate is insoluble in excess NaOH
NH3: no precipitate formed
what are the observations when NaOH and NH3 is added to and added in excess to Zn^2+ (zinc)?
NaOH: white precipitate of Zn(OH)2 is formed
NaOH in excess: precipitate is soluble in excess NaOH, forming a colourless solution
NH3: white precipitate of Zn(OH)2 is formed
NH3 in excess: precipitate is soluble in excess NH3, resulting in a colourless solution
what are the observations when NaOH and NH3 are added to and added in excess to Al^3+ (aluminium)?
NaOH: white precipitate of Al(OH)3 is formed
NaOH in excess: precipitate is soluble in excess NaOH, forming a colourless solution
NH3: white precipitate of Al(OH)3 is formed
NH3 in excess: precipitate is insoluble in excess NH3
why are zinc and aluminium hydroxides soluble in excess NaOH?
zinc and aluminium hydroxides have amphoteric characteristics and are able to react with both strong alkali and strong acid to form a stable compound
why is there no precipitate observed when aqueous ammonia is added to samples containing calcium ions?
unlike NaOH, aqueous ammonia is a weak alkali which dissociates to give a low concentration of OH- ions. hence, only a small amount of calcium hydroxide is formed. since calcium hydroxide is sparingly soluble, no obvious precipitate will be observed
what is the test for CO3^2- (carbonate)
and what are the observations of the test?
test: add dilute acid. bubble the gas given off into limewater
observations: effervescence observed. gas given off forms white precipitate (CaCO3) in limewater. carbon dioxide gas is produced
what is the test for NO3^- (nitrate) and what are the observations of the test?
test: add aqueous sodium hydroxide then add a piece of aluminium. warm the mixture carefully. test the gas given off with a piece of damp red litmus paper
observations: effervescence observed. gas given off turns damp red litmus paper blue. ammonia gas is produced
what is the test for Cl^- (chloride) and what is the observation of the test
test: add dilute nitric acid, then add aqueous silver nitrate
observation: white precipitate of silver chloride (AgCl) is formed
what is the test for I^- (iodide) and what is the observation of the test?
test: add dilute nitric acid, then add aqueous silver nitrate
observation: yellow precipitate of silver iodide (AgI) is formed
what is the test for SO4^2- (sulfate) and what is the observation of the test?
test: add dilute nitric acid, then add aqueous barium nitrate
observation: white precipitate of barium sulfate (BaSO4) is formed
what is the colour and odour, test and observation of the test of H2 (hydrogen)?
colour and odour: colourless and odourless
test: place a burning splint at the mouth of a test tube
observation: the burning splint extinguishes with a ‘pop’ sound
what is the colour and odour, test and observation of the test of O2 (oxygen)?
colour and odour: colourless and odourless
test: place a glowing splint at the mouth of the test tube
observation: the glowing splint rekindles
what is the colour and odour, test and observation of the test of CO2 (carbon dioxide)?
colour and odour: colourless and odourless
test: bubble the gas into limewater
observation: white precipitate (CaCO3) is formed in the limewater
what is the colour and odour, test and observation of the test of Cl2 (chloride gas)?
colour and odour: yellow green gas with pungent smell
test: place a piece of damp blue litmus paper at the mouth of the test tube
observations: damp blue litmus paper turns red and is then bleached
what is the colour and odour, test and observation of the test of SO2 (sulfur dioxide)?
colour and odour: colourless and pungent smell
test: place a piece of filter paper soaked with acidified potassium manganate (VII) at the mouth of the test tube
observations: purple acidified potassium manganate (VII) solution turns colourless
what is the colour and odour, test and observation of the test of NH3 (ammonia gas)?
colour and odour: colourless gas with a pungent smell
test: place a damp red litmus paper at the mouth of the test tube
observation: damp red litmus paper turns blue
why is dilute nitric acid added to salt samples?
it is to acidify the salt samples. it removes interfering ions, particularly carbonates and hydroxides as most carbonates and hydroxides are insoluble. presence of interfering ions could result in false positive results
why is dilute nitric acid used instead of other acids?
dilute nitric acid is used as its salts are always soluble
