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chem > chap 11: qualitative analysis > Flashcards

chap 11: qualitative analysis Flashcards

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1
Q

what are the similarities between NaOH and NH3?

A

-both are alkalis
-both dissociates in water to give OH- ions

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2
Q

what is the difference between NaOH and NH3?

A

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)

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3
Q

what are the observations when NaOH and NH3 is added to and added in an excess to NH4^+ (ammonium)?

A

NaOH: no precipitate formed. on warming, effervescence observed and gas evolved turns moist red litmus paper blue. ammonia gas is produced

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4
Q

what are the observations when NaOH and NH3 is added to and added in excess to Cu^2+ (copper)?

A

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

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5
Q

what are the observations when NaOH and NH3 is added to and added in excess to Fe^2+ (iron ii)?

A

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

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6
Q

what are the observations when NaOH and NH3 is added to and added in excess to Fe^3+ (iron iii)?

A

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

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7
Q

what are the observations when NaOH and NH3 is added to and added in excess to Ca^2+ (calcium)?

A

NaOH: white precipitate of Ca(OH)2 is formed
NaOH in excess: precipitate is insoluble in excess NaOH
NH3: no precipitate formed

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8
Q

what are the observations when NaOH and NH3 is added to and added in excess to Zn^2+ (zinc)?

A

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

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9
Q

what are the observations when NaOH and NH3 are added to and added in excess to Al^3+ (aluminium)?

A

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

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10
Q

why are zinc and aluminium hydroxides soluble in excess NaOH?

A

zinc and aluminium hydroxides have amphoteric characteristics and are able to react with both strong alkali and strong acid to form a stable compound

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11
Q

why is there no precipitate observed when aqueous ammonia is added to samples containing calcium ions?

A

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

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12
Q

what is the test for CO3^2- (carbonate)
and what are the observations of the test?

A

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

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13
Q

what is the test for NO3^- (nitrate) and what are the observations of the test?

A

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

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14
Q

what is the test for Cl^- (chloride) and what is the observation of the test

A

test: add dilute nitric acid, then add aqueous silver nitrate
observation: white precipitate of silver chloride (AgCl) is formed

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15
Q

what is the test for I^- (iodide) and what is the observation of the test?

A

test: add dilute nitric acid, then add aqueous silver nitrate
observation: yellow precipitate of silver iodide (AgI) is formed

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16
Q

what is the test for SO4^2- (sulfate) and what is the observation of the test?

A

test: add dilute nitric acid, then add aqueous barium nitrate
observation: white precipitate of barium sulfate (BaSO4) is formed

17
Q

what is the colour and odour, test and observation of the test of H2 (hydrogen)?

A

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

18
Q

what is the colour and odour, test and observation of the test of O2 (oxygen)?

A

colour and odour: colourless and odourless
test: place a glowing splint at the mouth of the test tube
observation: the glowing splint rekindles

19
Q

what is the colour and odour, test and observation of the test of CO2 (carbon dioxide)?

A

colour and odour: colourless and odourless
test: bubble the gas into limewater
observation: white precipitate (CaCO3) is formed in the limewater

20
Q

what is the colour and odour, test and observation of the test of Cl2 (chloride gas)?

A

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

21
Q

what is the colour and odour, test and observation of the test of SO2 (sulfur dioxide)?

A

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

22
Q

what is the colour and odour, test and observation of the test of NH3 (ammonia gas)?

A

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

23
Q

why is dilute nitric acid added to salt samples?

A

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

24
Q

why is dilute nitric acid used instead of other acids?

A

dilute nitric acid is used as its salts are always soluble

chem (8 decks)

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