Qualitative Analysis (Chapters 8.3, 24.5 and 29.1)

Question 1

What is qualitative analysis?

Answer 1

Analysis that relies on simple observations rather than measurements e.g. gas bubbles, precipitates, colour changes

Question 2

Describe the carbonate test

Answer 2

1) Add dilute HNO3 to carbonate in a test tube
2) If you see bubbles, shows gas is formed (could be CO2)
3) Bubble the gas through limewater (saturate Ca(OH)2 solution)
- CO2 reacts to form a fine white precipitate of CaCO3 which turns the limewater cloudy (milky)

Question 3

What are the two equations for the carbonate test?

Answer 3

1) Na2CO3(aq) + 2HNO3(aq) = 2NaNO3(aq) + CO2 + H2O

2) CO2(g) + Ca(OH)2(aq) = CaCO3(s) + H2O(l)

Question 4

Describe the sulfate test

Answer 4

1) Add Ba2+(aq) ions to unknown solution (usually aqueous BaCl2 or Ba(NO3)2)
2) If sulfate ions are present, a white precipitate of BaSO4 (a very insoluble sulfate) will form

Question 5

What is the equation for the sulfate test?

Answer 5

Ba2+(aq) + SO42-(aq) = BaSO4(s)

Question 6

Describe the halide test

Answer 6

1) Add AgNO3(aq) to an aqueous solution of a halide
2) If a halide is present, a silver halide (insoluble halide) will form
- if Cl- ions: white precipitate
- if Br- ions: cream precipitate
- if I- ions: white precipitate

Question 7

What is the equation for the halide test?

Answer 7

Ag+(aq) + X-(aq) = AgX(s)

Question 8

Describe how you would further distinguish between halides (because sometimes colours of precipitates are similar

Answer 8

1) Add aqueous ammonia to the silver halide
- if Cl- ions: will dissolve in dilute NH3
- if Br- ions: will dissolve in conc NH3
- if I- ions: will not dissolve in conc or dilute NH3

Question 9

Describe the correct sequence of tests

Answer 9

1) Carbonate test
2) Sulfate test
3) Halide test

Question 10

Why do you do the carbonate test first?

Answer 10

Because only carbonate ions produce bubbles with dilute acid

Question 11

Why do you do the sulfate test second?

Answer 11

• Because BaCO3 is also a white, insoluble precipitate

- Therefore, if you carry out a sulfate test on a carbonate, you will get a false positive

Question 12

Why do you do the halide test third?

Answer 12

• Because you are only looking for a precipitate and Ag2CO3 and Ag2SO4 are both insoluble and will form as precipitates
• Therefore, carrying out carbonate and sulfate tests first, avoids any false positives

Question 13

What is the sequence of tests if you are testing a mixture of ions?

Answer 13

The same (carbonate, sulfate, halide)

Question 14

Describe the ammonium (NH4+) test

Answer 14

1) NaOH(aq) is added to a solution of an ammonium ion
2) Ammonia gas is produced but you can’t see the bubbles as NH3 is very soluble
3) The mixture is warmed and NH3 is released
4) You can smell the ammonia or test with moist indicator paper - NH3 is alkaline and will therefore turn the paper blue

Question 15

How do you carry out the carbonate test on a mixture of ions?

Answer 15

1) If you see bubbles, continue adding dilute HNO3 until the bubbling stops
2) All carbonate ions will then have been removed and there will be none to react in the next tests

Question 16

Why do you have to use HNO3 in a carbonate test on a mixture of ions?

Answer 16

Because if you want to test for sulfate/halide, H2SO4 and HCl contain sulfate and chloride ions which will show up in the other tests

Question 17

How do you carry out the sulfate test on a mixture of ions?

Answer 17

1) Add an excess of barium nitrate to the solution left from the carbonate test
2) Filter the solution to remove the BaSO4

Question 18

Why do you have to use barium nitrate and not barium chloride in a sulfate test on a mixture of ions?

Answer 18

Because barium chloride contains chloride ions which will show up in the next test

Question 19

How do you carry out the halide test on a mixture of ions?

Answer 19

1) To any solution left from the sulfate test, add AgNO3(aq)
2) Any carbonate or sulfate ions initially present in the solution have been removed, therefore any precipitate formed must involve halide ions
3) Add aqueous ammonia to confirm which halide you have

Question 20

What is the chemical test and positive result for an alkene?

Answer 20

Test: add bromine water drop-wise
Observation: bromine water is decolourise from orange to colourless

Question 21

What is the chemical test and positive result for a haloalkane?

Answer 21

Test: add silver nitrate and ethanol and warm to 50 degrees in a water bath
Observation: chloroalkane - yellow ppt, bromoalkane - cream ppt, iodoalkane - yellow ppt

Question 22

What is the chemical test and positive result for a carbonyl (aldehyde and ketone)?

Answer 22

Test: add 2,4-DNP
Observation: orange ppt

Question 23

What is the chemical test and positive result for an aldehyde?

Answer 23

Test: add Tollen’s reagent and warm
Observation: silver mirror

Question 24

What is the chemical test and positive result for a primary/secondary alcohol and aldehyde?

Answer 24

Test: add acidified potassium dichromate (VI), K2Cr2O7/H2SO4 and warm in a water bath
Observation: colour change from orange to green

Question 25

What is the chemical test and positive result for a carboxylic acid?

Answer 25

Test: add aqueous sodium carbonate (Na2CO3(aq))
Observation: effervescence

Question 26

Describe how phenol can be identified

Answer 26

1) pH indicator paper as it is acidic
2) it is less acidic than COOH ∴ does not react with Na2CO3
3) it undergoes electrophilic substitution reactions with bromine at room temperature ∴ when bromine is aded to an aqueous solution of phenol, the bromine is decolourised and a white ppt is formed

Question 27

How can you identify transition metal ions?

Answer 27

Add NaOH - this produces ppts with transition metal ions

Question 28

Describe the test to identify NH4+ ions

Answer 28

1) when heated with OH- ions, NH4+ reacts to product ammonia gas - NH4+ + OH- => NH3 + H2O
2) if NH3 is evolved, damp red pH indicator paper (red litmus paper) will turn blue, confirming the presence of NH4+ ions (and ammonia has a strong smell)