Qualitative Analysis (Chapters 8.3, 24.5 and 29.1) Flashcards
What is qualitative analysis?
Analysis that relies on simple observations rather than measurements e.g. gas bubbles, precipitates, colour changes
Describe the carbonate test
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)
What are the two equations for the carbonate test?
1) Na2CO3(aq) + 2HNO3(aq) = 2NaNO3(aq) + CO2 + H2O
2) CO2(g) + Ca(OH)2(aq) = CaCO3(s) + H2O(l)
Describe the sulfate test
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
What is the equation for the sulfate test?
Ba2+(aq) + SO42-(aq) = BaSO4(s)
Describe the halide test
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
What is the equation for the halide test?
Ag+(aq) + X-(aq) = AgX(s)
Describe how you would further distinguish between halides (because sometimes colours of precipitates are similar
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
Describe the correct sequence of tests
1) Carbonate test
2) Sulfate test
3) Halide test
Why do you do the carbonate test first?
Because only carbonate ions produce bubbles with dilute acid
Why do you do the sulfate test second?
- 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
Why do you do the halide test third?
- 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
What is the sequence of tests if you are testing a mixture of ions?
The same (carbonate, sulfate, halide)
Describe the ammonium (NH4+) test
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
How do you carry out the carbonate test on a mixture of ions?
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
Why do you have to use HNO3 in a carbonate test on a mixture of ions?
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
How do you carry out the sulfate test on a mixture of ions?
1) Add an excess of barium nitrate to the solution left from the carbonate test
2) Filter the solution to remove the BaSO4
Why do you have to use barium nitrate and not barium chloride in a sulfate test on a mixture of ions?
Because barium chloride contains chloride ions which will show up in the next test
How do you carry out the halide test on a mixture of ions?
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
What is the chemical test and positive result for an alkene?
Test: add bromine water drop-wise
Observation: bromine water is decolourise from orange to colourless
What is the chemical test and positive result for a haloalkane?
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
What is the chemical test and positive result for a carbonyl (aldehyde and ketone)?
Test: add 2,4-DNP
Observation: orange ppt
What is the chemical test and positive result for an aldehyde?
Test: add Tollen’s reagent and warm
Observation: silver mirror
What is the chemical test and positive result for a primary/secondary alcohol and aldehyde?
Test: add acidified potassium dichromate (VI), K2Cr2O7/H2SO4 and warm in a water bath
Observation: colour change from orange to green
