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3.8 - Reactivity trends > Qualitative analysis > Flashcards

Qualitative analysis Flashcards

1
Q

What does qualitative analysis rely on?

A

Simple observations, rather than measurements

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2
Q

What may the observations in qualitative analysis be?

A

Gas bubbles, precipitates, colour changes, or identification of gases

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3
Q

What are the three tests for anions?

A

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

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4
Q

What is the carbonate test?

A

Carbonates react with acids to form carbon dioxide gas

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5
Q

What reaction is used as the basis for a test for the carbonate ion?

A

Reaction of dilute nitric acid with aqueous sodium carbonate

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6
Q

What is the equation for the reaction between nitric acid and sodium carbonate?

A

Na2CO3 (aq) + 2HNO3 (aq) –> 2NaNO3 (aq) + H2O (l)

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7
Q

What are the three steps of the carbonate test?

A

1) In a test tube, add dilute nitric acid to the solid or solution to be tested

2) If you see bubbles, the unknown compound could be a carbonate

3) To prove that the gas is carbon dioxide, bubble the gas through limewater

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8
Q

What is limewater?

A

A saturated aqueous solution of calcium hydroxide, Ca(OH)2

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9
Q

What indicates there is a carbonate in the solution?

A

Carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the limewater cloudy

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10
Q

What is the equation for the limewater test?

A

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

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11
Q

Which sulphate is used for the sulphate test and why?

A

Barium sulphate because it is very insoluble

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12
Q

What is the sulphate test?

A

Aqueous barium ions are added to a solution of an unknown compound

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13
Q

What is the basis of the sulphate test?

A

The formation of a white precipitate of barium sulphate

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14
Q

What is the ionic equation for the sulphate test?

A

Ba2+ (aq) + SO4 2- (aq) –> BaSO4 (s)

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15
Q

What are the Ba2+ ions added as?

A

Aqueous barium chloride or barium nitrate?

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16
Q

Which barium compound should be used if carrying out a halide test afterwards?

A

Barium nitrate

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17
Q

Why should barium nitrate be used if carrying out a halide test afterwards?

A

With barium chloride, you are introducing chloride ions to the solution

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18
Q

What is the halide test?

A

Aqueous silver ions react with aqueous halide ions to form precipitates of silver halides

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19
Q

Why are silver halides used for the halide test?

A

Most halides are soluble in water, but silver halides are insoluble

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20
Q

What is the general equation for the halide test?

A

Ag+ (aq) + X- (aq) –> AgX (s)

21
Q

What are the three steps of the halide test?

A

1) Add aqueous silver nitrate, AgNO3, to an aqueous solution of a halide

2) The silver halide precipitates are different colours - white, cream, and yellow

3) Add aqueous ammonia to test the solubility of the precipitate

22
Q

What colour is the silver chloride precipitate?

A

White

23
Q

What colour is the silver bromide precipitate?

A

Cream

24
Q

What colour is the silver iodide precipitate?

A

Yellow

25
Q

Why is adding ammonia to the precipitate useful?

A

The three precipitate colours can be difficult to tell apart

26
Q

What is the equation for the silver chloride test?

A

Ag+ (aq) + Cl- (aq) –> AgCl (s)

27
Q

What is the solubility of silver chloride?

A

Soluble in dilute NH3

28
Q

What is the equation for the silver bromide test?

A

Ag+ (aq) + Br- (aq) –> AgBr (s)

29
Q

What is the solubility of silver bromide?

A

Soluble in concentrated NH3

30
Q

What is the equation for the silver iodide test?

A

Ag+ (aq) + I- (aq) –> AgI (s)

31
Q

What is the solubility of silver iodide?

A

Insoluble in concentrated NH3

32
Q

What is the correct order for anion tests?

A

1) Carbonate, CO3 2-
2) Sulphate, SO4 2-
3) Halides, Cl-. Br-, and I-

33
Q

Why is it important to carry out the tests for anions in the correct order?

A

You could obtain confusing results and make an incorrect identification

34
Q

What is the purpose of the three anion tests?

A

To identify an unknown inorganic compound

35
Q

Why is the carbonate test done first?

A

You add a little dilute acid and look for effervescence of carbon dioxide gas
Neither sulphate nor halide ions produce bubbles with dilute acid, so the test can be carried out without the possibility of an incorrect conclusion - no bubbles = no carbonate

36
Q

Why is the sulphate test done second?

A

You add a solution containing Ba2+ ions and look for a white precipitate of BaSO4
Barium carbonate, BaCO3, is white and insoluble, so if a sulphate test is carried out on a carbonate a white precipitate forms too - important to carry out carbonate test first to know there is NO carbonate present

37
Q

Why is the halide test done last?

A

You add a solution containing Ag+ ions, as AgNO3, and look for a precipitate
Silver carbonate and silver sulphate are both insoluble so will form precipitates anyway - important to carry out halide test last to rule out carbonates and sulphates

38
Q

How is the carbonate test carried out on a mixture of ions?

A

If you see bubbles, continue adding dilute nitric acid until bubbling stops - all carbonate ions will be removed and none will be left to react in other tests

39
Q

Which acid should be used on a mixture if you intend to test for sulphate and halides afterwards?

A

Dilute nitric acid, HNO3

40
Q

Why is it important to use dilute nitric acid, rather than sulphuric or hydrochloric acid, for the carbonate test when testing a mixture?

A

Sulphuric acid contains sulphate ions and hydrochloric acid contains chloride ions, which will show up on the halide and sulphate tests

41
Q

How is the sulphate test carried out on a mixture of ions?

A

To the solution left from the carbonate test, add an excess of Ba(NO3)2 (aq) - any sulphate ions will precipitate out as barium sulphate
Filter to remove the barium sulphate

42
Q

Why is it important not to use BaCl2 (aq) in the sulphate test if you intent to test for halide ions when testing a mixture?

A

The chloride ions will show up in the halide tests

43
Q

How is the halide test carried out on a mixture of ions?

A

To the solution left from the sulphate test, add AgNO3 (aq) - carbonate and sulphate ions have already been removed so any precipitates must involve halide ions
Add NH3 to confirm which halide it is

44
Q

What is the one test for cations?

A

Test for ammonium ion, NH4+

45
Q

What is the test for ammonium ion?

A

When heated together, aqueous ammonium ions and aqueous hydroxide ions react to form ammonia gas, NH3

46
Q

What is the equation for the test for ammonium ion?

A

NH4+ (aq) + OH- (aq) –> NH3 (g) + H2O (l)

47
Q

What are the four steps of the test for ammonium ion?

A

1) Aqueous sodium hydroxide, NaOH, is added to a solution of an ammonium ion

2) Ammonia gas is produced - unlikely to see gas bubbles as ammonia is very soluble

3) Mixture is warmed and ammonia gas is released

48
Q

How are results obtained in the ammonia test?

A

Ammonia can be smelt, but it is easy to test the gas with moist pH indicator paper - ammonia is alkaline and will turn the paper blue

3.8 - Reactivity trends (3 decks)

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