CHEMICAL REACTIONS OF SELECTED ELEMENTS AND THEIR
COMPOUNDS
Key words: periodic table, toxicity of elements and their compounds, dissociation, anions,
cations, oxidation number, solubility of salts in water, solubility of salts in acids and bases,
solubility product, ionic equations, qualitative analysis, chemical nomenclature.
Reagents:
1. Hydrochloric acid diluted 1:1
2. Saturated water solution of sulfane !POISON!
3. Saturated water solution of ammonium sulfide
4. Ammonium carbonate solution 1 mol/l
5. Silver nitrate solution 0.1 mol/l
6. Barium nitrate solution 0.2 mol/l !POISON!
7. Nitric acid diluted 1:1
8. Sulphuric acid diluted 1:1
9. Concentrated water solution of ammonia
10. Sodium hydroxide solution 2 mol/l !CAUSTIC!
11. Potassium chromate solution 0.25 mol/l
12. Potassium iodide solution 0.5 mol/l
13. Ammonium chloride solution 1 mol/l
14. Potassium hexacyanoferrate (III) (ferricyanide) solution 0.01 mol/l
15. Potassium thiocyanate (rhodanide)solution 0.3 mol/l
16. Sodium hydrogenphosphate solution 0.3 mol/l
17. Ammonium molybdate solution in nitric acid (ammonium molybdate 7.5 g in
100 ml of water and 100 ml of nitric acid 320 g/l)
18. Diphenylamine solution in sulphuric acid 0.06 mol/l
19. Samples of solutions containing known cations
20. Samples of solutions containing known anions
21. Samples of solutions containing unknown compounds
1
The qualitative analysis
Analytical chemistry analyses unknown samples to answer two questions: what
substances are present and what is their amount. There are therefore two parts of analytical
chemistry: the qualitative and the quantitative analysis. We will deal here with qualitative
analysis of inorganic substances. The presented procedures are useful for studying
biological and toxic properties of selected elements.
Simple inorganic salts dissociate in water solutions to cations and anions. The
qualitative analysis is based on the fact that some combinations of cations and anions in water
give insoluble precipitates of colourless or coloured compounds.
For identification, cations and anions are classified according to their reactions with
group reagents. The group reagent precipitates only ions of a given group. In the given
group, reactions specific for cations or anions within this group are used. You will identify
only selected cations and anions.
Selected cations:
2+
2+
2+
2+
2+
2+
3+
2+
2+
2+
+
+
+
Ag , Pb , Hg2 , Hg , Cu , Zn , Fe , Fe , Ca , Ba , Mg , Na , K , NH4
+
Selected anions:
232HCO3 , CO3 , PO4 , Cl , I , SO4 , NO3
-
A list of group and specific reactions of these selected cations and anions follows.
Perform all the following reactions with known samples. Familiarity with these reactions will
help you in the further experiments - analysis of an unknown compound.
The basic rules for the solubility of salts:
Salts soluble in water:
1.
containing anions NO3-, Cl- (except at combination with cations Ag+, Pb2+,
Hg22+) and SO42- (except at combination with cations Ba2+, Pb2+)
2.
containing cations Na+, K+, NH4+
Salts insoluble in water:
containing anions S2-, CO32-, HPO42- and PO43- (exept for combination with
cations Na+, K+, NH4+)
Salts containing hydrogenanions are always more soluble in water.
Salts of weak acids are soluble in stronger acids solutions, the weak acid is liberated.
Salts of weak bases are soluble in stronger bases solutions, the weak base is liberated.
CUATION! Reagents are used at a max. volume of about 1 ml without pipetting (all
reactions are only qualitative ones).
2
1. Identification of selected cations
To perform the following reactions, use the solutions containing given cations. Use
always a new portion of the sample for each reaction.
+
2+
2+
1.1. Cations: Ag , Pb , Hg2
The group reagent is HCl. The resulting chlorides are insoluble. The sulfides of these
cations are also insoluble in water; they are insoluble even in diluted strong acid solutions
because of the very low value of their solubility product:
-10
KS:
AgCl = 1,8.10
-5
PbCl2 = 1,7.10
-18
Hg2Cl2 = 1,3.10
Procedure:
+
Ag :
+
Ag + HCl
Ag2S = 2,5.10
-50
PbS = 2,5.10
Ag2CrO4 = 1,1.10-12
-27
-45
Hg2S ~ 10
+
AgCl + H
+
white precipitate soluble in NH3 forming [Ag(NH3)2]
+
+
2 Ag + K2CrO4
Ag2CrO4 + 2 K
red-brown precipitate
Pb2+:
2+
Pb + 2 HCl
+
PbCl2 + 2 H
white precipitate soluble in hot water
+
PbCrO4 + 2 K
yellow precipitate
2+
Pb + K2 CrO4
Hg22+:
2+
Hg2 + 2 HCl
+
Hg2Cl2 + 2 H
white precipitate forming black Hg2O in the reaction with NH3
2+
2+
1.2. Cations: Hg , Cu
The group reagent is H2S. Chlorides of these cations are soluble, sulphides are
insoluble in water and in diluted strong acids. Their solubility product values are extremely
low:
KS:
CuS = 8,5.10
-45
HgS = 1,6.10
-52
Procedure:
CAUTION! The sample must be acidified before the addition of the H2S solution. Few drops
of diluted HCl are sufficient.
2+
Hg :
2+
Hg + H2S
+
HgS + 2 H
black precipitate insoluble in acids
3
2+
HgI2 + 2 K+
orange-red precipitate
soluble in excess of KI forming K2[HgI4]
Hg + 2 KI
2+
Cu :
2+
Cu + H2S
+
CuS + 2 H
brown-black precipitate insoluble in acids
+
Cu(OH)2 + 2 Na
light-blue precipitate soluble in NH3 forming dark blue complex
2+
cation [Cu(NH3)4]
2+
Cu + 2 NaOH
2+,
3+
1.3. Cations: Zn2+, Fe Fe
The group reagent is (NH4)2S. Chlorides are soluble, sulphides are insoluble. They
are soluble in diluted strong acids due to the higher solubility product values:
KS:
ZnS = 6,9.10
-26
FeS = 3,7.10
-19
Procedure:
CAUTION! Check the pH of the examined sample solution before the reaction with
(NH4)2S. In the case of acidic pH, neutralise it using a NH3 solution (use a dropper), check the
pH (the pH must not be alkaline). Then add 1 ml of NH4Cl solution to the neutral sample
solution.
2+
Zn :
2+
Zn + (NH4)2S
+
ZnS + 2 NH4
white precipitate soluble in diluted acids
2+
Fe :
2+
Fe + (NH4)2S
2+
+
FeS + 2 NH4
black precipitate soluble in diluted acids
+
Fe + 2 NaOH
Fe(OH)2 + 2 Na
white precipitate colouring blue-green and forming brown
oxidation product Fe(OH)3
Fe2++ K3[Fe(CN)6]
dark blue precipitate of “Prussian Blue”
Fe3+:
2 Fe3+ + 3 (NH4)2 S
3+
Fe + 3 NaOH
Fe3++ 3 KCNS
2 FeS + S + 6 NH4+
black precipitate soluble in diluted acids
+
Fe(OH)3 + 3 Na
brown precipitate
2+
+
[Fe(CNS)] + K
red complex
4
2+
2+
1.4. Cations: Ca , Ba
The group reagent is (NH4)2CO3. The resulting carbonates are insoluble, but are
soluble in diluted strong acids. Sulfides are soluble.
If these elements are in the form of volatile compounds, they colour the bunsen flame
(a principle of the flame fotometry, a method for rapid determination of elements).
Procedure:
CAUTION! Check the pH of the examined sample solution before the reaction with
(NH4)2CO3. In the case of acidic pH, neutralise it using a NH3 solution, check the pH (pH
must not be alkaline). Then add 1 ml of NH4Cl solution to the neutral sample solution.
Flame test:
1. Dip the looped end of platin or nichrome wire in concentrated hydrochloric acid and put it
into the hottest part of the bunsen flame until it no longer colours the flame.
2. Remove it from the flame and after slight cooling dip the wire into the sample solution.
3. Put the “loaded” wire at the edge of the flame and observe the colour.
2+
Ca :
Flame reaction: brick red
2+
Ca + (NH4)2CO3
CaCO3 + 2 NH4
+
white precipitate, soluble in diluted acids
+
CaHPO4 + 2 Na
white precipitate, soluble in diluted acids
2+
Ca + Na2HPO4
2+
Ba :
Flame test: apple green
2+
Ba + (NH4)2CO3
+
BaCO3 + 2 NH4
white precipitate, soluble in diluted acids
2+
+
Ba + H2SO4
BaSO4 + 2 H
white precipitate, insoluble in diluted acids
This explains why barium sulphate is harmless and can be used as a radio contrast substance
in X-ray examinations of the gastrointestinal tract. Other barium salts are soluble in water or
in diluted acids (HCl!) and are therefore toxic.
2+
+
+
+
1.5. Cations: Mg , Na , K , NH4
There is no group reagent. The chlorides, the sulphides and the carbonates are soluble.
Only specific reactions are used for the determination of these cations..
Procedure:
2+
Mg :
Add 1 ml of NH4Cl to keep the pH slightly acidic, otherwise a precipitate of magnesium
hydroxide will form.
5
2+
+
Mg + Na2HPO4+ NH3
MgNH4PO4 + 2 Na
white precipitate
Na+:
Flame test: yellow
K+:
Flame test: lilac (violet)
Note: Blue cobalt glass filter improves the visibility of this colour as it as it removes the
colour produced by the ubiquitous traces of sodium.
NH4+
+
+
NH4 + NaOH
NH3 + Na + H2O
Wet the indicator strip and detect the rising ammonia at the top of the tube (blue colour).
2. Identification of selected anions
To perform the following reactions, use the solutions containing given anions. Use
always a new portion of the sample for each reaction. Compare the reactions used for
detection of anions and analogical reactions described above as tests for cations.
-
2-
3-
2.1. Anions: HCO3 , CO3 , PO4
The group reagent is Ba(NO3)2. Barium salts are insoluble in water but soluble in
diluted acids.
Procedure:
HCO3-:
The original solution reacts weakly alkaline, pKB(HCO3-) = 7.48.
2 HCO3 + Ba(NO3)2
Ba(HCO3)2 + 2 NO3
the product is unstable, turns into BaCO3, see the following
part.
white precipitate soluble in diluted acids (CO2 is liberated).
CO32-:
The original solution reacts alkaline, pKB (CO3)2- = 3.6.
2CO3 + Ba(NO3)2
BaCO3 + 2 NO3
white precipitate soluble in diluted acids (CO2 is liberated).
3PO4 :
3-
2 PO4 + 3 Ba(NO3)2
3-
PO4 + (NH4)2MoO4
-
Ba3(PO4)2 + 6 NO3
white precipitate soluble in diluted acids.
yellow precipitate of ammonium phosphomolybdate
(the reaction mixture must be heated to boiling)
6
2.2. Anions: Cl-, IThe group reagent is AgNO3. Silver salts are soluble neither in water nor in diluted
acids.
Procedure:
Cl :
Cl + AgNO3
AgCl + NO3
-
white precipitate soluble in NH3 forming [Ag(NH3)2]
insoluble in diluted acids
+
-
I :
I + AgNO3
-
AgI + NO3
yellow precipitate insoluble in NH3
insoluble in diluted acids
2-
2.3. Anion SO4
This anion forms white precipitate in the reaction with Ba(NO3)2, described already
above as the test for barium. The precipitate is insoluble in diluted acids.
Procedure:
2SO4 :
2-
SO4 + Ba(NO3)2
-
BaSO4 + 2 NO3
white precipitate insoluble in diluted acids.
2.4. Anion NO3This anion forms no precipitate with or Ba(NO3)2 or AgNO3.
Procedure:
NO :
3
Use the diphenylamine solution in sulphuric acid to prove the nitrate anion.,A dark blue
colour will appear.
7
3. The analysis of a solution of an unknown inorganic compound
Determine the cation and anion in the solution of an unknown inorganic compound.
Following the method mentioned above, determine the class of cation and anion and then
determine the respective ions using special reactions, as done in tasks 1.1 and 1.2. (always use
a new portion of the sample!). Determine the pH value of the analysed solution as well.
3.1. Analysis of cations - determining a class
Procedure:
1 ml of the sample+HCl
precipitate
solution
Hg 22+, Pb2+, Ag+
Hg2+, Cu2+
Zn , Fe2+, Fe3+
Ca2+, Ba2+
2+
Mg , Na+, K+, NH4+
2+
+ H2S in excess
precipitate
solution
Hg2+, Cu2+
Zn2+, Fe2+, Fe3+
Ca2+, Ba2+
2+
Mg , Na+, K+, NH4+
8
1 ml of the new sample + possible neutralisation by NH3
CHEMICAL REACTIONS OF SELECTED
+NH4Cl + (NH4)2S
ELEMENTS AND THEIR
COMPOUNDS
Key words: periodic table, toxicity of elements and their
compounds, dissociation, anions, cations, oxidation number,
precipitate
solubility
of salts in water, solubilitysolution
of salts in acids and
bases, solubility product, ionic equations, qualitative analysis,
2+
3+
Zn chemical
, Fe2+, Fenomenclature.
Ca2+, Ba2+
Mg2+, Na+, K+, NH4+
Reagents:
1. hydrochloric acid diluted 1:1
2. saturated water solution of sulfane
3. saturated water solution of ammonium sulfide
4. ammonium carbonate solution 1 mol/l
5. 1silver
solution
mol/l neutralisation by NH
ml of nitrate
the new
sample0.1
+ possible
3
6. barium nitrate solution 0.2 mol/l !POISON!
7. nitric acid diluted 1:1
8. sulphuric acid diluted 1:1
+NH Cl + (NH4)2 CO3
9. concentrated water solution4 of ammonia
10. sodium hydroxide solution 2 mol/l !CAUSTIC!
11. potassium chromate solution 0.25 mol/l
12. potassium iodide solution 0.5 mol/l
13. ammonium chloride solution 1 mol/l
solution
precipitate
14. potassium hexacyanoferrate (III) (ferricyanide) solution
0.01 mol/l
15.2+potassium
thiocyanate solution 0.32+mol/l+ +
Ca , Ba2+
Mg , Na , K , NH +
16. sodium hydrogenphosphate solution 0.3 mol/l 4
17. ammonium molybdate solution in nitric acid (ammonium
After determining the cation class, use the specific reactions given in part 1. to identify the
molybdate 7.5 g in
cation.
100 ml of water and 100 ml of nitric acid 320 g/l)
18. 0.06 mol/l solution of diphenylamine in sulphuric acid
19. solutions of known cations
20. solutions of known anions
21. solutions of unknown compounds
The qualitative analysis
Analytical chemistry analyses unknown samples to
answer two questions: what substances are present and what
is their amount. There are therefore two parts of analytical
chemistry: the qualitative and the quantitative analysis.
Further we will deal here with qualitative analysis of
inorganic substances. Presented procedures are useful for
studying biological and toxic properties of selected elements.
Simple inorganic salts dissociate in water solution to
cations and anions. The qualitative analysis is based on the
fact that some combinations of cations and anions in water
give insoluble precipitates 9 of colourless or coloured
compounds.
For identification, cations and anions are distributed into
3.2. Analysis of anions - determining a class
Procedure:
1 ml of the sample + Ba(NO3)2
precipitate
3-
-
solution
2-
2-
-
PO4 , HCO3 , CO3 , SO4
+ HNO3
solution
PO4
SO4
-
+ AgNO3
precipitate
3-
-
Cl , I , NO3
precipitate
2-
-
Cl , I
solution
-
NO3
-
CO2
After determining the anion class, use the specific reactions given in part 2. to identify the
anion.
Write down the results of analysis of the unknown samples in the table.
Sample N°
pH
Formula
Name
10