How to determine the color of the sediment. Qualitative reactions in inorganic and organic chemistry. Chairman of the Ministry of Natural Disciplines

1. Qualitative reactions to cations.
1.1. Qualitative reactions to alkali metal cations (Li +, Na +, K +, Rb +, Cs +).
Alkali metal cations can be detected by adding a small amount of salt to the burner flame. This or that cation colors the flame in the corresponding color:
Li+ - dark pink.
Na+ - yellow.
K+ - purple.
Rb+ - red.
Cs+ - blue.
Cations can also be detected using chemical reactions. When a solution of lithium salt is combined with phosphates, an insoluble in water, but soluble in conc., is formed. nitric acid, lithium phosphate:
3Li + + PO4 3- = Li 3 PO 4 ↓
Li 3 PO 4 + 3HNO 3 = 3LiNO 3 + H 3 PO 4

K + and Rb + cations can be identified by adding their salts of fluorosilicic acid H 2 or its salts - hexafluorosilicates - to solutions:
2Me + + 2- = Me 2 ↓ (Me = K, Rb)

They and Cs+ precipitate from solutions when perchlorate anions are added:
Me + + ClO 4 - = MeClO 4 ↓ (Me = K, Rb, Cs).

1.2. Qualitative reactions to alkaline earth metal cations (Ca 2+, Sr 2+, Ba 2+).
Alkaline earth metal cations can be detected in two ways: in solution and by flame color. By the way, alkaline earth minerals include calcium, strontium, and barium.
Flame color:
Ca 2+ - brick red.
Sr 2+ - carmine red.
Ba 2+ - yellowish green.

Reactions in solutions. The cations of the metals in question have a common feature: their carbonates and sulfates are insoluble. The Ca 2+ cation is preferred to be detected by the carbonate anion CO 3 2-:
Ca 2+ + CO 3 2- = CaCO 3 ↓
Which easily dissolves in nitric acid, releasing carbon dioxide:
2H + + CO 3 2- = H 2 O + CO 2
Cations Ba 2+, Sr 2+ prefer to be identified by the sulfate anion with the formation of sulfates that are insoluble in acids:
Sr 2+ + SO 4 2- = SrSO 4 ↓
Ba 2+ + SO 4 2- = BaSO 4 ↓

1.3. Qualitative reactions to the cations of lead (II) Pb 2+, silver (I) Ag +, mercury (I) Hg +, mercury (II) Hg 2+. Let's look at them using lead and silver as an example.
This group of cations has one common feature: they form insoluble chlorides. But lead and silver cations can also be detected by other halides.

Qualitative reaction to a lead cation - the formation of lead chloride (white precipitate), or the formation of lead iodide (bright yellow precipitate):
Pb 2+ + 2I - = PbI 2 ↓

Qualitative reaction to a silver cation - the formation of a white cheesy precipitate of silver chloride, a yellowish-white precipitate of silver bromide, the formation of a yellow precipitate of silver iodide:
Ag + + Cl - = AgCl↓
Ag + + Br - = AgBr↓
Ag + + I - = AgI↓
As can be seen from the above reactions, silver halides (except fluoride) are insoluble, and bromide and iodide are colored. But this is not their distinguishing feature. These compounds decompose under the influence of light into silver and the corresponding halogen, which also helps to identify them. Therefore, containers containing these salts often emit odors. Also, when sodium thiosulfate is added to these precipitates, dissolution occurs:
AgHal + 2Na 2 S 2 O 3 = Na 3 + NaHal, (Hal = Cl, Br, I).
The same thing will happen when adding liquid ammonia or its conc. solution. Only AgCl dissolves. AgBr and AgI in ammonia are practically insoluble:
AgCl + 2NH 3 = Cl

There is also another qualitative reaction to the silver cation - the formation of black silver oxide when adding alkali:
2Ag + + 2OH - = Ag 2 O↓ + H 2 O
This is due to the fact that silver hydroxide does not exist under normal conditions and immediately decomposes into oxide and water.

1.4. Qualitative reaction to cations of aluminum Al 3+, chromium (III) Cr 3+, zinc Zn 2+, tin (II) Sn 2+. These cations are combined to form insoluble bases, which are easily converted into complex compounds. Group reagent - alkali.
Al 3+ + 3OH - = Al(OH) 3 ↓ + 3OH - = 3-
Cr 3+ + 3OH - = Cr(OH) 3 ↓ + 3OH - = 3-
Zn 2+ + 2OH - = Zn(OH) 2 ↓ + 2OH- = 2-
Sn 2+ + 2OH- = Sn(OH) 2 ↓ + 2OH - = 2-
Do not forget that the bases of the Al 3+, Cr 3+ and Sn 2+ cations are not converted into a complex compound by ammonia hydrate. This is used to completely precipitate cations. Zn 2+ when adding conc. ammonia solution first forms Zn(OH) 2, and in excess, ammonia promotes the dissolution of the precipitate:
Zn(OH) 2 + 4NH 3 = (OH) 2

1.5. Qualitative reaction to iron (II) and (III) cations Fe 2+, Fe 3+. These cations also form insoluble bases. The Fe 2+ ion corresponds to iron (II) hydroxide Fe(OH) 2 - a white precipitate. In air it immediately becomes covered with a green coating, so pure Fe(OH) 2 is obtained in an atmosphere of inert gases or nitrogen N 2 .
The Fe 3+ cation corresponds to iron (III) metahydroxide FeO(OH) of brown color. Note: compounds of Fe(OH) 3 composition are unknown (not obtained). But still, the majority adhere to the notation Fe(OH) 3.
Qualitative reaction to Fe 2+:
Fe 2+ + 2OH - = Fe(OH) 2 ↓
Fe(OH) 2, being a compound of divalent iron, is unstable in air and gradually turns into iron (III) hydroxide:
4Fe(OH) 2 + O 2 + 2H 2 O = 4Fe(OH) 3

Qualitative reaction to Fe 3+:
Fe 3+ + 3OH - = Fe(OH) 3 ↓
Another qualitative reaction to Fe 3+ is the interaction with the thiocyanate anion SCN -, which produces iron (III) thiocyanate Fe(SCN) 3, which colors the solution dark red (the “blood” effect):
Fe 3+ + 3SCN - = Fe(SCN) 3
Iron(III) rhodanide is easily “destroyed” when adding alkali metal fluorides:
6NaF + Fe(SCN) 3 = Na 3 + 3NaSCN
The solution becomes colorless.
Very sensitive reaction to Fe 3+, helps to detect even very small traces of this cation.

1.6. Qualitative reaction to manganese (II) cation Mn 2+. This reaction is based on the severe oxidation of manganese in an acidic environment with a change in the oxidation state from +2 to +7. In this case, the solution turns dark purple due to the appearance of permanganate anion. Let's look at the example of manganese nitrate:
2Mn(NO 3) 2 + 5PbO 2 + 6HNO 3 = 2HMnO 4 + 5Pb(NO 3) 2 + 2H 2 O

1.7. Qualitative reaction to cations of copper (II) Cu 2+, cobalt (II) Co 2+ and nickel (II) Ni 2+. The peculiarity of these cations is the formation of complex salts - ammonia - with ammonia molecules:
Cu 2+ + 4NH 3 = 2+
Ammonia gives solutions bright colors. For example, copper ammonia colors the solution bright blue.

1. Qualitative reactions to cations.
1.1.1 Qualitative reactions to alkali metal cations (Li +, Na +, K +, Rb +, Cs +).
Alkali metal cations can only be carried out with dry salts, because Almost all alkali metal salts are soluble. They can be detected by adding a small amount of salt to the burner flame. This or that cation colors the flame in the corresponding color:
Li+ - dark pink.
Na+ - yellow.
K+ - purple.
Rb+ - red.
Cs+ - blue.
Cations can also be detected using chemical reactions. When a solution of lithium salt is combined with phosphates, an insoluble in water, but soluble in conc., is formed. nitric acid, lithium phosphate:
3Li + + PO4 3- = Li 3 PO 4 ↓
Li 3 PO 4 + 3HNO 3 = 3LiNO 3 + H 3 PO 4

The K + cation can be removed by the hydrogen tartrate anion HC 4 H 4 O 6 - - by the tartaric acid anion:
K + + HC 4 H 4 O 6 - = KHC 4 H 4 O 6 ↓

K + and Rb + cations can be identified by adding their salts of fluorosilicic acid H 2 or its salts - hexafluorosilicates - to solutions:
2Me + + 2- = Me 2 ↓ (Me = K, Rb)

They and Cs+ precipitate from solutions when perchlorate anions are added:
Me + + ClO 4 - = MeClO 4 ↓ (Me = K, Rb, Cs).

1.1.2 Qualitative reactions to cations of alkaline earth metals (Ca 2+, Sr 2+, Ba 2+, Ra 2+).
Alkaline earth metal cations can be detected in two ways: in solution and by flame color. By the way, alkaline earth minerals include calcium, strontium, barium and radium. Beryllium and magnesium it is forbidden belong to this group, as they like to do on the Internet.
Flame color:
Ca 2+ - brick red.
Sr 2+ - carmine red.
Ba 2+ - yellowish green.
Ra 2+ - dark red.

Reactions in solutions. The cations of the metals in question have a common feature: their carbonates and sulfates are insoluble. The Ca 2+ cation is preferred to be detected by the carbonate anion CO 3 2-:
Ca 2+ + CO 3 2- = CaCO 3 ↓
Which easily dissolves in nitric acid, releasing carbon dioxide:
2H + + CO 3 2- = H 2 O + CO 2
The cations Ba 2+ , Sr 2+ and Ra 2+ prefer to be identified by the sulfate anion with the formation of sulfates that are insoluble in acids:
Sr 2+ + SO 4 2- = SrSO 4 ↓
Ba 2+ + SO 4 2- = BaSO 4 ↓
Ra 2+ + SO 4 2- = RaSO 4 ↓

1.1.3. Qualitative reactions to the cations of lead (II) Pb 2+, silver (I) Ag +, mercury (I) Hg 2 +, mercury (II) Hg 2+. Let's look at them using lead and silver as an example.
This group of cations has one common feature: they form insoluble chlorides. But lead and silver cations can also be detected by other halides.

A qualitative reaction to a lead cation is the formation of lead chloride (white precipitate) or the formation of lead iodide (bright yellow precipitate):
Pb 2+ + 2I - = PbI 2 ↓

Qualitative reaction to a silver cation - the formation of a white cheesy precipitate of silver chloride, a yellowish-white precipitate of silver bromide, the formation of a yellow precipitate of silver iodide:
Ag + + Cl - = AgCl↓
Ag + + Br - = AgBr↓
Ag + + I - = AgI↓
As can be seen from the above reactions, silver halides (except fluoride) are insoluble, and bromide and iodide even have color. But this is not their distinguishing feature. These compounds decompose under the influence of light into silver and the corresponding halogen, which also helps to identify them. Therefore, containers containing these salts often emit odors. Also, when sodium thiosulfate is added to these precipitates, dissolution occurs:
AgHal + 2Na 2 S 2 O 3 = Na 3 + NaHal, (Hal = Cl, Br, I).
The same thing will happen when adding liquid ammonia or its conc. solution. Only AgCl dissolves. AgBr and AgI in ammonia are practically insoluble:
AgCl + 2NH 3 = Cl

There is also another qualitative reaction to the silver cation - the formation of black silver oxide when adding alkali:
2Ag + + 2OH - = Ag 2 O↓ + H 2 O
This is due to the fact that silver hydroxide does not exist under normal conditions and immediately decomposes into oxide and water.

1.1.4. Qualitative reaction to cations of aluminum Al 3+, chromium (III) Cr 3+, zinc Zn 2+, tin (II) Sn 2+. These cations are combined to form insoluble bases, which are easily converted into complex compounds. Group reagent - alkali.
Al 3+ + 3OH - = Al(OH) 3 ↓ + 3OH - = 3-
Cr 3+ + 3OH - = Cr(OH) 3 ↓ + 3OH - = 3-
Zn 2+ + 2OH - = Zn(OH) 2 ↓ + 2OH- = 2-
Sn 2+ + 2OH- = Sn(OH) 2 ↓ + 2OH - = 2-
Do not forget that the bases of the Al 3+, Cr 3+ and Sn 2+ cations are not converted into a complex compound by ammonia hydrate. This is used to completely precipitate cations. Zn 2+ when adding conc. ammonia solution first forms Zn(OH) 2, and in excess, ammonia promotes the dissolution of the precipitate:
Zn(OH) 2 + 4NH 3 = (OH) 2

1.1.5. Qualitative reaction to iron (II) and (III) cations Fe 2+, Fe 3+. These cations also form insoluble bases. The Fe 2+ ion corresponds to iron (II) hydroxide Fe(OH) 2 - a white precipitate. In air it immediately becomes covered with a green coating, so pure Fe(OH) 2 is obtained in an atmosphere of inert gases or nitrogen N 2 .
The Fe 3+ cation corresponds to iron (III) metahydroxide FeO(OH) of brown color. Note: compounds of Fe(OH) 3 composition are unknown (not obtained). But still, the majority adhere to the notation Fe(OH) 3.
Qualitative reaction to Fe 2+:
Fe 2+ + 2OH - = Fe(OH) 2 ↓
Fe(OH) 2, being a compound of divalent iron, is unstable in air and gradually turns into iron (III) hydroxide:
4Fe(OH) 2 + O 2 + 2H 2 O = 4Fe(OH) 3

Qualitative reaction to Fe 3+:
Fe 3+ + 3OH - = Fe(OH) 3 ↓
Another qualitative reaction to Fe 3+ is the interaction with the thiocyanate anion SCN -, which produces iron (III) thiocyanate Fe(SCN) 3, which colors the solution dark red (the “blood” effect):
Fe 3+ + 3SCN - = Fe(SCN) 3
Iron(III) rhodanide is easily “destroyed” when adding alkali metal fluorides:
6NaF + Fe(SCN) 3 = Na 3 + 3NaSCN
The solution becomes colorless.
Very sensitive reaction to Fe 3+, helps to detect even very small traces of this cation.

1.1.6. Qualitative reaction to manganese (II) cation Mn 2+. This reaction is based on the severe oxidation of manganese in an acidic environment with a change in the oxidation state from +2 to +7. In this case, the solution turns dark purple due to the appearance of permanganate anion. Let's look at the example of manganese nitrate:
2Mn(NO 3) 2 + 5PbO 2 + 6HNO 3 = 2HMnO 4 + 5Pb(NO 3) 2 + 2H 2 O

1.1.7. Qualitative reaction to cations of copper (II) Cu 2+, cobalt (II) Co 2+ and nickel (II) Ni 2+. The peculiarity of these cations is the formation of complex salts - ammonia - with ammonia molecules:
Cu 2+ + 4NH 3 = 2+
Ammonia gives solutions bright colors. For example, copper ammonia colors the solution bright blue.

1.1.8. Qualitative reactions to ammonium cation NH 4 +. Interaction of ammonium salts with alkalis during boiling:
NH 4 + + OH - =t= NH 3 + H 2 O
When held up, wet litmus paper will turn blue.

1.1.9. Qualitative reaction to cerium (III) cation Ce 3+. Interaction of cerium (III) salts with an alkaline solution of hydrogen peroxide:
Ce 3+ + 3OH - = Ce(OH) 3 ↓
2Ce(OH) 3 + 3H 2 O 2 = 2Ce(OH) 3 (OOH)↓ + 2H 2 O
Cerium (IV) peroxohydroxide has a red-brown color.

1.2.1. Qualitative reaction to bismuth (III) cation Bi 3+. Formation of a bright yellow solution of potassium tetraiodobismutate (III) K when a solution containing Bi 3+ is exposed to excess KI:
Bi(NO 3) 3 + 4KI = K + 3KNO 3
This is due to the fact that insoluble BiI 3 is first formed, which is then bound with I - into a complex.
This is where I will finish the description of identifying cations. Now let's look at qualitative reactions to some anions.

Qualitative analysis designed to detect individual elements or ions that make up a substance.

Analytical reactions are accompanied by an analytical effect that allows one to obtain information about the presence of the element being determined. Analytical effects include: precipitation or dissolution of a precipitate, release of gaseous products, change in the color of the solution, and the formation of crystals of a certain shape.

To determine the presence of substances, anions, cations, qualitative reactions. Having carried them out, you can clearly confirm their presence. These reactions are widely used in qualitative analysis, the purpose of which is to determine the presence of substances or ions in solutions or mixtures. We present the minimum quality reactions required to pass the Unified State Exam.

I. Qualitative reactions to cations.

1. Hydrogen cation H +, change in color of indicators: red litmus, pink-red - methyl orange.

2. Ammonium ion:

NH + 4 + OH → NH 3 + H 2 O (smell or blue discoloration of wet litmus paper).

3. Fe 2+ ion:

3Fe 2+ + 2 2 ↓ (Turnboolean blue); Fe 2+ + 2OH = Fe(OH) 2 ↓ . (greenish precipitate).

4. Fe 3+ ion:

4Fe 3+ + 3 4- → Fe 4 3 ↓ (Prussian blue);

Fe 3+ + 3CNS → Fe(CNS) 3 (blood red);

Fe 3+ + 3OH - = Fe(OH) 3 ↓ (brown sediment).

5. IonA1 3+:

Al 3+ + 3OH - →A1(OH) 3 ↓ (white precipitate, dissolves in excess alkali).

6. Ion Ba 2+:

Ba 2+ + SO 4 2- → BaSO 4 ↓ . (white precipitate).

7. Ca 2+ ion:

Ca 2+ + CO 3 2- →CaCO 3 ↓ . (white precipitate).

8. Ion Cu 2+:

Cu 2+ + 2OH - → Cu(OH) 2 ↓ (blue precipitate).

9. Ag+ ion:

Ag + + CI - → AgCl ↓ (white cheesy sediment).

10. Flame color:

II. Qualitative reactions to anions.

1. Hydroxide ion:OH -: change in color of indicators: litmus - blue, phenolphthalein - crimson, methyl orange - yellow.

2. Halide ions:

F - + Ag + → no precipitate is formed;

C1 - + Ag + → AgC ↓ - white precipitate

Br - + Ag + →AgBr ↓ - yellowish-white precipitate

I - + Ag + →AgI ↓ - bright yellow sediment

3. Sulfide ion:

H 2 S + Pb(NO 3) 2 →PbS ↓ + 2HNO 3 ;

CuSO 4 + H 2 S (Na 2 S) → H 2 SO 4 (Na,SO 4) + CuS ↓ (black residue).

4. Sulfate ion:

BaCI 2 + H,SO 4 →BaSO 4 ↓ + 2HC1; Ba 2+ + SO 4 2- = BaSO 4 ↓ (white precipitate).

5. Nitrate ion:

Сu 2+ + NO 3 - + 2Н + →Сu 2+ + NO 2 + Н 2 O (brown gas).

6. Phosphate ion:

PO 4 3- + 3Ag + → Ag 3 PO 4 ↓ (yellow precipitate, which, unlike AgBr precipitate, is soluble in mineral acids).

7. Chromate ion:

CrO 4 2- + Ba 2+ → BaCrO 4 ↓ . (yellow precipitate).

8. Carbonate ion, detection of C0 2:
CO 3 2- + 2H + → CO 2 + H 2 O;

CO 2 + Ca(OH) 2 →CaCO 3 + H 2 O;

CaCO 3 + CO 2 + H 2 O →Ca(HCO 3) 2.

III. Qualitative reaction to ozone:

2KI + O 3 + H 2 O → I 2 ↓ + 2KON + O 2 ; KI + O 2 → does not work

The formation of iodine can be proven by a change in the color of the solution in the presence of starch: blueing occurs.

Identification of organic compounds

1. Qualitative reactions to compounds containing double and triple bonds (alkenes, alkadienes, alkynes, etc.). Discoloration of potassium permanganate:

3CH 2 = CH 2 + 2KMnO 4 + 4H 2 O → 3CH 2 OH - CH 2 OH + 2MnO 2 + 2KOH;

3C H = CH + 8KMpO 4 → 3KOOS-SOOC + 8MpO 2 +2KOH + 2H 2 O.

Discoloration of bromine water:

H 3 C-CH 2 -CH = CH 2 + Br 2 → H 3 C-CH 2 -CH-CH 2 ;

CH≡CH + 2Br 2 → CHBr 2 -CHBr 2

CH 2 = CH-COOH + Br 2 → CH 2 Br-CHBg-COOH.

Qualitative reactions to polyhydric alcohols, mono- and disaccharides.

Interaction with Cu(OH) 2 in the cold is qualitative reaction for polyhydric alcohols, as well as for mono- and disaccharides:

Monosaccharide (disaccharide) + Cu(OH) (blue precipitate) → blue solution:

3. Qualitative reaction to phenols.

C 6 H 5 OH + FeCl 3 → complex compound of dark purple color.

4. Qualitative reactions “Silver Mirror” and with freshly prepared Cu(OH)2 precipitate on the aldehyde group:

CH 3 CHO + Ag 2 O(NH 3) → CH 3 COOH + 2Ag |;

HCNO + 2Ag 2 O(NH 3) → CO 2 + H 2 O + 4Ag ↓

CH 2 OH-(CHOH) 4 -CHO+Ag 2 O(NH 3) → CH 2 OH-(CHOH) 4 -COOH + 2Ag ↓ ;

CH 3 CHO + 2Cu(OH) 2 →CH 3 COOH + Cu 2 O ↓ + 2H 2 O

5. Qualitative reactions to organic acids:
CH 3 COOH: red litmus;

CH 3 COOH + Na 2 CO 3 → CH 3 COONa + H 2 O + CO 2 (gas evolution);

NCOUN: red litmus;

2HCOOH + Na 2 CO 3 → 2HCOONa + H 2 O + CO 2 (gas evolution);

HCOOH + Ag 2 O(NH 3) → CO 2 + H 2 O + 2Ag ↓

6. Qualitative reaction with iodine to starch:

(C 6 H |0 O 5) n + I 2 →blue color.

Qualitative reactions to proteins

a) biuret reaction.

When the protein is treated with a concentrated alkali solution and a solution of copper sulfate, a red-violet color appears, caused by the formation of a copper complex of the protein (reaction to a peptide bond);

b) xanthoprotein reaction.

When exposed to concentrated nitric acid, the protein turns yellow. The reaction is associated with the presence of aromatic groups in the protein molecule, which are nitrated under mild conditions;

c) sulfhydryl reaction.

When lead (II) acetate and sodium hydroxide are added to a protein solution upon heating, a black precipitate of lead sulfide precipitates due to the presence of thiol (sulfhydryl) groups in the protein.

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