Signs of qualitative reactions of organic substances. Qualitative reactions to inorganic and organic substances (Grade 9). Qualitative reactions to cations

Lecture: Qualitative responses to organic matter and ions

Qualitative reactions are used to determine substances in mixtures. With their help, you can distinguish a substance from others, as well as find out its quantitative content. For example, such reactions are those in which a specific precipitation or evolution of gas occurs, as well as reactions in which the color of the solution occurs. Their use is possible when the substances contained in the mixture, except for the one being determined, do not give similar signs during the reaction.

The tables present various options for detecting specific ions:


Qualitative reactions to cations

Cation

Reagent

Sign of reaction

Ba 2+

Ba 2+ + SO 4 2- → BaSO 4

Cu2+

1) Precipitation of blue color:

Cu 2+ + 2OH - → Cu(OH) 2

2) Precipitation of black color:

Cu 2+ +S 2- → Cu S

Pb 2+

Precipitation of black color:

Pb 2+ + S 2- → PbS

Ag+

Precipitation of a white precipitate, insoluble in HNO 3, but soluble in ammonia NH 3 H 2 O:

Ag + + Cl − → AgCl↓

Fe2+

2) Potassium hexacyanoferrate (III) (red blood salt) K 3

1) Precipitation of a white precipitate that turns green in air:

Fe 2+ + 2OH - → Fe(OH) 2 ↓

2) Precipitation of a blue precipitate (turnbull blue):

K + + Fe 2+ + 3- → KFe↓

Fe3+

2) Potassium hexacyanoferrate (II) (yellow blood salt) K 4

3) Rhodanide ion SCN −

1) Precipitation of brown color:

Fe 3+ + 3OH - → Fe(OH) 3 ↓

2) Precipitation of a blue precipitate (Prussian blue):

K + + Fe 3+ + 4- → KFe↓

3) The appearance of intense red (blood red) staining:

Fe 3+ + 3SCN - → Fe(SCN) 3

Al 3+

Alkali (hydroxide amphoteric properties)

Precipitation of a white precipitate of aluminum hydroxide when a small amount of alkali is added:

OH − + Al3 + → Al(OH) 3

and its dissolution upon further addition:

Al(OH) 3 + NaOH → Na

NH4+

OH − , heating

Emission of gas with a pungent odor:

NH 4+ + OH - → NH 3 + H 2 O

Blue wet litmus paper

H+
(acid environment)

Indicators:

− litmus

− methyl orange

Red staining

Qualitative reactions to anions

Anion

Impact or reagent

Reaction sign. Reaction equation

SO 4 2-

Precipitation of a white precipitate, insoluble in acids:

Ba 2+ + SO 4 2- → BaSO4 ↓

NO 3 −

1) Add H 2 SO 4 (conc.) and Cu, heat

2) A mixture of H 2 SO 4 + FeSO 4

1) Formation of a blue solution containing Cu 2+ ions, brown gas evolution (NO 2)

2) The appearance of the color of nitroso-iron sulfate (II) 2+. Violet to brown color (brown ring reaction)

PO 4 3-

Precipitation of a light yellow precipitate in a neutral medium:

3Ag + + PO 4 3- → Ag3PO4↓

CrO4 2-

Precipitation of a yellow precipitate, insoluble in acetic acid, but soluble in HCl:

Ba 2+ + CrO 4 2- → BaCrO 4 ↓

S 2-

Black precipitation:

Pb 2+ + S 2- → PbS↓

CO3 2-

1) Precipitation of a white precipitate, soluble in acids:

Ca 2+ + CO 3 2- → CaCO3 ↓

2) Emission of a colorless gas ("boiling"), causing the lime water to become cloudy:

CO 3 2- + 2H + → CO 2 + H 2 O

Ca(OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O

CO 2

Lime water Ca(OH) 2

Precipitation of a white precipitate and its dissolution upon further passage of CO 2:

Ca(OH) 2 + CO 2 → CaCO 3 ↓ + H 2 O

CaCO 3 + CO 2 + H 2 O → Ca(HCO3) 2

SO 3 2-

SO 2 gas evolution with a characteristic pungent odor (SO 2):

2H + + SO 3 2- → H 2 O + SO 2

F −

Precipitation of a white precipitate:

Ca 2+ + 2F − → CaF2 ↓

Cl −

Precipitation of a white cheesy precipitate, insoluble in HNO 3 but soluble in NH 3 H 2 O (conc.):

Ag + + Cl → AgCl↓

AgCl + 2(NH 3 H 2 O) → }

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