Colored rain. Color Chemistry of Qualitative Reactions Basic Precipitation in Chemistry

Almost all chromium compounds and their solutions are intensely colored. Having a colorless solution or a white precipitate, we can conclude with a high degree of probability that chromium is absent. Compounds of hexavalent chromium are most often colored yellow or red, while trivalent chromium is characterized by greenish tones. But chromium is also prone to the formation of complex compounds, and they are painted in a variety of colors. Remember: all chromium compounds are poisonous.

Potassium dichromate K 2 Cr 2 O 7 is perhaps the most famous of the chromium compounds and is the easiest to obtain. A beautiful red-yellow color indicates the presence of hexavalent chromium. Let us carry out several experiments with it or with sodium dichromate very similar to it.

We strongly heat in the flame of a Bunsen burner on a porcelain shard (a piece of crucible) such an amount of potassium dichromate that will fit on the tip of a knife. Salt will not release water of crystallization, but will melt at a temperature of about 400 ° C with the formation of a dark liquid. Let's warm it up for a few more minutes on a strong flame. After cooling, a green precipitate forms on the shard. We will dissolve part of it in water (it will turn yellow), and leave the other part on the shard. The salt decomposed when heated, resulting in the formation of soluble yellow potassium chromate K 2 CrO 4, green chromium oxide (III) and oxygen:

2K 2 Cr 2 O 7 → 2K 2 CrO 4 + Cr 2 O 3 + 3/2O 2
Due to its tendency to release oxygen, potassium dichromate is a strong oxidizing agent. Its mixtures with coal, sugar or sulfur ignite vigorously on contact with the flame of a burner, but do not give an explosion; after combustion, a voluminous layer of green is formed - due to the presence of chromium oxide (III)-ash.

Carefully! Burn no more than 3-5 g on a porcelain shard, otherwise the hot melt may start to splatter. Keep your distance and wear safety goggles!

We scrape off the ash, wash it with water from potassium chromate and dry the remaining chromium oxide. Let's prepare a mixture consisting of equal parts of potassium nitrate (potassium nitrate) and soda ash, add it to chromium oxide in a ratio of 1:3 and melt the resulting composition on a shard or magnesia stick. Dissolving the cooled melt in water, we get a yellow solution containing sodium chromate. Thus, molten saltpeter oxidized trivalent chromium to hexavalent. By fusion with soda and saltpeter, all chromium compounds can be converted into chromates.

For the next experiment, let's dissolve 3 g of powdered potassium dichromate in 50 ml of water. To one part of the solution, add a little potassium carbonate (potash). It will dissolve with the release of CO2, and the color of the solution will become light yellow. Chromate is formed from potassium dichromate. If we now add a 50% solution of sulfuric acid in portions (Caution!), Then the red-yellow color of the bichromate will appear again.

Pour 5 ml of potassium dichromate solution into a test tube, boil with 3 ml of concentrated hydrochloric acid under draft or in the open air. Yellow-green poisonous chlorine gas is released from the solution, because chromate will oxidize HCl to chlorine and water. Chromate itself will turn into green trivalent chromium chloride. It can be isolated by evaporating the solution, and then, fusing with soda and nitrate, converted to chromate.

In another test tube, carefully add 1-2 ml of concentrated sulfuric acid to potassium dichromate (in an amount that fits on the tip of a knife). (Caution! The mixture may splatter! Wear safety goggles!) We heat the mixture strongly, as a result, brownish-yellow hexavalent chromium oxide CrOz is released, which is poorly soluble in acids and well in water. It is anhydride of chromic acid, but sometimes it is called chromic acid. It is the strongest oxidizing agent. Its mixture with sulfuric acid (chromium mixture) is used for degreasing, since fats and other hard-to-remove contaminants are converted into soluble compounds.

Attention! Extreme care must be taken when working with the chromium mixture! If splashed, it can cause severe burns! Therefore, in our experiments, we will refuse to use it as a cleaning agent.

Finally, consider the reactions of detection of hexavalent chromium. Place a few drops of potassium dichromate solution in a test tube, dilute it with water and carry out the following reactions.

When a solution of lead nitrate is added (Caution! Poison!) Yellow lead chromate (chrome yellow) precipitates; when interacting with a solution of silver nitrate, a red-brown precipitate of silver chromate is formed.

Add hydrogen peroxide (properly stored) and acidify the solution with sulfuric acid. The solution will take on a deep blue color due to the formation of chromium peroxide. The peroxide, when shaken with some ether (Caution! Fire hazard!) will turn into an organic solvent and turn it blue.

The latter reaction is specific for chromium and is very sensitive. It can be used to detect chromium in metals and alloys. First of all, it is necessary to dissolve the metal. But, for example, nitric acid does not destroy chromium, as we can easily verify by using pieces of damaged chromium plating. With prolonged boiling with 30% sulfuric acid (hydrochloric acid can be added), chromium and many chromium-containing steels are partially dissolved. The resulting solution contains chromium (III) sulfate. To be able to conduct a detection reaction, we first neutralize it with caustic soda. Gray-green chromium (III) hydroxide will precipitate, which will dissolve in excess NaOH and form green sodium chromite.

Filter the solution and add 30% hydrogen peroxide (Caution! Poison!). When heated, the solution will turn yellow, as chromite is oxidized to chromate. Acidification will result in a blue color of the solution. The colored compound can be extracted by shaking with ether. Instead of the method described above, thin filings of a metal sample can be alloyed with soda and nitrate, washed, and the filtered solution tested with hydrogen peroxide and sulfuric acid.

Finally, let's test with a pearl. Traces of chromium compounds give a bright green color with brown.

Lesson objectives:

• determination of the factors that cause the coloring of chemicals;
• expansion and systematization of knowledge on the chemical foundations of the theory of the origin of color;
• development of cognitive interest in the study of qualitative reactions.

Formed competencies of students:

• the ability to analyze the phenomena of the surrounding world in chemical terms;
• the ability to explain chemical phenomena associated with the appearance of color solutions;
• willingness to work independently with information;
• willingness to interact with colleagues and speak in front of an audience.

"All living things strive for color." W. Goethe

Knowledge update

In previous lessons, we have studied the properties of inorganic and organic substances, often using qualitative reactions that indicate the presence of a particular substance by color, smell, or sediment. The crossword puzzle offered to you consists of the names of chemical elements that have color differences.

Crossword solution:

Vertically:

1) Substance that turns flames purple (potassium).

2) The lightest silvery metal (lithium).

Horizontally:

3) The name of this element is "green branch" (thallium)

4) The metal that stains glass blue (niobium)

5) The name of the metal means sky blue (cesium)

6) Violet vapors of this substance were first obtained by Courtois thanks to his cat (iodine).

Motivation of educational activity.

Please note that the solution to the crossword puzzle was related to the color of the substances. But not only chemicals, but the world around us is colorful.

"All living things strive for color." These words of the great genius of poetry truly reflect the peculiarity of the emotions that this or that color evokes in us. We perceive it associatively, i.e. recall something familiar and familiar. The perception of color is accompanied by certain emotions. (Demonstration of paintings by artists).

Students answer questions about emotions on the perception of color.

• The blue color evokes calmness, it is pleasant, it increases the assessment of self-affirmation.
• Green - the color of green plants, the mood of peace, tranquility.
• Yellow is the spirit of happiness, fun, associated with the sun.
• Red is the color of activity, action, you want to achieve results.
• Black - causes sadness, irritation.

Why is the world around us so colorful?

Today we are trying to find the answer to the question "What is a color?" in terms of chemistry.

The topic of the lesson is "Color Chemistry of Qualitative Reactions".

Determination of color factors

It is impossible to consider the chemical essence of color without knowledge of the physical properties of visible light. Without light there is no coloring of objects, everything seems dark. Light is electromagnetic waves. How much joy a rainbow in the sky brings to both children and adults, however, it appears only if the sun's rays are reflected in water droplets and return to the human eye with a multi-colored spectrum. We owe the great English physicist Isaac Newton the fact that he explained this phenomenon: white is a combination of rays of different colors. Each wavelength corresponds to a certain energy that these waves carry. The color of any substance is determined by the wavelength, the energy of which prevails in this radiation. The color of the sky depends on how much sunlight reaches our eyes. Rays with a short wavelength (blue) are reflected from the molecules of air gases and scattered. Our eye perceives them and determines the color of the sky - blue, blue (Table 1.)

Table 1 - Color of substances having one absorption band in the visible part of the spectrum.

The same happens in the case of colored substances. If a substance reflects rays of a certain wavelength, then it is colored. If the energy of light waves of the entire spectrum is equally absorbed or reflected, then the substance appears black or white. From biology lessons, you know that the human eye contains an optical system: the lens and the vitreous body. The retina contains light-sensitive elements: cones and rods. The cones allow us to distinguish colors.

Thus, what we call color is the result of two physical and chemical phenomena: the interaction of light with the molecules of a substance and the effect of waves coming from a substance on the retina of the eyes.

1 color formation factor is light.

Consider examples of the next factor - the structure of substances.

Metals have a crystalline structure, they have an ordered structure of atoms and electrons. Color is related to the mobility of electrons. When illuminating metals, reflection predominates, their color depends on the wavelength they reflect. (Demonstration of the collection of metals). The white luster is due to the uniform reflection of almost the entire set of visible rays. This is the color of aluminum, zinc. Gold has a reddish-yellow color because it absorbs blue, indigo and violet rays. Copper also has a reddish color. Magnesium powder is black, which means that this substance absorbs the entire spectrum of rays.

Let's see how the color of a substance changes from the state of the structure using sulfur as an example.

Demonstration of the video film "Chemical elements".

We conclude: sulfur in the crystalline state is yellow, and in the amorphous state it is black, i.e. in this case, the color factor is the structure of the substance.

What happens to the color of substances when the structure is destroyed, for example, during the dissociation of salt molecules, if these solutions are colored.

CuS0 4 (blue) Cu 2+ + SO 4 2-

NiS0 4 (green) Ni 2+ + SO 4 2-

CuCI 2 (blue) Cu 2+ + 2CI -

FeCI 3 (yellow) Fe 3+ +3CI -

In these solutions, the same anions, different cations give color.

The following solutions have the same cation, but different anions, so the anions are responsible for the color:

K 2 Cr 2 O 7 (orange) 2K + +Cz 2 O 4 2-

K 2 Cr0 4 (yellow) 2K + + Cz0 4 2-

KMnO 4 (violet) K + + Mn04 -

The 3rd factor in the appearance of color is the ionic state of substances.

The color also depends on the environment around the colored particles. Cations and anions in solution are surrounded by a shell of a solvent that affects the ions.

We carry out the following experiment. There is a solution of beet juice (crimson color). Add the following to this solution:

1. an experience. Beet juice solution and acetic acid
2. an experience. Beet juice solution and NH 4 0H solution
3. an experience. A solution of beet juice and water.

In experiment 1, an acidic medium causes a color change to purple, in experiment 2, an alkaline medium changes the color of the beets to blue, and the addition of water (neutral medium) does not cause color changes.

A well-known indicator for the determination of an alkaline environment is phenolphthalein, which changes the color of alkali solutions to crimson.

Experience is being made:

NaOH + phenolphthalein -> crimson color

We conclude: the 4th color change factor is the environment.

Let us consider the case of the environment of an atom of one element by various complexes.

An experiment is being carried out: a qualitative reaction to the Fe 3+ ion:

FeCl 3 + KCNS -> red color

FeCl 3 + K 4 (Fe(CN) 6) -> p-p dark blue

A historical fact is associated with a change in the color of the iron ion when surrounded by potassium thiocyanate in a bloody color.

Student messages.

In 1720, political opponents of Peter I from the clergy organized a "miracle" in one of the St. Petersburg cathedrals - the icon of the Mother of God began to shed tears, which was commented on as a sign of her disapproval of Peter's reforms. Peter I carefully examined the icon and noticed something suspicious: he found small holes in the eyes of the icon. He also found the source of the tears: it was a sponge soaked in a solution of iron thiocyanate, which has a blood-red color. The weight evenly pressed on the sponge, squeezing out drops through a hole in the icon. "Here is the source of miraculous tears," said the Emperor.

We are experimenting.

We write words on paper with solutions of CuS0 4 (blue) and FeСI 3 (yellow), then we process the sheet with yellow blood salt K 4 (Fe (CN) 6). The word CuSO 4 (cyan) turns red and the word FeCI 3 (yellow) turns blue-green. There is no change in the oxidation state of the metal, only the environment changed:

2CuS0 4 + K 4 (Fe(CN) 6) Cu 2 (Fe(CN) 6) + 2K 2 SO 4

4FeCl 3 + 3 K 4 (Fe(CN) 6) Fe 4 (Fe(CN) 6) 3 +12 KCI

5th color factor - environment of ions by complexes.

Conclusion.

We have identified the main factors influencing the manifestation of the color of substances.

We realized that color is the result of the absorption of a certain part of the visible spectrum of sunlight by a substance.

A qualitative reaction is a special reaction that detects ions or molecules by color.

Messages of students on the topic "Color serves people".

Animal blood and leaf greens contain similar structures, but blood contains iron ions - Fe, and plants - Mg. This ensures the color: red and green. By the way, the saying "blue blood" is true for deep-sea animals, whose blood contains vanadium instead of iron. Also, algae that grow in places where there is little oxygen have a blue color.

Plants with chlorophyll are able to form organomagnesium substances and use the energy of light. The color of photosynthetic plants is green.

Iron-containing hemoglobin is used to carry oxygen throughout the body. Hemoglobin with oxygen stains the blood bright red, and without oxygen gives the blood a dark color.

Paints and dyes are used by artists, decorators and textile workers. Harmony of color is an integral part of the art of "design". The most ancient paints were charcoal, chalk, clay, cinnabar and some salts such as copper acetate (verdigris).

Phosphor paints are used for road signs and advertisements, rescue boats.

For the purpose of bleaching, substances are introduced into the composition of washing powders that give the fabric a bluish fluorescence.

The surface of all metal objects under the influence of the environment is destroyed. Their protection is most effective with colored pigments: aluminum powder, zinc dust, red lead, chromium oxide.

Reflection.

1. What factors cause the color of chemicals?

2. What substances can be determined by qualitative reactions by color change?

3. What factors determine the color of potassium and copper salts?

Nature, of which chemicals are a part, surrounds us with mysteries, and trying to solve them is one of life's greatest joys.

Today we tried to approach the truth "Chemistry of color" from one side, and maybe you will discover another. The most important thing is that the world of color is cognizable.

Man is born
To create, dare - and nothing else,
To leave a good mark in life
And solve all the difficult problems.
For what? Look for your answer!

Homework.

Give examples of qualitative reactions to iron ions by color change.

Let's imagine the following situation:

You work in a lab and decide to do an experiment. To do this, you opened the cabinet with reagents and suddenly saw the following picture on one of the shelves. Two jars of reagents had their labels peeled off, which were safely left lying nearby. At the same time, it is no longer possible to determine exactly which jar corresponds to which label, and the external signs of the substances by which they could be distinguished are the same.

In this case, the problem can be solved using the so-called qualitative reactions.

Qualitative reactions called such reactions that allow you to distinguish one substance from another, as well as to find out the qualitative composition of unknown substances.

For example, it is known that the cations of some metals, when their salts are added to the burner flame, color it in a certain color:

This method can only work if the substances to be distinguished change the color of the flame in different ways, or one of them does not change color at all.

But, let's say, as luck would have it, the substances you determine do not color the color of the flame, or color it in the same color.

In these cases, it will be necessary to distinguish substances using other reagents.

In what case can we distinguish one substance from another with the help of any reagent?

There are two options:

• One substance reacts with the added reagent, while the other does not. At the same time, it must be clearly seen that the reaction of one of the starting substances with the added reagent has really passed, that is, some external sign of it is observed - a precipitate has formed, a gas has been released, a color change has occurred, etc.

For example, it is impossible to distinguish water from a sodium hydroxide solution using hydrochloric acid, despite the fact that alkalis react perfectly with acids:

NaOH + HCl \u003d NaCl + H 2 O

This is due to the absence of any external signs of a reaction. A transparent colorless solution of hydrochloric acid, when mixed with a colorless hydroxide solution, forms the same transparent solution:

But on the other hand, water can be distinguished from an aqueous solution of alkali, for example, using a solution of magnesium chloride - a white precipitate forms in this reaction:

2NaOH + MgCl 2 = Mg(OH) 2 ↓+ 2NaCl

2) Substances can also be distinguished from each other if they both react with the added reagent, but do so in different ways.

For example, a solution of sodium carbonate can be distinguished from a solution of silver nitrate using a solution of hydrochloric acid.

hydrochloric acid reacts with sodium carbonate to release a colorless, odorless gas - carbon dioxide (CO 2):

2HCl + Na 2 CO 3 \u003d 2NaCl + H 2 O + CO 2

and with silver nitrate to form a white cheesy precipitate AgCl

HCl + AgNO 3 \u003d HNO 3 + AgCl ↓

The tables below show different options for detecting specific ions:

Qualitative reactions to cations

Cation	Reagent	Sign of reaction
Ba 2+	SO 4 2-	Ba 2+ + SO 4 2- \u003d BaSO 4 ↓
Cu2+		1) Precipitation of blue color: Cu 2+ + 2OH - \u003d Cu (OH) 2 ↓ 2) Precipitation of black color: Cu 2+ + S 2- \u003d CuS ↓
Pb 2+	S2-	Precipitation of black color: Pb 2+ + S 2- = PbS↓
Ag+	Cl-	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 - \u003d 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 - \u003d 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 - + Al 3+ \u003d 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 - \u003d 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-	Ba 2+	Precipitation of a white precipitate, insoluble in acids: Ba 2+ + SO 4 2- \u003d BaSO 4 ↓
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-	Ag+	Precipitation of a light yellow precipitate in a neutral medium: 3Ag + + PO 4 3- = Ag 3 PO 4 ↓
CrO 4 2-	Ba 2+	Precipitation of a yellow precipitate, insoluble in acetic acid, but soluble in HCl: Ba 2+ + CrO 4 2- = BaCrO 4 ↓
S2-	Pb 2+	Black precipitation: Pb 2+ + S 2- = PbS↓
CO 3 2-		1) Precipitation of a white precipitate, soluble in acids: Ca 2+ + CO 3 2- \u003d CaCO 3 ↓ 2) Emission of a colorless gas ("boiling"), causing the lime water to become cloudy: CO 3 2- + 2H + = CO 2 + H 2 O
CO2	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 \u003d Ca (HCO 3) 2
SO 3 2-	H+	SO 2 gas evolution with a characteristic pungent odor (SO 2): 2H + + SO 3 2- \u003d H 2 O + SO 2
F-	Ca2+	Precipitation of a white precipitate: Ca 2+ + 2F - = CaF 2 ↓
Cl-	Ag+	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) =)