The reaction of the silver mirror is qualitative for. What is the silver mirror reaction? Silver mirror reaction using formaldehyde

Conducting an elective course "Experiment in Organic Chemistry"

Lesson study

Practical work No. 3 "The reaction of the silver mirror"

Topic: Study of the interaction of organic compounds of various classes with silver (I) compounds.

The purpose of the lesson:

general educational

Investigate the interaction of classes of organic compounds with silver compounds, find out the reason for the choice of this interaction.

Educational

Development of thinking skills, comparison, analysis, synthesis of cognitive skills. Ability to put and ask a question, formulate a problem, draw conclusions; improving the skills of independent behavior.

Didactic

To form competencies in the field of independent cognitive activity, teamwork skills, the ability to see the problem and outline ways to solve it.

Educational

The formation of worldview thinking, broadening of horizons, the formation of a positive attitude towards the study of the subject, the habit of helping comrades, a conscientious attitude to the task being performed, and developing exactingness towards oneself and comrades.

Methods: research, work with educational literature, partial search, laboratory work, group work.

Equipment: On the students' tables, there are instruments and reagents for laboratory work “Silver Mirror Reaction”, task cards, presentation for the lesson.

During the classes

1. Organizational moment

“The time for miracles has passed, and we

Looking for reasons

Everything that happens in the world"

W. Shakespeare

(slide number 1)

Introductory speech of the teacher : Everyone who has an idea about the science of chemistry knows that an integral part of it is a chemical experiment. Entertaining experiments and practical exercises help develop chemical thinking and contribute to the assimilation of chemistry. The elective course "Studying organic chemistry through a system of experimental problems" is designed for 17 hours 12 - practical work. Most of the practical work is done by small groups of students. The course work is structured as follows. Students receive assignments on the topic of the upcoming practical work. At home, they get acquainted with the theoretical material, study the content of the work itself.

Today we are doing practical work No. 3 (see title above)

(slide number 2)

The purpose of this work: to experimentally investigate the interaction of classes of organic compounds with silver compounds, to find out the reason for the choice of this interaction, to consolidate the practical skills necessary in the activities of the experimenter.

(slide number 3)

The students received a preliminary task for the lesson.

Teacher : The interaction reaction of the silver compound is called the "silver mirror reaction". Tell the story of getting this mirror.

1 student : Mirror acquisition history. Mirrors… Segoday is a very common thing in our lives. Few people know that mirrors appeared long before our era. At first, they were metal plates polished to a shine made of gold, silver, copper or bronze. The manufacture of mirrors of the modern type (on glass) was initiated in 1858 by the German chemist Justus Liebig. He used for this the ability of silver cations to form numerous complex compounds.Once, after washing the inner surface of the flask with a solution of soda, and then with water, ethyl alcohol and diethyl ether, Liebig poured into it a little diluted formalin - a 10% formaldehyde solution. Then, adding to the formalin a solution of silver ammonia of the composition, he carefully heated the flask.After a few minutes, the flask became a mirror.

The complex silver-containing cation is reduced to metal (silver), and formaldehyde is oxidized to formic acid HCOOH. Subsequently, instead of formalin, Justus Liebig used glucose to obtain a "silver mirror".

Teacher : Tell us about the history of obtaining Tollens' reagent, which is widely used in analytical chemistry.

2 student: The history of obtaining Tollens' reagent. In 1881, the German chemist Bernhard Christian Tollens proposed using a silver complex compound to detect compounds with an aldehyde group in solution. When an aqueous solution of ammonia is added to a solution of silver nitrate, a gray-black precipitate of oxide occurs, since silver hydroxide does not exist under normal conditions.

2AgNO 3 + 2NH 4 OH → Ag 2 O + 2NH 4 NO 3 + H 2 O

Silver oxide dissolves in excess ammonia water, forms a complex unity.

Ag 2 O + 4NH 4 OH → 2 OH + 3H 2 O

The resulting colorless solution was named Tollens' reagent.

2. Main body

“It is impossible to become a chemist in any way,

Without seeing the practice itself

and not taking on chemical operations"

M.V. Lomonosov

(slide number 4)

A) Role play: The class is a research laboratory that begins its work on the study of the interaction of various classes of organic compounds with silver (I) compounds. Students are divided into three creative groups. Each creative group has an information department and a chemical laboratory. Groups receive theoretical and practical tasks and work (5-7 min). Along with the task, instructions for the implementation of the experiment are issued. (slide number 5)

Task 1 creative group

Task 2 for the creative group

Does it interact? Why?

Information department: Which classes of organic compounds do the issued substances belong to? What functional groups do they have?

Task 3 for the creative group

Information department: Which classes of organic compounds do the issued substances belong to? What functional groups do they have?

(slide number 6)

“The thinking mind does not feel happy,

until it is possible to tie together disparate facts,

observed by him"

D. Hevesy.

(slide number 7)

B) Discussion of the reports of each group

Teacher: And now the creative groups should report on the work done. A student of the chemical laboratory talks about his observations that occur during the experiment, making a conclusion about the class with which the silver compound interacts. At this time, the student who makes up the information department goes out and writes down the reaction equation for the interaction of organic matter with silver (I) oxide, highlighting the functional group of each class.

The reaction record is checked on the corresponding slide No. 8, 9,10

A general conclusion is formulated that Tollens' reagent (silver (I) compound) is qualitative for the aldehyde group, which is found in aldehydes, glucose and formic acid. (slide number 11)

The lesson is summed up.

"The joy of seeing and understanding

is the most beautiful gift

A. Einstein

(slide number 12)

The reaction of the silver mirror is used for the manufacture of mirrors, silvering decorations and Christmas decorations.

Qualitative analysis of substances is an important topic in the study of organic chemistry. Knowing it helps not only chemists, but also doctors, ecologists, biologists, epidemiologists, pharmacists, and food industry workers.

Conclusion. Summing up, grading. Thanks for attention!

Instructions for laboratory experiment

Silver mirror reaction

Pour 2 ml of 1% solution of silver nitrate AgNO 3 into a test tube and add 10% ammonia water NH 4 OH in small portions until the resulting precipitate of silver oxide is completely dissolved. (Or use ready-made Tollens reagent)

Add 1 ml to the resulting solution. the substance under study. Heat the contents by rotating the test tube around the flame, heating the walls rather than the bottom. Hold the tube upright.

1. What do you observe in a test tube?

2. Why does the glass surface become mirror-like?

3. Write the reaction equation.

Instructions for laboratory experiment

Silver mirror reaction

Add 1 ml to the resulting solution. the substance under study. Heat the contents by rotating the test tube around the flame, heating the walls rather than the bottom. Hold the tube upright.

1. What do you observe in a test tube?

2. Why does the glass surface become mirror-like?

3. Write the reaction equation.

Instructions for laboratory experiment

Silver mirror reaction

Add 1 ml to the resulting solution. the substance under study. Heat the contents by rotating the test tube around the flame, heating the walls rather than the bottom. Hold the tube upright.

1. What do you observe in a test tube?

2. Why does the glass surface become mirror-like?

3. Write the reaction equation.

Task 1 creative group

Chemical laboratory: There are two organic substances in the test tubes: glucose and ethanol, carry out the reaction of the interaction of these substances with an ammonia solution of silver oxide (I). Which of the substances does not interact? Why?

Information department: Which classes of organic compounds do the issued substances belong to? What functional groups do they have?

Task 2 for the creative group

Chemical laboratory: There are two organic substances in the test tubes: formaldehyde (formalin, methanal) and acetic acid, carry out the reaction of the interaction of these substances with an ammonia solution of silver oxide (I). Which of the substances interacts? Why?

Information department: Which classes of organic compounds do the issued substances belong to? What functional groups do they have?

Task 3 for the creative group

Chemical laboratory: There are two organic substances in the test tubes: formic acid and glycerin, carry out the reaction of the interaction of these substances with an ammonia solution of silver oxide (I). Which of the substances does not interact? Why?

Information department: Which classes of organic compounds do the issued substances belong to? What functional groups do they have?

It is necessary to understand what is the reaction of a silver mirror? The silver mirror reaction is the process of reducing metallic silver from an ammonia solution of silver oxide.

Ag2O + 4NH4OH ↔ 2OH + H2O

Silver oxide in an aqueous solution of ammonia dissolves to form a complex silver compound - silver diamine hydroxide (I) OH.

By adding any aldehyde (formaldehyde) to the silver complex compound, metallic silver is formed as a result of the redox reaction. As a result of the reaction on the walls of a glass test tube, a beautiful mirror coating of silver or a mirror will form.

R-CH=O + 2OH → 2Ag ↓ + R-COONH4 + 3NH3 + H2O

You can read in any chemistry textbook that the silver mirror reaction can be used to detect aldehydes. For example, glucose gives a "silver mirror" reaction, but fructose does not. However, there are many chemicals that, like aldehydes, can undergo a silver mirror reaction.

How, in practice, can a silver mirror reaction be carried out?

At first glance, it seems that it is very easy to conduct a silver mirror reaction, but this is not entirely true. Everything seemed simple, you can take an ammonia solution with some aldehyde, it can be a solution of formaldehyde or glucose and carry out a silver mirror reaction. However, this is a simple and primitive approach to understanding what is the reaction of a silver mirror? This reaction can lead to disappointment. Instead of the expected specular coating on the glass, a black or brown suspension of silver in solution may form.

Usually the reaction, in such a simple way, in most cases ends in failure. Even if you manage to create a mirror, then it comes out of very poor quality. The silver layer is fragile and uneven. Why it happens? There are many reasons for such an unsuccessful reaction. Of these, two main reasons can be distinguished: non-observance of the necessary conditions for the reaction or a poorly prepared glass surface for silvering.

As a result of the reaction, a positively charged silver ion is formed, which combines with the aldehyde group to form the smallest or colloidal silver particles. Such tiny particles can adhere strongly to the glass surface or remain in solution as a suspension of silver.

In order for the colloidal particles of silver to reliably adhere to the glass and form a strong and uniform layer of silver, that is, a mirror, the glass surface must first be degreased before silvering. The surface of the glass must not only be perfectly clean, but also as smooth as possible.

The main contaminant of glass is fatty deposits, which must be removed. To remove fat, an alkali solution, a hot chromium mixture are used, and then the glass is repeatedly washed with distilled water. If you don't have lye, you can use a regular synthetic dish detergent as a last resort. After degreasing, it is useful to rinse the glass with a solution of tin chloride and distilled water.

All solutions should be made with distilled water. If distilled water is not available, rainwater can be used as a last resort. To reduce metallic silver, in the silver mirror reaction, reducing agents are often used: formaldehyde or glucose. The choice of these two substances depends on the purpose for which the chemical reaction will be used.

Silver mirror reaction using formaldehyde

To demonstrate the experience, the reaction of the silver mirror can be carried out with the participation of formaldehyde. If you need to make a high-quality mirror with a durable and uniform surface, it is better to use glucose.

For silvering glass, it is better to take silver, as part of the silver salt - silver nitrate. Ammonia and alkali solutions are added to silver nitrate. The deposition of silver on glass must take place in an alkaline solution. In this case, the alkali solution should not be too much, since its excess is also undesirable. Depending on the technique, the silver mirror reaction is carried out at room temperature or under heating.

When the solution turns brown, this means that tiny colloidal particles of silver have formed in the solution. Later, on the surface of the glass, a thin mirror coating is formed, made of reduced silver. It is very easy to carry out the reaction of a silver mirror, but it can be very difficult to obtain a high-quality mirror. In order to obtain a high-quality silver coating - a silver mirror, you need to spend a lot of work, while you need to be careful and very persistent.

For experience, you can carry out a simple silvering of glass, in order to first familiarize yourself and find out what the reaction of a silver mirror is? When demonstrating this reaction, a mirror of not very good quality may turn out.

To carry out the reaction, we need: a clean glass flask with a capacity of 50 - 100 ml, an ammonia solution at a concentration of 2.5 to 4 percent, a 2 percent solution of silver nitrate and a formalin solution.

Before silvering, prepare the flask for a chemical reaction. We clean the flask from mechanical impurities by wiping it with a brush with soap, and then rinse the flask with distilled water. Then we wash it with a chromium mixture, and then rinse it again with distilled water.

Pour a 2% solution into one quarter of the flask, then gradually add ammonia solution to this solution. An ammonia solution is prepared from the calculation, we take a 25% ammonia solution and dilute it with distilled water 8-10 times. We gradually add an ammonia solution to the silver nitrate, until the precipitation is completely dissolved in its excess. To the solution that has formed, gradually along the wall, add formalin solution - 0.5 - 1 ml. We place the glass flask in a container with hot or even better with boiling water. Soon, it will begin to form on the flask, which forms a pretty silver mirror.

The main disadvantage of this technique is that not only ammonia solution, but also alkali (this can be sodium hydroxide - NaOH or potassium hydroxide - KOH) must be added to the silver nitrate solution. To obtain a high-quality mirror coating, it is necessary to add ammonia first, and then alkali.

To simply demonstrate the reaction of a silver mirror, you can do the opposite: first add alkali to silver nitrate until the formation of a brown precipitate - (Ag2O) stops, and then - an ammonia solution until the precipitate is completely dissolved:

2Ag+ + 2OH - = Ag2O + H2O

Ag2O + 4NH3 + H2O = 2OH

When carrying out this technique, a white precipitate may form, most likely it will be urotropine (or hexamethylenetetramine):

6CH2O + 4NH3 = (CH2)6N4 + 6H2O

The formation of a white precipitate is a bad sign that does not contribute to obtaining a high-quality mirror.

It should be especially noted that the reaction of the silver mirror must necessarily take place in an alkaline environment, and not in an acidic one. In an acidic environment, the silver mirror reaction will proceed without the formation of a silver coating. Sometimes it happens that when mixing reagents, the medium suddenly becomes acidic. One has only to add an excess of alkali to the reaction mixture, as a silver mirror is immediately formed on the walls of a glass test tube.

Silver mirror reaction using glucose

The silver mirror reaction using glucose can be carried out, not only to demonstrate a chemical reaction, but also to obtain a high-quality silver mirror or conductive surface.

Everyone is well aware that the silver mirror reaction is a chemical process of reducing metallic silver from an ammonia solution of silver oxide (Tollens' reagent).

The reaction of the silver mirror is at the heart of the production of silver mirrors. In order for metallic silver (Ag) not to oxidize, not form black (Ag2S), that is, it does not become dull and scratched, it is coated on top with a protective varnish. In order to silver the mirror, you need to prepare two freshly prepared solutions (A and B).

Solution - A

In 100 ml of distilled water, dissolve 6 grams of silver nitrate - (AgNO3), add an aqueous solution of ammonia to this solution, until the initially formed precipitate dissolves. Then we add alkali to the resulting solution - 70 ml of a 3% solution of sodium hydroxide (NaOH), and again pour an aqueous solution of ammonia until the solution is completely clear (without excess). The resulting solution is diluted with distilled water to 500 ml.

Solution - B

In 25 ml of distilled water, dissolve 1.3 grams of glucose (one drop of concentrated nitric acid - HNO3 is added to the resulting solution) and boil the resulting solution for two minutes. Then the solution is cooled and diluted with the same volume of alcohol.

Solutions: A and B are mixed before use in a ratio of 10:1. After mixing the solutions, a thick film of silver forms on the glass within 30 minutes.

Before silvering a mirror, you need to clean the glass well. This is a very important and necessary condition. To obtain a high-quality mirror coating, this condition cannot be neglected. The glass surface is cleaned with a hot mixture - HNO3 + K2Cr2O7, then the glass is rinsed in distilled water and treated with alcohol.

In order to obtain a thicker layer of silver, the surface treatment of silvered glass is repeated again with freshly prepared portions of solutions, one or two more times. Then washed with water and alcohol, the resulting precipitate of silver.

Silver mirror reaction using sucrose

The silver mirror reaction can be carried out (if there is no formaldehyde or glucose) with sucrose. Before carrying out the reaction, sucrose is subjected to hydrolysis, using sulfuric and nitric acids diluted to 10 percent concentration. An acid solution is added to an aqueous solution of sugar, in the ratio: 10 ml of acid per 100 grams of sugar. The resulting solution is boiled for 15 - 20 minutes. The hydrolyzed sucrose turns into a mixture of glucose and fructose.

Silver mirror reaction using starch

Using starch instead of glucose in the reaction fails. This happens because starch does not completely convert to glucose as a result of hydrolysis. With such partial hydrolysis of starch, dextrins are formed - polysaccharides, which, like starch, consist of glucose units that have a lower molecular weight, unlike starch. Dextrins, at the end of the chains, have aldehyde groups that reduce the silver ion, but this produces a black colloidal solution of silver, instead of the expected specular coating. Metallic silver does not settle on the surface of the glass, apparently because the long linear molecules of dextrins stabilize the colloidal solution of silver. These molecules, in other words, perform the function of a protective colloid. In order to prevent the formation of a black colloidal solution of silver, complete hydrolysis of starch is necessary.

We continue the topic of chemical experiments, because we hope that you will definitely like them. This time we present to your attention another fascinating experience, during which we will receive a silver mirror.

Let's start by watching the video

We will need:
- capacity;
- silver nitrate;
- hot water;
- ammonia solution 10%;
- glucose;
- alcohol burner

Let's start with silver nitrate. We take about a gram from it and dilute it in a small amount of hot water.

Next, sodium hydroxide is added to the resulting solution. During this reaction, silver oxide is formed, which precipitates.

Next, add a 10% ammonia solution to the precipitate of silver oxide. It is necessary to pour in the ammonia solution until the precipitate dissolves.

During this reaction, silver ammonia is formed. Add 5 grams of glucose to the resulting solution.

Now you need to heat the resulting mixture. To do this, we light the alcohol burner and put a glass on it so that the mixture gradually heats up. During this reaction, a very large amount of ammonia is released, so this reaction must be carried out either under a fume hood or outdoors. During the reaction, silver nitrite can also be formed, which is a very dangerous substance, so the dishes must be thoroughly washed after the reaction.

After some time, a thin layer of silver gradually begins to settle on the walls of the cup. The reaction is completed after about 15 minutes after the start of heating.

In order to get a more even layer of silver, you need to put a glass with the mixture in a large container, pour hot water into the container and put it on an alcohol burner. The temperature will thus be distributed evenly and the result will be more spectacular.

To begin with, each class of organic substances has a certain reaction by which its representatives can be distinguished from other substances. The school chemistry course assumes the study of all qualitative reagents for the main classes of organic substances.

Aldehydes: structural features

Representatives of this class are derivatives of saturated hydrocarbons in which the radical is connected to the aldehyde group. The isomers of aldehydes are ketones. Their similarity lies in belonging to the class of carbonyl compounds. When performing a task that involves isolating aldehyde in a mixture, a “silver mirror” reaction will be required. Let us analyze the features of this chemical transformation, as well as the conditions for its implementation. The silver mirror reaction is a process of reduction from silver diamine (1) hydroxide to metallic silver. In a simplified form, this complex compound can be written in a simplified form of silver oxide (1).

Separation of carbonyl compounds

To form a complex compound, silver oxide is dissolved in ammonia. Given that the process is a reversible reaction, the silver mirror reaction is performed with a freshly prepared ammonia solution of silver oxide (1). When a complex compound of argentum is mixed with an aldehyde, a redox reaction occurs. The completion of the process is indicated by the precipitation of metallic silver. With the correct implementation of the interaction of ethanal and the ammonia solution of silver oxide, the formation of a silver coating is observed on the walls of the test tube. It was the visual effect that gave this interaction the name of the "silver mirror".

Definition of carbohydrates

The silver mirror reaction is qualitative for an aldehyde group, which is why it is also mentioned in organic chemistry courses as a way to recognize carbohydrates such as glucose. Given the specifics of the structure of this substance, which exhibits the properties of an aldehyde alcohol, thanks to the “silver mirror” reaction, glucose can be distinguished from fructose. Thus, this is not only a qualitative reaction to aldehydes, but also a way to recognize many other classes of organic substances.

Practical application of the "silver mirror"

It would seem, what difficulties can arise with the interaction of aldehydes and an ammonia solution of silver oxide? You just need to purchase silver oxide, stock up on ammonia and pick up aldehyde - and you can safely proceed to the experiment. But such a primitive approach will not lead the researcher to the desired result. Instead of the expected mirror surface, you will see (at best) a dark brown silver suspension on the walls of the tube.

The essence of interaction

A qualitative reaction to silver implies compliance with a certain algorithm of actions. Often, even when signs of a mirror layer appear, its quality clearly leaves much to be desired. What are the reasons for this failure? Is it possible to avoid them? Among the many problems that can lead to an undesirable result, there are two main ones:

violation of the conditions for conducting chemical interaction;
poor quality surface preparation for silvering.

During the interaction of the starting substances in solution, silver cations are formed, which combine with the aldehyde group, resulting in the formation of colloidal fine silver particles. These grains are capable of adhering to glass, but may remain in solution as a silver suspension. In order for the precious metal particles to adhere to the glass, to form a uniform and durable layer, it is important to pre-degrease the glass. Only in the presence of an ideally clean initial surface of the test tube can one count on the formation of a uniform silver layer.

Possible problems

The main contaminant of glassware is greasy coating, which must be disposed of. An alkali solution will help solve the problem, as well as a hot chromium mixture. Next, the tube is washed with distilled water. In the absence of alkali, you can use a synthetic dishwashing detergent. After degreasing is completed, the glass is washed with a solution of tin chloride, rinsed with water. Distilled water is used to prepare solutions. In case of its absence, you can use rainwater. Glucose and formaldehyde are used as reducing agents that allow a pure substance to precipitate from a solution. With aldehyde, it is difficult to count on obtaining a high-quality silver coating, but the monosaccharide (glucose) gives a uniform and durable silver layer on the surface of the mirror.

Conclusion

To silver glass, it is desirable to use silver nitrate. Alkali and ammonia solution are added to this salt solution. The condition for a full-fledged reaction and silver deposition on glass is the creation of an alkaline environment. But with an excess of this reagent, side effects are possible. Depending on the chosen method of conducting the experiment, a qualitative reaction is obtained by heating. The brown coloring of the solution indicates the formation of the smallest colloidal particles of silver. Further, a mirror coating appears on the surface of the glass. In case of successful completion of the process, the metal layer will be even and durable.