Silver mirror equation. What is the silver mirror reaction? Chemical processes in the "silver mirror" reaction
Silver oxide dissolves to form a complex compound - diamminesilver(I) hydroxide OH
when an aldehyde is added to which, a redox reaction occurs with the formation of metallic silver:
If the reaction is carried out in a vessel with clean and smooth walls, then silver precipitates in the form of a thin film, forming a mirror surface. In the presence of the slightest contamination, silver is released in the form of a gray loose precipitate.
The silver mirror reaction can be used as a qualitative test for aldehydes. Thus, the "silver mirror" reaction can be used to distinguish between glucose and fructose. Glucose belongs to aldoses (containing an open aldehyde group), and fructose belongs to ketoses (containing an open keto group). Therefore, glucose gives a "silver mirror" reaction, while fructose does not.
Literature
- Nekrasov B.V. Fundamentals of General Chemistry. - 3rd ed., Rev. and additional - M .: "Chemistry", 1973. - T. 2. - 688 p.
- Nesmeyanov A. N., Nesmeyanov N. A. Beginnings of organic chemistry. In 2 volumes. - 2nd ed., trans. - M .: "Chemistry", 1974. - T. 1. - 624 p.
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The silver mirror reaction is a reaction to reduce silver from an ammonia solution of silver oxide.
In an aqueous solution of ammonia, silver oxide dissolves to form a complex compound - diammine silver(I) hydroxide OH.
Ag2O + 4NH3 * H2O = 2OH + 3H2O.
When aldehyde is added to which, a redox reaction occurs with the formation of metallic silver.
If the reaction is carried out in a vessel with clean and smooth walls, then silver precipitates in the form of a thin film, forming a mirror surface. In the presence of the slightest contamination, silver is released in the form of a gray loose precipitate.
The silver mirror reaction can be used as a qualitative test for aldehydes.
One of the possibilities for a visual demonstration of chemical transformations is the reaction of obtaining pure silver from silver oxide in an ammonia solution, which takes place with the addition of aldehyde, which is usually taken as glucose or formaldehyde. It is called the "silver mirror reaction".
Preparation for the reaction
This reaction is very capricious. Its successful implementation requires some preliminary preparation, namely:
- the dissolution of the reagents must be carried out exclusively in distilled water, since even minor impurities will affect the course of the reaction, and a brown precipitate will form instead of silver.
- This reaction should be carried out in a perfectly clean, and therefore fat-free and well-washed chemical glassware, with ideally smooth walls.
- Before carrying out the reaction, the flask should be treated with a solution of tin chloride, and then rinsed several times with distilled water.
- An attempt to replace glucose with fructose will not be successful, since the silver mirror reaction involves the aldehyde group, which is present in glucose openly, and in the composition of fructose molecules only in a closed form. Remember that fructose is a ketone and glucose is an aldehyde. Ketones do not give a silver mirror reaction.
Compliance with all the above rules will allow the colloidal silver particles formed during the reaction to reliably adhere to the glass and to each other, which will lead to the appearance of a silver layer on the walls of the flask, which is clearly visible to listeners.
Chemical processes in the "silver mirror" reaction
The first stage of this reaction is the interaction of silver oxide with an ammonia solution, with the formation of silver diamine hydroxide OH. Square brackets in the formula of this substance mean that it is a complex compound:
Ag + 2 O + 4 NH 4 OH → 2 OH + H 2 O.
Further interaction of silver diamine hydroxide with aldehyde leads to the reduction of pure silver, which is deposited on the walls of the flask. This reaction with glucose will be written as:
HOCH 2 (CHOH) 4 HC \u003d O + 2OH → HOCH 2 (CHOH) 4 COOH + 2Ag 0 ↓ + 3NH 3 + H 2 O.
Lab #5
Propertiescarbohydrates
Experiment 1. Silver mirror reaction is a recovery reaction silver from ammonia solution silver oxide (Tollens reagent).
in aqueous solution ammonia silver oxide dissolves to form a complex compound - diamminesilver(I) hydroxide OH
when added to which aldehyde a redox reaction occurs with the formation of metallic silver:
If the reaction is carried out in a vessel with clean and smooth walls, then silver precipitates in the form of a thin film, forming a mirror surface.
In the presence of the slightest contamination, silver is released in the form of a gray loose precipitate.
The silver mirror reaction can be used as a qualitative test for aldehydes. Thus, the reaction of the "silver mirror" can be used as a distinguishing between glucose And fructose. Glucose belongs to aldoses (containing an open aldehyde group), and fructose belongs to ketoses (containing an open keto group). Therefore, glucose gives a "silver mirror" reaction, but fructose does not. But if an alkaline medium is present in the solution, then ketoses isomerize into aldoses and also give positive reactions with an ammonia solution silver oxide (Tollens reagent).
Qualitative reaction of glucose with ammonia solution of silver oxide. You can prove the presence of an aldehyde group in glucose using an ammonia solution of silver oxide. Add a glucose solution to the ammonia solution of silver oxide and heat the mixture in a water bath. Soon, metallic silver begins to deposit on the walls of the flask. This reaction is called the silver mirror reaction. It is used as a quality for the discovery of aldehydes. The aldehyde group of glucose is oxidized to a carboxyl group. Glucose is converted to gluconic acid.
CH 2 OH - (CHOH) 4 – SLEEP+Ag 2 O= CH 2 OH - (CHOH) 4 – COOH + 2Ag↓
The order of the work.
Pour 2 ml into two test tubes. ammonia solution of silver oxide. 2 ml is added to one of them. 1% glucose solution, in another - fructose. Both tubes are boiling.
An ammonia solution of silver hydroxide is obtained by reacting silver nitrate with sodium hydroxide and ammonium hydroxide:
AgNO3+ NaOH → AgOH↓+ NaNO3,
AgOH + 2 NH4 OH→[Ag(NH3)2] OH + H2O,
ammonia solution
OH + 3 H2 → Ag2O + 4 NH4 OH.
The principle of the method. A mirror is formed on the walls of the test tube with glucose as a result of the release of metallic silver.
Work design: Write the conclusion, as well as the course and equations of the reaction in a notebook.
Experience 3. Qualitative reaction to fructose
The principle of the method. When a sample with fructose is heated in the presence of resorcinol And of hydrochloric acid up to 80 ° C after a while a bright red color appears in a test tube with fructose.
When a sample with fructose is heated in the presence of resorcinol And of hydrochloric acid a cherry red color appears. The assay is also applicable to the detection of other ketosis. Aldoses under the same conditions, they interact more slowly and give a pale pink color or do not interact at all. open F. F. Selivanov in 1887. It is used in the analysis of urine. The test is positive for fructosuria of metabolic or transport origin. In 13% of cases, the test is positive with a food load of fruits and honey. Chem. formula fructose - C 6 H 12 O 6
The cyclic formula of fructose
Acyclic form 
fructose
Painted connection
R-residue
hydroxymethylfurfural
The order of the work.
2 ml are poured into two test tubes: in one - 1% glucose solution, in the other - 1% fructose solution. Both tubes are filled with 2 ml of Selivanov's reagent: 0.05 g of resorcinol is dissolved in 100 ml of 20% hydrochloric acid. Both tubes are gently heated to 80°C (before boiling). A red color appears.
Conclusions: the results of the experiment and the reaction equation are written in a notebook.
Very visual is the experience with a beautiful effect of formation on the glass of a mirror coating. For this reaction, you need to stock up on experience and patience. In this article, you will learn about the necessary and specific features of equipment preparation, and also see what reaction equations this process follows.
The essence of the silver mirror reaction is the formation of metallic silver as a result of a redox reaction during the interaction of an ammonia solution of silver oxide in the presence of aldehydes.
"Silver Mirror" (test tube on the left)To create a durable silver layer you will need:
- glass flask with a capacity of up to 100 ml;
- ammonia solution (2.5-4%);
- silver nitrate (2%);
- aqueous solution of formaldehyde (40%).
Instead, you can take the ready-made Tollens reagent - an ammonia solution of silver oxide. To create it, you need to add 1 gram of silver nitrate to 10 drops of water (if the liquid will be stored for a long time, you need to place it in a dark place or in a glass container with dark walls). Immediately before the experiment, the solution (about 3 ml) must be mixed in a 1:1 ratio with a 10% aqueous solution of sodium hydroxide. Silver can precipitate, so it is diluted by slowly adding an ammonia solution. We recommend that you conduct another spectacular experiment with an ammonia solution and print a “chemical photograph”.
The reaction is carried out at room temperature. A prerequisite for a successful final is perfectly clean and smooth walls of a glass vessel. If there are the smallest particles of pollution on the walls, the sediment resulting from the experiment will become a loose layer of black or dark gray.
To clean the flask, you need to use different types of alkali solutions. So, for processing, you can take a solution that, after cleaning, has to be washed off with distilled water. It is necessary to rinse the flask from the cleaning agent many times.
Why is cleanliness so important?
The fact is that the colloidal silver particles formed at the end of the experiment must firmly adhere to the glass surface. On its surface there should be no fats and mechanical particles. water does not contain salts and is ideal for the final cleaning of the flask. It can be prepared at home, but it is easier to buy ready-made liquid.
Silver mirror reaction equation:
Ag₂O + 4 NH₃ H₂O ⇄ 2OH + 3H₂O,
where OH is diammine silver hydroxide, obtained by dissolving the metal oxide in an aqueous solution of ammonia.
Diamminesilver complex molecule
Important! The reaction works at low concentrations of ammonia - carefully observe the proportions!
This is how the final stage of the reaction proceeds:
R (any aldehyde)-CH=O + 2OH → 2Ag (precipitated silver colloid) ↓ + R-COONH₄ + 3NH₃ + H₂O
The second step of the reaction is best carried out by carefully heating the flask over a burner flame - this will increase the chances that the experiment will be successful.
What can a silver mirror reaction show?
This interesting chemical reaction demonstrates not only certain states of matter - it can be used to perform a qualitative determination of aldehydes. That is, such a reaction will solve the question: is there an aldehyde group in the solution or not.
General structural formula of aldehydes
For example, in a similar process, you can find out what is contained in the solution: glucose or fructose. Glucose will give a positive result - you will get a “silver mirror”, and fructose contains a ketone group and you cannot get a silver precipitate. In order to carry out the analysis, instead of a formaldehyde solution, it is necessary to add a 10% glucose solution. Consider why and how dissolved silver turns into a solid precipitate:
2OH + 3H₂O + C₆H₁₂O₆ (glucose) = 2Ag↓+ 4NH₃∙H₂O + C₆H₁₂O₇ (gluconic acid is formed).
Before starting a chemical experiment, it is necessary to find out what the aldehyde is, the presence of which is to be determined. Aldehydes are a group of organic compounds in which a carbon atom has a double bond with an oxygen atom. Each such compound contains a >C=O group. The essence of the reaction is that as a result, metallic silver is formed, which is deposited on the surface. The reaction is carried out with substances containing an aldehyde group in an aqueous solution by heating in the presence of ammonia. Most often, they are used in the reaction, and ordinary sugar is used as an aldehyde. As a substance containing ammonia, it is usually used.Silver salts must be handled with care as they leave black marks. Experiment with gloves.
How does the reaction take place
?Reagents for the experiment can be found at any pharmacy. Silver nitrate is a lapis pencil. You can also buy formaldehyde and ammonia there. Among other things, you need chemical glassware. The substances that you will have to deal with are non-aggressive, but any chemical experiments are best done in chemical glass test tubes and flasks. Of course, the dishes must be thoroughly washed. Make an aqueous solution of silver nitrate AgNO3. Add ammonia to it, that is, ammonium hydroxide NH4OH. You form silver oxide Ag2O, which precipitates in the form of a brown precipitate. The solution then becomes clear and an OH complex compound is formed. It is he who acts on the aldehyde during the redox reaction, which results in the formation of the ammonium salt. The formula for this reaction looks like this: R-CH=O + 2OH --> RCOONH4 + 2Ag + 3NH3 + H2O. If a glass rod or plate is left in the jar during the reaction, after about a day it will be covered with a shiny layer. The same layer is formed on the walls of the vessel.
The reaction can be written in a simplified way: R-CH=O + Ag2O --> R-COOH + 2Ag.
