What is the essence of the silver mirror reaction. A mirror finish is created by the reaction of the silver mirror As a result of the reaction of the silver mirror
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.
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.
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).
The silver mirror reaction is a beautiful name for a chemical reaction that results in the precipitation of a thin layer of silver on the walls of the vessel where the process took place. Once upon a time, all surfaces were treated in this way, where a mirror coating was needed.
Now this method of obtaining a thin metal coating on glass or ceramics is used only if it is necessary to create a conductive layer on dielectrics, as well as in the production of optics for telescopes, cameras, etc. This reaction can also be used to obtain. Such a poetic name for a simple chemical reaction is based on the hype that occurs when it comes to precious metals - gold and silver.
In order to carry out the recovery of silver from its oxide not in laboratory conditions, it is necessary to dissolve silver nitrate in water. You can take it at the pharmacy. This is a pencil pencil. It is better to use distilled water. It can be obtained by simple condensation of water evaporating from a boiling kettle. Based on a half-liter container, then ammonia (1 tsp) must also be dissolved in such an amount of silver nitrate solution. Here you also need to add 2-3 drops of formaldehyde - formalin.
All reagents do not react immediately, so shake the solution well and leave it alone for about a day. If everything goes well, then during this period your jar will be covered with a thin metal layer. The same layer will cover the object that you place in the jar.
Sometimes something goes wrong and instead of a mirror, the reaction gives gray precipitated flakes. This suggests that the reagents were not quite pure. Most often, claims must be made to water and the cleanliness of dishes. Particular attention should be paid to the acidity of water, since most surprises occur in an alkaline environment.
Reaction indicator function
This reaction is used to determine the presence of aldehydes in a solution. This group includes such organic substances that have an aldehyde group. Otherwise they are called alcohols devoid of hydrogen. The presence of aldehyde in solution gives the effect of a mirror.
An ammonia solution of silver oxide is used to determine monosaccharides and disaccharides. The first group includes glucose in all its isomeric states, the second group includes lactose and maltose. The silver mirror reaction is especially characteristic of glucose, which is reflected in the methods for detecting glucose and fructose.
Despite the similarity of these substances and the fact that fructose is isomeric to glucose, they are still different. In the open form, the aldehyde group is present only in glucose. Accordingly, silver will precipitate only in the presence of glucose, while fructose will not give such a reaction. But in an alkaline environment, fructose can also give a positive reaction.
Thus, silver oxide as a reagent can be used as an indicator of the presence of a certain group of substances in a solution. In addition, with the help of the described reaction, you can get pure silver, a silver mirror and a plate coated on both sides with a metallic coating, which is not only funny, but often useful.
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.