In this lab students will coat the inside of a small test tube with metallic silver, making a shiny mirror in a dramatic transformation. This procedure creates a keepsake for the student and demonstrates some interesting chemistry. Silver compounds undergo interesting transformations: a clear solution is mixed with another clear solution, producing a dark gray powder. Another clear solution is added, which redissolves the powder. When a glucose solution is added the solution turns dark again and after a few moments the inner surface of the test tube turns bright with deposited silver metal.
To begin, organic, or carbon-based, molecules are classified and understood based on their wide variety of structures. Identifiable parts of organic compound structures are called functional groups. A simple carbon chain with only single bonds between the carbon atoms is called an alkane (fig. 2). This is the simplest functional group. Such molecules are very simple and relatively unreactive. They are mostly burned as fuels.
More interesting molecules incorporate other atoms into their structures, such as oxygen. Oxygen atoms play a central role in several organic functional groups. In alcohols (fig. 3) an oxygen atom has a single bond to a hydrogen atom and a carbon atom. Simple alcohols such as ethanol make excellent solvents that lie on a spectrum between polar, like water, and non-polar. Ethanol is the kind of alcohol in beverages such as beer and wine. Sugar molecules have an alcohol group, also known as a hydroxyl group (—OH), attached to nearly every carbon atom in a chain.
Ketones are another functional group involving oxygen. Their structure involves a double bond between carbon and oxygen C⚌O) known as a carbonyl group. In ketones, the carbon atom of the carbonyl group is bonded to two other carbon atoms (fig. 4). Aldehydes also include a carbonyl group with the difference being that the carbon atom is bonded to one carbon atom and a hydrogen atom (fig. 5). Sugars are a large class of related molecules differentiated from one another in part based on whether their structure includes a ketone or an aldehyde. Sugars which include a ketone group are called ketoses and sugars which include an aldehyde group are called aldoses.
When oxygen is added to an aldehyde in a chemical reaction it is transformed into a carboxylic acid. A carboxylic acid is a functional group with a carbonyl carbon bonded to one carbon atom and one hydroxyl group (fig. 6). It appears to be an aldehyde in which —OH has replaced —H.
As part of determining the structure of various sugars it was important to distinguish whether the sugar molecule included an aldehyde group or a ketone group. The aldoses are called reducing sugars because they are capable of reducing ions or molecules in the process of becoming oxidized and transformed into carboxylic acids. A common sugar which falls into this category is glucose (also known as dextrose, fig. 8). Non-reducing sugars, or ketoses, include a ketone group instead of an aldehyde. One example is table sugar, or sucrose, and another is fructose (fig. 9).
In the silver coating reaction an aldose sugar is oxidized into a carboxylic acid. The original purpose of the reaction was to detect qualitatively whether a sugar was an aldose, which would cause the silver to appear, or a ketose, which would not. It was published by the German chemist Bernhard Tollens (1841 - 1918) in 1882 and is widely known as Tollens’ Test. (2)
Modern chemistry laboratories no longer need this test to help to determine the molecular structure of sugars. Instead a variety of methods can be used: Nuclear Magnetic Resonance (NMR) spectrometry, infrared spectrometry, gas chromatography, and mass spectrometry. But at the time Tollens published his investigations of reactions between silver and sugar molecules, none of these technologies had been invented. As a result, the ability to tell with a quick addition of readily available chemicals whether a sugar included an aldehyde group was very useful. In modern times it provides an opportunity to learn about the underlying chemistry, which Tollens himself may not have been aware of.
Silver is a well-known metal used in jewelry and to make mirrors. The shiny metal is in a state known to chemists as ‘reduced’. Metals form positive ions in compounds and when in ionic form they make crystalline ionic compounds which may dissolve in water. Reduction is the process by which an ion, atom, or molecule gains one or more electrons. In the case of metals this means that they are transformed from ionic form to neutral atoms, or metallic form.
Equation 1 Ag+ + 1e– → Ag
Equation 1 shows the reduction of silver ions, which always have a +1 charge. The silver ion combines with one electron to become a neutral silver atom. This is a so-called half-reaction, showing only the reduction. The process of oxidation is when an atom, ion, or molecule loses one or more electrons. All reduction half-reactions must be coupled with an oxidation to provide the electron(s). In the reaction that creates the silver mirror it is the aldose sugar molecule glucose which provides the electron.
In order to create a mirror, however, it is not sufficient to mix solutions of silver ions and glucose. When those two substances are mixed the result is a fine powdered form of metallic silver which settles to the bottom of the test tube. No mirror forms. The problem is that the reaction is too fast: the silver ions do not have enough time to find their way to the wall of the test tube before they are reduced to metallic form. Because of this, some additional steps are required.
The first step of the procedure is to add sodium hydroxide solution. The reduction of silver by glucose works best in a solution that is strongly basic, with a high concentration of hydroxide ions (OH–). Silver ions cannot remain dissolved in the solution when the pH is too high and so the silver oxide precipitate forms when we add the sodium hydroxide (Equation 2).
Equation 2 2AgNO3(aq) + 2NaOH(aq) → Ag2O(s) + H2O(l) + 2NaNO3(aq)This silver compound traps all of the silver ions in the solid phase, meaning that they are no longer available to be reduced on the glass to make the mirror. In order to make the silver ions available again for the reduction reaction another step is required.
The second step of the procedure is to add strong ammonia (NH3) solution. The silver ions form a complex ion with the ammonia molecules with the formula Ag(NH3)2+ (Equation 3).
Equation 3 Ag2O(s) + 4NH3(aq) + H2O(l) → 2Ag(NH3)2+(aq) + 2OH–(aq)Complex ions are combinations of metal ions with negative ions or neutral molecules which may be more soluble in water than the metal ion alone. By forming bonds with the ammonia molecules the silver ions redissolve and the gray silver oxide precipitate redissolves. The resulting complex ion has the formula 2Ag(NH3)2+ and is called diamminesilver(I). Besides redissolving the silver ions, the formation of the complex ion has another benefit. The silver ions alone reduce too easily, forming a metallic precipitate. The complex ion, on the other hand, is harder to reduce(3) and so it requires contact with the test tube walls for the reaction to take place (Equations 4a and 4b). The numbers come from the study of electrochemistry and thermodynamics and they indicate that the reduction of plain silver ions is more spontaneous than the reduction of the silver-ammine complex. If this means nothing to you, you can safely ignore it.
Equation 4a Ag+ + 1e– → Ag E° = +0.799 V Equation 4b Ag(NH3)2+ + 1e– → Ag + 2NH3 E° = +0.373 V (3)
Once the solution is strongly basic and the silver ions are returned to their dissolved state by forming a complex-ion, the sugar will be added. The sugar used is glucose (also called dextrose), which is an aldose sugar. Aldehyde groups in organic molecules are good at providing electrons to reduce ions or other molecules. When they do so, the aldehyde group is transformed into an organic acid group. Sucrose, or table sugar, is a ketose and so would have no effect. The overall reaction is given here as Equation 5.
Equation 5 C6H12O6 + 2Ag(NH3)+ + 3OH– → C6H11O7– + 2Ag + 4NH3 + 2H2O
Answer these questions before coming in to the lab. If you do not know how to make the drawing or answer the question then that is a hint that you should go find out. Study the figures in the text for help and inspiration. Most of what you need to know to answer these questions can be found in the introductory text in this lab handout. If you do outside research, cite your sources, make sure they are legitimate, and do not copy and paste. Read and understand and write in your own words, always.
Be aware of the chemical hazards as you follow this procedure. If done carefully there is little chance of harm.
If desired the test tube can be used as a small vase for dried or fake flowers but be aware that this may scratch the mirror’s surface. Do not add water to the test tube or this may ruin the silver, too. If you wish to use it for live flowers with water or if you want to protect the surface you will have to find a waterproof finish for the interior surface. Clear nail polish may work but the author has not tested this suggestion.
Another way to display the silvered test tube is to glue the stopper into the top and to add a hook to make it into an ornament.
For fun, answer the following calculation questions.