SILVER MIRROR / SILVER BOTTLE
AP CHEMISTRY
We will again demonstrate several different reactions that will produce a souvenir silver bottle.
Mirrors have been known since ancient times. The earliest mirrors were made by polishing disks of a meal such as bronze. These simple mirrors did not last very long due to oxidation of the metal and abrasion from everyday use. In the middle ages, beautiful mirrors were made by backing glass with thin sheets of metal foil, usually silver. Mirrors produced in this manner were very expensive. Most household mirrors are made with silver because light reflected from a silvered mirror has a slight pink tinge to it, which enhances skin tones.
In 1835, the German chemist Justus von Liebig (1803−1873) invented the silvering process used in this lab…which was later employed by Bernhard Tollens as a test for aldehydes.
Tollens’ test is a qualitative test used by chemists to determine whether a carbonyl functional group is part of an aldehyde or ketone. Treatment of an aldehyde with a solution of silver nitrate in an ammoniacal sodium hydroxide solution (or ammoniacal potassium hydroxide) produces a silver mirror on a very clean glass surface. This process does not require any electricity and is called “electro-less plating.” Glucose (or its isomer dextrose), is a reducing sugar, and is used to reduce silver ions in Tollens’ reagent to silver metal, which is then deposited on the inside of a clean the glass bottle.
Glucose, with a flexible carbon backbone, and both alcohol and aldehyde functional groups can react with itself to form a ring structure. Because of this reaction ability, only a small percentage of the glucose molecules are in the open-chain (aldehyde) form in aqueous solution.
This low percentage of glucose molecules existing as open-chains may be one reason why it is so widely used in biochemical systems. Most aldohexoses (a hexose with an aldehyde group) have a rather significant tendency to react with the amine portions of proteins and thus disable the proteins. This reaction is called glycation and according to authors at this ability to disable proteins may be the cause for the long-term effects (blindness, renal failure) of diabetes. Again, glucose has a limited ability for this reaction, due to its more stable cyclic form.
Tollens set out to discover whether glucose was indeed an aldohexose. He knew, in essence, that an aldehyde could be oxidized to a carboxylic acid (-CHO → -COO- ) Thus, Tollens proposed if, glucose (or dextrose) had an aldehyde formation, it could be used as a reducing sugar. The proof is the ability of the sugar to be oxidized and have those lost electrons reduce silver ion back into silver metal.
The silver metal produced adheres to the interior glass surface.
The generalized aldehyde has a chain of carbons (represented by R), or a simple hydrogen atom, terminating in a carbonyl group and hydrogen atom.
For an aldehyde, this functional group of atoms must be terminal (at one end of the molecule on the “first C” ...as opposed to a ketone or ester where thecarbonyl groupis bound to an interior carbon of the parent chain).
The Tollens’ test is specific for reducing sugars, like glucose (or its isomer, dextrose). As implied, a reducing sugar is a carbohydrate which becomes oxidized in a chemical reaction (or rather, a reducing sugar is a carbohydrate which acts as a reducing agent)
The primary idea of this exercise is to oxidize glucose and allow the lost electrons to reduce silver ion (Ag+1) to silver atom or metal (Ag0).
Ag+1+1 e- Ag0
The source of electrons, theoretically is any molecule with an aldehyde structure.
Tollens' theory went something like this: Were glucose (C6H12O6) an aldehyde then it should be oxidized and act as the source of the electrons for the reduction of a cation such as silver ion back to its metallic form. Thus, if silver metal were produced, then glucose would be proven to be an aldehyde. The Tollens Test therefore is essentially a standard test for aldehyde structure.
There are a few problems, however with testing the theory.
Glucose will become oxidized, only when the molecules are dissolved in a strongly alkaline aqueous solution However,silver ion reacts with the hydroxide polyatomic ion (OH)-1 of the strong base, to produce silver hydroxide, a precipitate where Ksp = 1.52 x 10-8
Ag+1(aq) + OH-1(aq) AgOH(s)↓ Ksp = 1.52 x 10-8
silver hydroxide
When the silver ion is bonded to the hydroxide in the silver hydroxide precipitate, the silver ion cannot be reduced by the oxidizing aldehydes. This defeats the means of proving the molecular structure for glucose.
So, Tollens' genius was in using the silver diamine ion;a complex ionwith the silver ion and a ligand of ammonia. The complex ion is called the Tollens’ Reagent.
The silver ion, bonded in the Tollens Reagent, will not react directly with the potassium hydroxide (the strong base) used in the reaction.
By creating the Tollens’ Reagent, and by promoting the decomposition of the silver diamine ion reagent, at the correct point of the reaction, the silver ion is free to be reduced (despite the presence of hydroxide ions), as the glucose becomes oxidized. Hence, the Tollens’ Reagent is an important means of keeping the silver ion, viable (& available) until the point of reduction.
There are (at least!) two critical chemical reactions which occur.
Reaction 1: Ag+1 ions of the silver nitrate solution react with concentrated ammonia solution (NH3(aq)) and
produce a brown solid (a precipitate, Ag2O) . This brown colored precipitate dissolves in an excess
of ammonia. This reaction produces what is called the Tollens’ Reagent.
Reaction 1: Adding Silver Nitrate and Ammonia
Ag+1(aq)+ 2 NH3(aq) Ag(NH3)2+1
brownish solid
which then dissolves
as more NH3 is added
Reaction 2: Adding a basic solution of glucose to the Ag(NH3)2+1 enables the reduction of the Ag+1 of the
Tollens’ Reagent back to Ag0.
Reaction 2: Adding the Glucose (in a basic solution: KOH) to the Tollens’ Reagent
Glucose(aq) + Ag(NH3)2+1(aq) Ag(s) + Gluconic acid(aq) + NH3(aq) + H2O(l) (unbalanced)
(dissolved in the base KOH) Tollens’ Reagent
Thus the overall reaction is:
[Please note that the above reaction is an unbalanced “net” reaction and does not account for the remaining potassium ion and NH3 at the end of the reaction]
Based on work and concepts found in:
Shakhashiri B.Z. Chemical Demonstrations: A Handbook for Teachers of ChemistryUniversity of Wisconsin Press,
Madison Wisconsin, 1992 p. 240 -241
Katz, D.A. at:
Keusch P: from the University of Regensburg, GR at:
PROCEDURE
1) Put on your goggles
2) Bring your bottle up to the instructor and get approximately 20.0 mL of 0.25 MAgNO3 (aq)
3) Go to the hood and add concentrated ammonia (NH3), using a pipette. A brown precipitate will form.
Keep adding and swirling the mixture in your bottle, until the brown solid dissolves, and there is a clear
(but possible brown-tinged colored) solution.
Visualize step 3
Add NH3 to the AgNO3 Abrown
precipitate should form. Keep adding
NH3 until the solution becomes clear.
4) Go to the teacher station and using a graduated cylinder, measure out 10 mL of 0.80 MKOH(aq) .
Pour the potassium hydroxide base into the bottle.
If you get another brownish color or even another precipitate, go back to the fume hood
and add more concentrated ammonia, until the solution in the bottle goes clear again.
5) Go to the teacher station and measure 10.0 mL of glucose solution using a graduated cylinder. Add this
toyour bottle.
Visualize step 5
Add Glucose to the solution in the bottle
6) TIGHTLY SEAL the bottle and begin agitating the contents. (Don’t dawdle). You are on the
right track when there is another color change from the clear solution to a black-brown-gray solution.
7) Continue to agitate the contents, coating the interior, until a silver foil (mirror) is produced.
8) Open the bottle and pour all fluid the contents down the drain.
a) Carefully rinse your bottle in distilledwater (if possible, tap water if necessary).... Be sure you
avoid the problem of the pressurized stream of water shooting waste water all over the place!!!
You also don’t want to rub off / tear the silver mirror! Rinse the bottle a minimum of 2 times...
Questions: In a word-processed piece answer the following questions. Please be sure to include the question and then the answer to each.
1) What visual evidence was there that (a) chemical reaction(s) took place? Include at least 2 reasons.
(Consider using your notes and the consider the 5 visual means of determining a chemical reaction… and/or the vocabulary
associated with physical changes and chemical reactions.)
2) Compare this to the brass penny (alchemy) lab. Did you really produce silver metal in this lab? Using the
reactants, the law of the conservation of matter, and your grasp of redox, design and write a short response
which supports your answer.
3) Using the ideas found in the reading and your grasp of redox, what is the oxidizing agent for the overall
reaction? What is the reducing agent?
4) Glucose is described as a “reducing sugar”. From a chemical point of view, what is implied by the term,
reducingsugar?
O
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5) Glucose is a water-soluble carbohydrate (a sugar). Carbohydrates contain an aldehyde group (-C-H) at the
end of a string of carbons, or they contain a ketone group (just a C=O group), found somewhere in the
middle of the molecule’s carbon chain (not at the end). Aldehydes, like glucose can be oxidized to reduce
metal ions to metal atoms. Ketones can not be oxidized to reduce metal ions. You used glucose in this
lab. Sucrose (table sugar) is another water-soluble carbohydrate BUT, sucrose does NOT work in this lab.
Make at least one conclusion about sucrose.
6) Using OR
and/or
OR
a) Discuss how a silvered-mirror is produced (briefly). Identify another material that could be used in
amodern-day mirror other than silver.
b) Discuss (briefly) how a one-way mirror, is, well, made to be “one way”.
SOLUTIONS:
14.5 M [concentrated] ammonium hydroxide
0.80 MKOH (44.8 g/L) assuming a GFM of 56 g/mol. Only 0.50 Liter is required, thus dissolve
22.4 grams for a 500 mL solution.
0.75 MGlucose (135 g/L) assuming a GFM of 180 g/mol. But, since only 0.50 Liter is required,
use only 67.5 grams for 500 mL of solution )
0.25 M silver nitrate (42.5 g/L) assuming a GFM of 170 g/mol and 20.0 mL/student. With 60 students
this requires 1.2 L of solution … adjustments can be made obviously.
Use 51 grams or limit the # of trials and use 42.5 grams to make only
1 L of solution.