Letter 14502

How does aluminum foil clean silverware  

Dear Helper, I am in year 10 at St Pius X College Chatswood. At present I have an assessment task for science which requires me to carry out a scientific investigation. I chose to research and report on a method of cleaning tarnished silverware. The method involves submerging silver items into a bowl lined with aluminum and filled with boiling water and a teaspoon of salt (dissolved). (no baking soda) I have found that the salt is only a catalyst, as the silver and aluminium combo works as well. I am writing this e-mail to ask you if you could explain the process that is occurring and provide me with an equation (words and symbols) Thank you for taking the time to read my e-mail. Yours sincerely, I am sorry if this is inconvenient but I really need a reply. Please help me. Patrick Robinson - Sydney, NSW, Australia   The principle behind this is that most metals corrode with exposure to oxygen. For example, iron rusts with time because of its exposure to moisture in the air, oxygen. You may also have noticed that cars in the northeast rust a lot faster because of the salt used on the roads in winter. The whole process involves electrons moving between the metal and oxygen atoms. It happens spontaneously with most metals (gold being a notable exception). Silver does pretty much the same thing, only that tarnish is the combination of silver with sulfur and not oxygen. Silver is special with its more complex reactions, but the principle by which silver tarnishes is the same by which iron rusts. Aluminum also does the same thing, only it happens more readily than silver...(see where this is going?) So we need to move the electrons to and from the metals and we need a metal that will take the electrochemical abuse for the silver. The aluminum takes the hit for the silver, and the salt water allows the electrons to move between the silver and aluminum. I would explain more, but this is not a general chemistry article. This of course raises the question of why airplanes (which have aluminum) don't dissolve in midflight... Lisa Robbinson - NSW, Australia

Explanation: Silver tarnishes because it undergoes a chemical reaction with sulfur-containing substances in the air. Silver combines with sulfur to form silver sulfide, which is black, and darkens the silver.The silver can be made shiny again by removing the silver sulfide coating from the surface.

Two ways to remove the silver sulfide are to: remove it from the surface, or reverse the chemical reaction and turn silver sulfide back into silver. The first method involves polishes that remove some of the silver during polishing. The above demo uses a chemical reaction (which is sped up by heating the water) to convert the silver sulfide back into silver, without removing any silver.

Aluminum has a lower ionization energy (energy required to remove electrons from an atom of the element) than silver. As a result, aluminum is oxidized (loses electrons and oxidation number increases), and silver is reduced (gains electrons and oxidation number is reduced). Depending on the amount of tarnish, the silver will be bright and the aluminum foil may be brown with tarnish (aluminum oxide), in a short while. The silver tarnish is "transferred" to the aluminum via reactions, which occur instantaneously, as follows: 3 Ag2S(s) + 2 Al(s)+ 3 H2O(l) --> 6 Ag(s) + Al2O3(s) + 3 H2S(aq)

silver sulfide + aluminum + water --> silver + aluminum oxide + hydrogen sulfide
(* Note, this reaction can be done without the baking soda, but it takes longer to see results).

The baking soda (sodium bicarbonate) reacts with the (sulfur-smelling) H2S: 3 NaHCO3(aq) + 3 H2S(aq) --> 3 NaHS(aq)+3 H2O(l)+ 3 CO2(g) baking soda + hydrogen sulfide --> sodium hydrosulfide + water + carbon dioxide The CO2 gas can be observed escaping from the most tarnished parts of the silver.

The silver and aluminum must be in contact with each other because a small electric current flows between them during the reaction.This type of reaction, which involves an electric current (because atoms are charged), is called an electrochemical reaction, and is used in batteries to produce electricity.

Tania R. Chase
- Topsham, ME, United States

Should the aluminium foil be shiny side up or dull side up?

SHIV ANAND
- Houston, Texas

The aluminum foil should be bright side up.

Robert Franklin Kynor
- Newburyport, Massachusetts

Tania, I do believe you got the reaction correct. Very nice explanation! However, I don't think the silver needs to be in contact with the aluminum because the saltwater acts as an electrolyte.

Susan Ammons
- Bowling Green, KY