Intro: Demonstrating Nitric Acid Acts Upon a Copper Penny Experiment
AKA-Penny Operated Nitric Acid Fountain Nitric acid acts upon copper to yield nitric oxide in an exothermic reaction. The expanding gas displaces water from another flask; actually bubbles in this set up. When the gas cools, the water is drawn back into the flask, dissolving the nitric oxide and forming a blue solution with the copper ions still there. Very cool reaction.
500 mL florence flask
large ring placed below the flask
small ring that fits over the neck of the flask
one-hole rubber stopper
60 cm glass tubing
large container of water
The glass tube is bent in such a way as to connect the top of the flask with the bottom of the water container. The water in the container can be stirred by hand or with a magnetic stirrer. It needs to be stirred, however, or else the NO2 gas collects above the liquid.
Safety and Disposal
The solution is highly acidic. I pour it out into a large beaker or battery jar and add excess sodium carbonate. The carbon dioxide bubbles indicate neutralization and the resulting copper carbonate precipitate is filtered, placed in a baggie and thrown away. The neutralized filtrate can be disposed of as you would any simple salt solution. Procedures may vary from location to location.
Nice Reading While Doing this Experiment-
While reading a text book of chemistry, I came upon the statement, "nitric acid acts upon copper." I was getting tired of reading such absurd stuff and I determined to see what this meant. Copper was more or less familiar to me, for copper cents were then in use. I had seen a bottle marked "nitric acid" on a table in the doctors office where I was then 'doing time'! I did not know its peculiarities, but I was getting on and likely to learn. The spirit of adventure was upon me. Having nitric acid and copper, I had only to learn what the words "act upon" meant. Then the statement "nitric acid acts upon copper", would be something more than mere words. All was still. In the interest of knowledge I was even willing to sacrifice one of the few copper cents then in my possession. I put one of them on the table; opened the bottle marked "nitric acid"; poured some of the liquid on the copper; and prepared to make an observation. But what was this wonderful thing which I beheld? The cent was already changed, and it was no small change either. A greenish blue liquid foamed and fumed over the cent and over the table. The air in the neighborhood of the performance became colored dark red. A great cloud arose: This was disagreeable and suffocating--how should I stop this? I tried to get rid of the objectionable mess by picking it up and throwing it out the window, which I had meanwhile opened. I learned another fact--nitric acid not only acts upon copper but it acts upon fingers. The pain led to another unpremeditated experiment. I drew my fingers across my trousers and another fact was discovered. Nitric acid acts upon trousers. Taking everything into consideration, that was the most impressive experiment, and, relatively, probably the most costly experiment I have ever performed. I tell of it even now with interest. It was a revelation to me. It resulted in a desire on my part to learn more about that remarkable kind of action. Plainly the only way to learn about it was to see its results, to experiment, to work in a laboratory.
-- Ira Remsen (1846-1927)
Oxidation of copper metal with a strong oxidizing agent, conc. nitric acid.
In a classic experiment, copper metal is turned into copper(II) ion while the nitrogen(V) in the nitrate ion becomes nitrogen(IV) in the nitrogen dioxide gas.
As the temperature from the reaction warms the gas, it expands. Later, as it cools, the gas contracts.
Nonmetal oxides are acid anhydrides (also link to acid rain)
Although the nitrogen dioxide gas is noxious and toxic, it dissolves readily in water and make the solution acidic. This can be shown by adding a little indicator to the water and making the water slightly basic before the copper is added to the acid.
As the pressure in the flask is decreased as it cools, the outside pressure pushes the water up the tubing toward the flask. The nitrogen dioxide gas is not pulling the water in.
Descriptive chemistry--copper solutions are green and blue
The colored solutions come from complexes of copper(II) ion in solution. Aqueous copper ion is blue, Cu(H2O)42+ The green must be copper surrounded by nitrates.