14 - 18 years
Preparation: 15 mins
Activity: 20 mins
Youtube ID: -iLGo8zUkws
Customize Activity to Venue
Identify Safety Practices
React Copper Metal with Nitric Acid
React Copper Metal with Sulfur
Here are some key themes to explore with the audience once they've completed the activity. Adjust the details to match the level of your audience.
Copper (Cu) is element #29 on the periodic table. It is found in nature in its pure form or extracted from minerals, such as chalcopyrite and bornite.
Copper is soft, malleable, and ductile reddish-yellow metal that can be polished to a high sheen. Consequently, it has been used throughout history in both coins and jewelry. Its high thermal and electrical conductivity are second only to silver among pure metals. However, copper is far more abundant than silver, making it a popular choice for cookware and electrical wires.
Reaction of copper and nitric acid
Copper metal reacts with nitric acid to form copper(II) nitrate, nitrogen dioxide, and water:
Cu(s)+ 4 HNO3(aq) →Cu(NO3)2(aq) + 2 NO2(g) + 2 H2O(l)
Copper(II) nitrate is initially a deep green compound that turns the reaction mixture green. Copper(II) nitrate coordinates easily with water, so adding water or simply allowing the reaction to continue will yield a brilliant blue hydrated copper(II) nitrate.
Nitrogen dioxide is a noxious brown gas with an irritating odor. It is what gives smog its characteristic color and smell.
During the reaction, the copper is oxidized (gave up electrons) from an oxidation state of 0 to 2+, and the nitrogen is reduced (accepted electrons) from 5+ to 4+. At the same time, the H+ in the HNO3 molecule is replaced with copper. A reaction involving both electron transfer and a replacement is called a “redox replacement” reaction.
Reaction of copper and sulfur
Sulfur comes in a variety of forms and colors. In yellow sulfur powder, it is mostly rings of eight sulfur atoms each. Heating melts the sulfur, which then oxidizes the copper to a 1+ oxidation state, while the sulfur is reduced to 2-:
16 Cu(s) + S8(l) → 8 Cu2S(s)
In contrast to the shiny, malleable metal, copper(I) sulfide is a gray, brittle crystalline solid.
Copper outside of the lab
Copper oxidizes easily to form Cu1+ and Cu2+ ions. It reacts with oxygen in the air to form brown-black copper oxides, with CO2 dissolved in rainwater to form green, and sulfur compounds to form copper sulfides.
Copper left outside, such as architectural decorations, will oxidize to form a surface layer patina consisting of copper oxides, sulfides, or other compounds, depending on what is in the air.
As seen in the demonstration, copper compounds have properties that are vastly different from the properties that make copper metal so valuable for technology, cookware, coins, and jewelry. Fortunately, compounds formed on the surface of copper actually constitute a protective layer that prevents further corrosion underneath. Some manufacturers and artists will deliberately oxidize the surface of the copper for this reason, in a process called “passivation.”