React with Copper

ACS International Student Chapter at Qatar University Presents: React with Copper

Concepts to Explore

• Redox reactions: Copper metal loses electrons sulfur to fom copper(I) sulfate and to nitric acid to produce copper(II) nitrate and nitric oxide
• Replacement reactions: When copper reacts with nitric acid, the hydrogen in the acid is replaced with a copper ion
• Copper reactions: The reactions of copper with acid and sulfur also create the patina seen on many copper objects

Safety & Other Considerations

• Reactions generate noxious gas and must be performed in a fume hood
• This activity should be done either as a video or a demonstration with the audience at least 10 ft from the presenter(s)
• Potential hazards include: 
  • Acids and bases
  • Allergens and sensitizers
  • Broken glassware
  • Hot surfaces and liquids
  • Inhalation hazards
  • Oxidizers
  • Spills and splashes        
• Conduct your own RAMP assessment prior to presenting the activity.

• Learn more about Safety in Outreach Settings and review a sample RAMP worksheet.

Materials Required

Preparation

Prior to Activity

On-Site

Instructions & Talking Points

Explore the Chemistry

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.

What is copper?

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 HNO_3(aq) →Cu(NO₃)_2(aq) + 2 NO_2(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) + S_8(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.”

References 

• American Chemical Society, 2023
• ACS International Student Chapter at Qatar University