Copper in Concentrated Strong Acids
Pieces of copper are added to hot concentrated solutions of hydrochloric, phosphoric, sulfuric and nitric acids. The reactions are demonstrated, showing differences in the strengths of the oxidizing agents.
(170 )
Keywords
acids/bases, ammonia, copper, descriptive chemistry, halogens/halides/hydrohalic acids, nitrogen/nitrogen oxides/nitric acid, oxidizing acid, phosphorus/phosphorus oxides/phosphoric acid, redox reaction, strong acid, strong oxidizing agent, sulfur/sulfur oxides/sulfuric acid
Multimedia
Play movie
(QuickTime 3.0 Sorenson, duration 113 seconds, size 7.7 MB)
Pieces of copper are added to hot concentrated solutions of hydrochloric, phosphoric, sulfuric and nitric acids. A reaction clearly takes place between copper and nitric acid.
The presence of copper ions can be observed by adding each solution to a dilute solution of ammonia. If copper ions were present, we would see the blue color of the copper-ammonia complex. Sulfuric acid has oxidized the copper metal, as indicated by the blue color. Nitric acid is a stronger oxidizing agent (and produces a higher concentration of copper(II) ions), as indicated by the darker blue color.
 Reaction between copper and nitric acid on the right. |
 Each solution is added to a dilute solution of ammonia. |
 Left to right: hydrochloric, phosphoric, sulfuric and nitric acids in dilute solution of ammonia. |
Additional still images for this movie
Discussion
Of the four industrially important strong acids, only two, nitric and sulfuric, are strong enough oxidizing agents to dissolve copper. The concentration of copper(II) ions is larger with the nitric acid, because it is a stronger oxidizing agent than sulfuric acid. This is demonstrated by reaction the copper(II) ions with ammonia to form the deep blue copper-ammonia complex ion.
The equations for the reactions are
Cu(s ) + 4 H+(aq ) + 2 NO3-(aq ) --> Cu2+(aq ) + 2 NO2(g ) + 2 H2O(l )
Cu(s ) + 4 H+(aq ) + SO42-(aq ) --> Cu2+(aq ) + SO2(g ) + 2 H2O(l )
Cu2+(aq ) + 4 NH3(aq ) --> Cu(NH3)42+(aq )
Demonstration Notes, Warnings, Safety Information, etc.
Exam and Quiz Questions
1. Write down as many different observations as you can make during the reaction of concentrated nitric acid with the copper.
2. Identify the gaseous product of the reaction.
3. Write a balanced equation for the reaction of concentrated nitric acid with copper.
4. Write down as many different observations as you can make during the reaction of concentrated sulfuric acid with copper.
5. What experiments could you do to confirm that the gaseous product of the reaction is sulfur dioxide?
6. Write a balanced equation for the reaction of concentrated sulfuric acid with copper.