Introduction

The spontaneity of redox reactions is demonstrated using copper, zinc, and silver, and their cations.

When solid zinc is placed in Cu2+ solution, the blue color of the solution becomes less intense as Cu2+ is reduced to copper metal.  Orange copper is visible on the surface of the zinc.

Zn(s) + Cu2+(aq) —> Zn2+(aq) + Cu(s)

When solid copper is placed in Ag+ solution, the solution becomes faintly blue as copper metal is oxidized to Cu2+.  White slivers of silver metal appear on the surface of the copper as Ag+ is reduced to silver metal.

Cu(s) + Ag+(aq) —> Cu2+(aq) + Ag(s)

When solid zinc is placed in Ag+ solution, the zinc is seen to grow black and dissolve as it is oxidized to Zn2+.  Long, shiny slivers of silver metal begin to grow on the dissolving zinc as the Ag+ is reduced to silver metal.

Zn(s) + Ag+(aq) —> Zn2+(aq) + Ag(s)

No reaction takes place between silver metal and Cu2+ or Zn2+.

No reaction takes place between copper metal and Zn2+.

Half reaction reduction potentials:

Ag+ + e- -à Ag           eo= +0.80V

Cu2+ + 2e- -à Cu        eo= +0.34V

Zn2+ + 2e-  -à Zn        eo= -0.76V

To Conduct Demonstration:

Note:  Perform steps one and two at the beginning of lecture and check back on the reactions after about 30 minutes.

  1. Pour 0.2 M solutions of Cu2+, Ag+, and Zn2+ into two petri dishes per solution (six total) to a depth of about 3/8".
  2. Add a clean sample of each metal to the cation solutions of the other two metals (zinc to copper and silver cation solutions, etc.).
  3. Allow the metal samples to remain undisturbed for about 30 minutes.  (To be able to see the reactions on the overhead, be prepared to let the reactions proceed for most of the  lecture period.)
  4. Observe results - this is most frequently shown on an overhead projector, however sometimes the bench light is used.  Encourage the students to come up at the end of the lecture to see the reactions close-up as some of these reactions are difficult to observe on the overhead.  More recently, the document projector is being used with very good results.  Students can see the reaction results projected on the screen.

Demo Time: 5 minutes to set up. ~30 - 50 minutes for the reaction to proceed.

Chirstmas tree variation:

A copper sheet is cut in the shape of a Christmas tree which is suspended from the top of a 1L beaker.  To suspend, use wire through a small hole in the metal sheet and twist around a stir rod spanning the top of the beaker.  Fill the beaker to the top of the tree with 0.2 M Ag(NO3).  Start the demo at the beginning of lecture over light box to allow the silver “icicles” to form.  Leave the tree up after lecture for students to see. Alternate small scale version: cut out copper foil Christmas tree to fit a petri dish and perform demo on document camera.