Everything Science Grade 12

4.2 CHAPTER 4. ELECTROCHEMICAL REACTIONS

Half-cell reactions in theZn-Cu cell ESCBJ

The experiment above demonstrated a zinc-copper cell. This was made up of a zinc
half cell and a copper half cell.

DEFINITION: Half cell

A half cell is a structurethat consists of a conductive electrode sur-

rounded by a conductive electrolyte. For example, a zinc half cell

could consist of a zincmetal plate (the electrode) in a zinc sulphate

solution (the electrolyte).

How do we explain what has just been observedin the zinc-copper cell?

• Copper plate
  At the copper plate, there was an increase in mass. This means that Cu2+ions
  from the copper sulphate solution were deposited onto the plate as atoms of
  copper metal. The half-reaction that takes placeat the copper plate is:

Cu2++ 2e−→ Cu (Reduction half reaction)

Another shortened wayto represent this copperhalf-cell is Cu2+/Cu.

• Zinc plate
  At the zinc plate, there was a decrease in mass. This means that some of the zinc
  goes into solution as Z2+ions. The electrons remain on the zinc plate, giving it
  a negative charge. The half-reaction that takes place at the zinc plate is:

Zn→ Zn2++ 2e−(Oxidation half reaction)

The shortened way to represent the zinc half-cell is Zn/Zn2+.

The overall reaction is:

Zn + Cu2++ 2e−→ Zn2++ Cu + 2e−or, if we cancel the electrons:

Zn + Cu2+→ Zn2++ Cu

For this electrochemicalcell, the standard notation is:

Zn|Zn2+||Cu2+|Cu

where

| = a phase boundary (solid/aqueous)

|| = the salt bridge