Voltaic Cells

Question 1

How does a zinc/copper voltaic cell work?

Answer 1

Zn (s) rod is in zinc sulfate (aq) solution in one half-cell, Cu (s) rod is in copper sulfate solution (aq) in another.

Wire connects the two half-cells. Both the Zn2+ ions in solution and the Cu2+ ions in solution want electrons. Cu2+ has greater pull for electrons, so they move across wire, creating electricity. These electrons come from the Zn (s) being oxidised and becoming Zn2+ ions.

Eventually, Zn (s) rod starts to break down and Cu (s) starts to become bigger.

Question 2

Why is a salt bridge needed in voltaic cells?

Answer 2

A salt bridge is needed as when the Zn (s) starts to form Zn2+ ions (aq), the positive charge builds up in the half-cell.
When the Cu2+ ions (aq) start to add to the Cu (s) rod, a negative charge builds up in the half-cell.
SO4 2- (aq) ions are in the solution as zinc/copper sulfate was added to provide the ions. Na+ and Cl- ions from the salt bridge become attracted to the positive/negative charges forming, and move into the half-cells to balance out the charges.

Question 3

Why is there cotton wool in the salt bridge?

Answer 3

So all of the salt doesn’t enter half-cells at once.

Question 4

What is the anode in a voltaic cell?

Answer 4

Site of oxidation.
In Zn/Cu cell, this is the piece of zinc.

Question 5

What is the cathode in a voltaic cell?

Answer 5

Site of reduction.
In Zn/Cu cell, this is piece of copper.

Question 6

What is reduced in Zn/Cu voltaic cell? Give the equation.

Answer 6

Copper.
Cu2+ (aq) + 2e- -> Cu (s)

Question 7

What is oxidised in Zn/Cu voltaic cell? Give the equation.

Answer 7

Zinc.
Zn (s) -> Zn2+ (aq) + 2e-

Question 8

Give the cell notation for the oxidation of zinc in a zinc half-cell.

Answer 8

Zn (s) l Zn2+ (aq)

Question 9

Give the cell notation for the reduction of copper in a copper half-cell.

Answer 9

Cu2+ (aq) l Cu (s)