Look at the cell on the left. At the anode more and more positive zinc ions are being formed. While at the cathode more and more electrons are being pumped into it.
The result is an excessive positive charge that builds up at the anode that attracts electrons(negative) and prevents them moving away. While at the cathode the negative build up repels the electrons. As a consequence of this build up of charge, no electron flow occurs and the cell eventually fails.

To remedy the situation we use a salt bridge. The salt bridge contains ions that complete the circuit by moving freely from the bridge to the half cells. These ions are carefully selected so as not to interfere with the reaction taking place.

Click to see a simple animation of how the salt bridge works.
Click to see a flash movie on the electrochemicall cell

After viewing the salt bridge animation answer the questions below. Answer the questions by assuming the animation is correct.
1) Out of the nickel and copper electrodes which one is the anode and which one is the cathode?
2) What charge builds up at the anode?
3) What charge builds up at the cathode?
4) What is the charge on the ions coming from the salt bridge into the nickel half cell?
5) What is the charge on the ions coming from the salt bridge into the copper half cell?

6) A galvanic cell(similar to the one above) is set up by connecting the Ni²⁺_(aq) | Ni_(s) and Mg²⁺_(aq) | Mg_(s half cells via a wire and a salt bridge. The nickel electrode is noticed to have increased in mass while the magnesium electrode has decreased. Draw the galvanic cell indicating clearly the:
- cathode and anode and the polarity.
- direction of electron movement
- the charge on the ions flowing in each half cell
- the reactions occurring in each half cell.
Solution to question 6)

7) Draw a possible set up of a galvanic cell using metal strips of zinc and copper and solutions of copper sulfate and zinc nitrate. The salt bridge consists a filter paper soaked in potassium nitrate.
Indicate clearly the:
- cathode and the anode;
- direction of electron movement;
- the direction of cations and anions flowing from the salt bridge.
Discuss the purpose of the salt bridge.
Solution

More exercises of setting up electrochemical cells from half cells.

Continue with exercises to convert overall equations into an electrochemical cell