Voltaic Cell – Simple Cell

Simple Voltaic Cell

  1. A simple voltaic cell consists of two metal plates of different metals immerse in an electrolyte.
  2. The metal which is higher in electrochemical series will become the negative terminal.
  3. The atom of the metal which is higher in electrochemical series will release electron to form negative ions.
  4. The electrons are then move to the other metal plate through the wire.
  5. The metal which is lower in electrochemical series will become the positive terminal.
  6. In positive terminal, the electrons will be released to the positive ions in the electrolyte.
  7. In a voltaic cell, the negative terminal is the anode while the positive terminal is the cathode.
 Example
At Anode
Since magnesium is higher than copper in electrochemical series, hence magnesium is the negative electrode. In voltaic cell, the negative electrode is the anode.
Chemical Reaction
Magnesium atoms release 2 electrons to form magnesium ions.
Mg → Mg2+ + 2e
Observation
The magnesium strip (anode) erodes. Note: The electrons released are transferred to the cathode (copper) through the wire. As a result, current is produced and the galvanometer deflects.
At Cathode
Copper is lower than magnesium in electrochemical series, hence copper becomes the positive electrode. In voltaic cell, the positive electrode is the cathode.
Chemical Reaction
The positive ions that presence in the electrolyte is magnesium ions and the hydrogen ions. The hydrogen ions is lower than magnesium in electrochemical series, hence it will be selected to be discharged.
2H+ + 2e  H2
Observation
Bubbles of colourless gas are evolved around the copper metal.

Daniel Cell

  1. The Daniell cell was invented in 1836 by a British chemist name John Frederic Daniell.
  2. It was invented to eliminate the hydrogen bubble problem found in the simple voltaic cell.
  3. The Daniell cell consist of a zinc electrode and a copper electrode.
  4. The zinc electrode is immersed in zinc sulphate solution (or sulphuric acid) whereas the copper electrode is immersed in copper(II) sulphate solution.
  5. Figure below shows the illustration of the Daniell cell.
  1. Since zinc is more electropositive than copper, hence it acts as the anode and be come the negative therminal of the cell. Copper is the cathode and the positive therminal of the cell.

At Anode (Zinc Electrode)

The zinc atoms release 2 electrons and become zinc ions

Zn  Zn2+ + 2e

Observation:
The zinc electrode erodes (become thinner).

At Cathode (Copper Electrode)<

The copper(II) ions in the electrolyte are discharged and become zinc atom

Cu2+ + 2e  Cu

Observation:
The copper electrode become thicker.

Other Observation:

  1. The pointer of the galvanometer deflect.
  2. The intensity of the blue colour of copper(II) sulphate solution decreases because the concentration of the copper(II) ions deceases.

Function of the Porous Pot:

The function of the porous pot are

  1. to allow the flow of ions to complete the circuit
  2. to separate the two electrolytes and hence prevent them from react with each other.

Weaknesses of Daniel Cell

  1. The electrolyte can easily spill out.
  2. Difficult to be carried around.
  3. The voltage produced decreases rapidly owing to the polarity of the cell.

Cell with Salt Bridge

Cell with Salt Bridge


  1. A Daniel Cell can be substitute by a design as showing in diagram above. The chemical reaction remains the same as the Daniel cell.
  2. Using salt bridge can reduce the internal resistance of the cell.
  3. The function of the salt bridge is to allow ions follow from one solution to another to complete the circuit.

At Anode (Zinc Electrode)

The zinc atoms release 2 electrons and become zinc ions

Zn  Zn2+ + 2e
Observation:
The zinc electrode erodes (become thinner).

At Cathode (Copper Electrode)

The copper(II) ions in the electrolyte are discharged and become zinc atom

Cu2+ + 2e  Cu

Observation:
The copper electrode become thicker.


Other Observation:

  1. The pointer of the galvanometer deflect.
  2. The intensity of the blue colour of copper(II) sulphate solution decreases because the concentration of the copper(II) ions deceases.

Voltage of the Cell

Voltage of the cell

  1. The voltage of a voltaic cell depends on the difference of the electropositivity of the two metals.
  2. The greater the difference in electrochemical series, the bigger the voltage produced.
  3. For example, the voltage of the cell consist of zinc and copper as the electrodes is higher than the voltage of the cell that uses iron and copper as its electrodes.


The Electrochemical Series

  1. The electrochemical series is an arrangement of metals based on the tendency of each metal atom to donate electrons.
  2. The higher the position of a metal in the Electrochemical Series, the greater the tendency of the metal atoms to donate electrons.
  3. The Electrochemical Series can be constructed based on
    1. The votage between two metals
    2. The ability of a metal to displace another metal from its salt solution