Types of ElectroChemical Cells, Half Cells, Salt Bridge, Anode, Cathode

When two half cells are connected with each other, an electrochemical cell can be created. Salt bridge is used to connect those two half cells to keep the current flow from one cell to other cell. There are different types of electrochemical cells in the world and according to the requirement, right choice should be made. Electrochemical cell will give current when two half cells are connected with each other.


In this tutorial, we will study followings.

  1. Definition of half cell
  2. Types of half cells
  3. Electrochemical cell
  4. Salt bridge in electrochemical cells
  5. Anode and cathode of electrochemical cell


Definition of half cell

A reductant in equilibrium with an oxidant on a metal surface under standard conditions of temperature and pressure is known as an electrochemical half-cell. Here temperature should be kept at 298K and pressure at 1atm and all concentrations of ions at 1 mol dm-3. There are three possible types of half cells.



Types of half cells

There are three types in electrochemical cells as below. These categorization is done by considering metals, ions and phases of matter.

  • A metal/metal ion half cell
  • Oxidant and reductant half cell
  • Gas/ion half cell



A metal/metal ion half cells (Electrodes)

A metal and its ions are at equilibrium at 1 atm pressure and 298k temperature. As the following figure, when Zinc metal is dissolved in a aqueous solution in mentioned physical conditions where Zn2+ ion concentration is 1.0 mol dm-3, its is defined as Zn|Zn2+ ion half cell.

Zinc electrode half cell

Oxidant and reductant half cells

There are two different ions which can independently function as oxidant and reductant. Electrode is made of a inactive anode such as Platinum (Pt). (Both the Oxidant and reductant must be available in the electrolyte).


Half cell of Manganate ion (MnO4-) ions and Manganese (2+) (Mn2+) in the presence of Platinum electrode

  • Electrolyte which contains Manganate ion (MnO4-) ions and Manganese (2+) (Mn2+) ions can behave as an oxidant - reductant half cell.
  • MnO4- ions are reduced to Mn2+ cations and Mn2+ cations are oxidized to MnO4- ions in half cell.
  • Concentrations of both MnO4- and Mn2+ ions should be 1.0 mol dm-3 at 298K.

Half cell of Ferrous (+2) ion (Fe2+) ions and Ferric (2+) (Fe3+) in the presence of Platinum electrode

  • Electrolyte which contains Ferrous (+2) ion (Fe2+) ions and Ferric (2+) (Fe3+) ions can behave as an oxidant - reductant half cell.
  • Fe3+ ions are reduced to Fe2+ cations and Fe2+ cations are oxidized to Fe3+ ions in the half cell.
  • Concentrations of both Fe3+ and Fe2+ ions should be 1.0 mol dm-3 at 298K.
oxidant and reductant half cells

Gas/ion half cells (Electrodes)

A gas is adsorbed on to a Platinum plate and allow reaching to the equilibrium with its ions solution. Concentration of ions should be 1.0 mol dm-3 at 298K. Pressure of gaseous phase should be kept at 1 atm.

Gas ion half cells.jpg

Electrochemical cell

If any two different half cells are connected with each other through a salt bridge and a volt meter/an electric bulb and both half cells are kept under standard conditions. Then it is referred to as a standard electrochemical cell. If the metal plates of two half cells are connected to a bulb, it will produce light because generated current due to the potential difference of half cells is flown through the bulb. If they are connected through a voltmeter it will produce a deflection at the voltmeter. (Current could be produced by the electric cell)



Salt bridge in electrochemical cells

Salt bridge is used to keep the current flow between electrochemical cells. Therefore, a good electrical conductivity should be in the salt bridge. The salt bridge takes the form of either,

  1. Trampled filter paper dipped in concentrated solution of Potassium nitrate (KNO3) / Potassium chloride (KCl) and the ends dipping in the two solutions.
  2. A "U" shaped glass tube filled with salts absorbed in two solutions.


Main functions and uses of salt bridge

  1. Carrying charges: It completes the circuits by allowing ions carrying charge to move towards one half cells to the other through the solution. When the salt bridge is removed the current ceases since the charge can no longer flow around the circuit.

  2. Replacing anions and cations in half cells: When electrochemical cell releases energy by generating current, cations and anions are converted to other chemical compounds. (As an example, in Cu/Zn cell, Zn is oxidized to Zn2+ cations and Cu2+ cations and reduced to Cu.) It provides cations and anions to replace those consumed at the electrodes to balance the charges on any ion formed from the electrode.



Anode and cathode of electrochemical cell


Definition of anode

The system (Half cell) with the algebraically lower reduction potential (Electrode potential) will be the negative terminal of the cell and this is known as the anode of the cell. Anode is the electrode where the oxidation takes place in an electrochemical cell.


Definition of cathode

The system (Electrode) with the algebraically higher reduction potential (Electrode Potential) will be the positive terminal of the cell and known as the cathode where the reduction takes place in an electrochemical cell.




Characteristics of anode and cathode

  • Since the electron density at the anode is high, anode is negatively charged.
  • Cathode can attract more cations to reduce at its surface, therefore it is positively charged.
  • Therefore a stream of electrons will flow from anode to cathode through the external circuit.




Questions


Can I use silver chloride for salt bridge in a electrochemical cell?

Silver chloride is a precipitate. Therefore, it is a weak electrolyte which does not carry electricity properly as a strong electrolytes do.







Electromotive force of cells Different types of electrochemical cells Electrochemical cells Electrochemical series Electrolysis of copper sulfate (CuSO4)