Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes
Electrochemical Cell or Galvanic Cell or Voltaic Cell :
→ Those apparatus or cells or electro-chemical devices by which we convert chemical energy into electric energy, are called Galvanic or voltaic or electrochemical cells. These were first discovered by L. Galvani and A. Volta. So, these were named as Galvanic or Voltaic cell on their names.
→ In these cells, oxidation or reduction reactions take place in different cells, which are called half-cells. Redox reaction takes place spontaneously here. Electrical energy is produced during these reactions. Here, spontaneous redox reactions takes place indirectly.
→ Example: This redox reaction can be explained by working of an electro-chemical cell. This cell is made up of two electrodes, which is called Galvanic or Voltaic or Daniell cell. These two electrodes are attached with porous plate or salt bridge. The rod of Zn metal is dipped in 1.0 M ZnSO4 solution in one electrode and Cu rod is dipped in 1.0 M CusO4 solution in other electrode.
→ These metallic rods present in beaker are attached with ammeter by a connecting wire and a key is also attached in this path. Here, the solutions of both beakers are connected by salt bridge in which the saturated solution of KCl, KNO3 or NH4 NO3 is filled. This salt bridge is made up of U-tube. The saturated solution of KCl, KNO3 or NH4 NO3 does not represent any chemical reaction during the process. This electrolyte is called inert electrolyte.
→ Here, the saturated solution of inert electrolyte is generally formed in gelatin. U-tube is made up of glass. Both open ends of U-tube are closed by a porous substance like glass wool or cotton. It is found on completion of circuit by pressing key in circuit that electric current is passed through the circuit, which can be seen in ammeter.
→ Following observations are seen during the reaction:
- The weight of zinc rod decreases slowly.
- The concentration of Zn2+ in ZnSO4 solution increases.
- More amount of Cu is liberated on copper rod.
- The concentration of Cu2+ in CuSO4 solution decreases.
- Here, electron flows from zinc rod to copper rod in external circuit. So, electric current flows from copper
rod to zinc rod. We can explain following on the basis of these observations:
→ Here, the oxidation of Zn takes place and Zn2+ ions go in the solution. As a result, the weight of rod decreases.
Zn → Zn2+ + 2e2- (Oxidation)
→ When electrons are released from zinc electrode then these move towards Cu electrode. Here, Cu2+ ions of CuSO4 solution accept these electrons and liberated at Cu rod on reduction.
Cu2+ + 2e2- → Cu (Reduction)
→ The rod at which oxidation occurs, is called anode and where reduction occurs, that is called cathode. Since, electrons move from zinc rod to copper rod. So, zinc rod is called negative terminal and copper rod is called positive terminal. Apparatus in which oxidation and reduction takes place is called half-cell. Zinc rod dipped in ZnSO3 solution is called oxidation half-cell and copper rod dipped in CuSO4 solution is called reduction half-cell.
- Trick Remember : LOAN
- L = Left
- O = Oxidation
- A = Anode
- N = Negative terminal
→ The reaction obtained by addition of oxidation reaction of Zn electrode and reduction reaction of Cu electrode is called cell reaction.
Zn + Cu2+ – Zn2+ + Cu (Redox Reaction)