Electrochemical cell: It is a device to convert chemical energy of a spontaneous redox reaction into chemical energy.It is also called Galvanic or Voltaic cell.One such example of an Electrochemical cell is DANIEL CELL.
Deniel's cell converts the chemical energy liberated during the redox reactions.
Oxidation Half-reaction-
At Zn(s)→Zn^2+(aq)+2e^-
Reduction Half-Reaction:In the right hand beaker reduction occurs.Cu^2+ ions from the solution deposit on the copper plate.It gains weight.
At Cathode: Cu^2+(aq)+2e^- →Cu(s)
Electron flow from zinc plate to copper plate in the external circuit.Conventional current flows from copper to zinc plate(as shown above)
Combining the two half-reactions,we get the complete cell reaction or redox reaction.
Zn(s)+Cu^2+(aq)→Zn^2+(aq)+2e^-
Electrolytical Cell-It is a device to convert electrical energy from an external source to produce chemical change.In the above cell if Eext is >1.1 the reaction will again start,but in the opposite reaction.it now will function as an electrolytical cell.
In the electrochemical cell at each electrode-electrolyte interface there is a tendency of metal ions from the solution to deposit on the metal electrode trying to make it positively charged.At the same time,metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged.At equilibrium, there is a separation of charges and depending on the tendencies of the two opposing reactions,the electrode may be positively or negatively charged with respect to the solution.A potential difference develops between the electrode and the electrolyte which is called Electrode potential.
When concentration of all species involved in half-cells is unity,the electrode potential is called STANDARD ELECTRODE POTENTIAL.
According to IUPAC conventions,standard reducion potentials are now called standard electrode potentials.
