NEET Chemistry Notes Electrochemistry – Representation of a Cell

NEET Chemistry Notes Electrochemistry – Representation of a Cell

 Representation of a Cell

Representation of a Cell

• The anode is written on the left hand side and cathode on the right hand side.
• A vertical line (|) or semicolon (;) indicates a contact between two phases.
• The anode of the cell is represented by writing metal first and then the metal ion present in the electrolytic solution.
• The cathode of the cell is represented by writing the cation of the electrolyte first and then metal.
•  The salt bridge which separates the two half-cells is indicated by two parallel vertical lines.
• Sometimes, molar concentration or signs are also indicated on the electrodes.
  Consider the Daniel cell with following cell reaction:
  

Difference between Electrolytic and Electrochemical Cells
The main points of difference between these two cells are

• The device in which electrolysis (chemical reaction involving oxidation and reduction) is carried out by using electricity or in which conversion of electrical energy into chemical energy is done is called electrolytic cell while galvanic or voltaic cell is a device in which redox reaction is used to convert chemical energy into electrical energy.
•  In electrolytic cell, anode is positive electrode, while cathode is negative electrode. On the other hand, in galvanic cell, anode is negative electrode and cathode is positive electrode.
  In both the cells, anode is always the site of oxidation and cathode is of reduction.
• In electrolytic cell, ions are discharged at both the electrodes, while in galvanic cell, ions are discharged only at cathode.
• In electrolytic cell, both the electrodes are fitted in same compartment, while in galvanic cell, both the electrodes are fitted in different compartments.
  
• Besides, salt-bridge in both the cells, both the electrodes are connected externally with the help of a wire connected through a voltmeter. Flow of current and electrons occur through this wire.

EMF of Cell

In electrochemical cell, the electrodes in different half-cells have different reduction potential. As a result of this, different flow of electrons is seen from the electrode with higher tendency to lose electrons to other electrode. This difference in electrode potential of electrodes is called electromotive force or cell potential of a cell. This is the driving force for all cell reactions.

Standard Electrode Potential and EMF

•  If we connect two different electrodes, electrons will flow from the electrode of higher negative electric charge density to the electrode with a lower negative electric charge density. A property closely related to the density of negative electric charge is called the electrode potential. Potential difference between the metal and the metal ion in which electrode is dipped, is called electrode potential denoted as E.
• In the standard state, when pressure is 1 atm (latest IUPAC correction use 1 bar) and concentration is 1 M, electrode potential is called standard electrode potential denoted as E°. Temperature is generally taken as 298 K (i.e. 25°C).
•  According to international convention, standard reduction potentials are now called standard electrode potentials. If the standard electrode potential of a half-cell is 0.34 V, it means it is the standard reduction potential , of the half-cell
  
• E°cell or Ecell is the potential difference between the two half-cells. Since, the potential difference is the driving force for electrons, it is also called the electromotive force (emf) of the cell or the cell potential or the cell voltage.
•  This driving force pushes the negatively charged electrons away from the anode (-ve electrode) and pulls them towards the cathode (+ve electrode). The SI unit of cell potential is volt (V) and the potential of a galvanic cell is defined as the positive quantity
  
  For such cases, take values according to the reaction. Electrons should be equal in both half-cell reactions.
•  Standard Hydrogen Electrode, SHE is used as a reference electrode. Its reduction potential is taken as zero. It is represented as
  Pt, H₂(l atm) | H⁺(1M) or H⁺ (1 M)|H₂ (1 atm) ,Pt depending upon whether it acts as anode or cathode respectively.
•  The difference between the potential required for the evolution of gas and the standard electrode potential of that gas is called overvoltage/overpotential.

Electrochemical Series

The arrangement of metals in decreasing order of tendency to lose electrons is called electrochemical series or it is the series in which the elements are arranged on the basis of the values of their standard reduction potential at 25°C.

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