Introduction to Electrochemistry - Class 12, Chemistry IIT JAM Notes | EduRev

IIT JAM : Introduction to Electrochemistry - Class 12, Chemistry IIT JAM Notes | EduRev

 Page 1


1 
 
Electrochemistry Electrochemistry Electrochemistry Electrochemistry    
What are half reactions? 
Let us consider the reaction 2Na + Cl2 ? 2Na
+
 + 2Cl
–
. It occurs by the transfer of electrons from 
Na to Cl. Na loses an electron and is said to be oxidized to Na
+
 ion. At the same time,                                     
Cl gains an electron and is reduced to Cl
–
 ion. Such a reaction which is brought about by loss of 
electrons (oxidation) and gain of electrons (reduction) simultaneously is called an                                       
Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox 
reaction no free electrons are generated. 
The redox reaction can be considered as made up of two reactions. For example, the redox 
reaction 2Na + Cl 2 ? Na
+
 + 2Cl
–
 is composed of two half-reactions: 
2Na ? 2Na
+
 + 2e
–
 (oxidation) 
Cl 2 + 2e
– 
? 2Cl– (reduction) 
Each of the two reactions shows just its oxidation or just the reduction portion of the overall 
redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.                
The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction. 
The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.                  
When the two half-reactions are added together, the sum is the net redox reaction. 
Electrochemical cells 
A device for producing an electrical current from a chemical reaction (redox reaction) is called 
an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is 
dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).                      
The net reaction is  Zn + Cu
2+
 ??? Zn
2+
 + Cu 
 
 
Fig. 1:  Redox reaction taking place on zinc bar itself 
Page 2


1 
 
Electrochemistry Electrochemistry Electrochemistry Electrochemistry    
What are half reactions? 
Let us consider the reaction 2Na + Cl2 ? 2Na
+
 + 2Cl
–
. It occurs by the transfer of electrons from 
Na to Cl. Na loses an electron and is said to be oxidized to Na
+
 ion. At the same time,                                     
Cl gains an electron and is reduced to Cl
–
 ion. Such a reaction which is brought about by loss of 
electrons (oxidation) and gain of electrons (reduction) simultaneously is called an                                       
Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox 
reaction no free electrons are generated. 
The redox reaction can be considered as made up of two reactions. For example, the redox 
reaction 2Na + Cl 2 ? Na
+
 + 2Cl
–
 is composed of two half-reactions: 
2Na ? 2Na
+
 + 2e
–
 (oxidation) 
Cl 2 + 2e
– 
? 2Cl– (reduction) 
Each of the two reactions shows just its oxidation or just the reduction portion of the overall 
redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.                
The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction. 
The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.                  
When the two half-reactions are added together, the sum is the net redox reaction. 
Electrochemical cells 
A device for producing an electrical current from a chemical reaction (redox reaction) is called 
an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is 
dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).                      
The net reaction is  Zn + Cu
2+
 ??? Zn
2+
 + Cu 
 
 
Fig. 1:  Redox reaction taking place on zinc bar itself 
2 
 
This is a redox reaction and the two half-reactions are: 
Zn ??? Zn
2+
 + 2e
–
 
Cu
2+
 + 2e
–
 ??? Cu 
In this change, Zn is oxidized to give Zn
2+
 ions and Cu
2+
 ions are reduced to Cu atoms. The 
electrons released in the first half-reaction are used up by the second half-reaction. Both the half 
reactions occur on the zinc bar itself and there is no net charge. 
Now, let the two half-reactions occur in separate compartments which are connected by a wire 
(See Fig. 2) The electrons produced in the left compartment flow through the wire to the other 
compartment. However the current will flow for an instant and then stop. The current stops flowing 
because of the charge build up in the two compartments. The electrons leave the left compartment and 
it would become positively charged. The right compartment receives electrons and becomes negatively 
charged. Both these factors oppose the flow of electrons (electrical current) which eventually stops. 
 
Fig. 2:  Separate half-reactions cause flow of 
electrons (Current) in the wire connecting them 
This problem can be solved very simply. The solutions in the two compartments may be 
connected, say, by a salt bridge. The salt bridge is a U-tube filled with an electrolyte such as NaCl, 
KCl, or K
2
SO
4
. It provides a passage to ions from one compartment to the other compartment without 
extensive mixing of the two solutions. With this ion flow, the circuit is complete and electrons pass 
freely through the wire to keep the net charge zero in the two compartments. 
 
Page 3


1 
 
Electrochemistry Electrochemistry Electrochemistry Electrochemistry    
What are half reactions? 
Let us consider the reaction 2Na + Cl2 ? 2Na
+
 + 2Cl
–
. It occurs by the transfer of electrons from 
Na to Cl. Na loses an electron and is said to be oxidized to Na
+
 ion. At the same time,                                     
Cl gains an electron and is reduced to Cl
–
 ion. Such a reaction which is brought about by loss of 
electrons (oxidation) and gain of electrons (reduction) simultaneously is called an                                       
Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox 
reaction no free electrons are generated. 
The redox reaction can be considered as made up of two reactions. For example, the redox 
reaction 2Na + Cl 2 ? Na
+
 + 2Cl
–
 is composed of two half-reactions: 
2Na ? 2Na
+
 + 2e
–
 (oxidation) 
Cl 2 + 2e
– 
? 2Cl– (reduction) 
Each of the two reactions shows just its oxidation or just the reduction portion of the overall 
redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.                
The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction. 
The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.                  
When the two half-reactions are added together, the sum is the net redox reaction. 
Electrochemical cells 
A device for producing an electrical current from a chemical reaction (redox reaction) is called 
an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is 
dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).                      
The net reaction is  Zn + Cu
2+
 ??? Zn
2+
 + Cu 
 
 
Fig. 1:  Redox reaction taking place on zinc bar itself 
2 
 
This is a redox reaction and the two half-reactions are: 
Zn ??? Zn
2+
 + 2e
–
 
Cu
2+
 + 2e
–
 ??? Cu 
In this change, Zn is oxidized to give Zn
2+
 ions and Cu
2+
 ions are reduced to Cu atoms. The 
electrons released in the first half-reaction are used up by the second half-reaction. Both the half 
reactions occur on the zinc bar itself and there is no net charge. 
Now, let the two half-reactions occur in separate compartments which are connected by a wire 
(See Fig. 2) The electrons produced in the left compartment flow through the wire to the other 
compartment. However the current will flow for an instant and then stop. The current stops flowing 
because of the charge build up in the two compartments. The electrons leave the left compartment and 
it would become positively charged. The right compartment receives electrons and becomes negatively 
charged. Both these factors oppose the flow of electrons (electrical current) which eventually stops. 
 
Fig. 2:  Separate half-reactions cause flow of 
electrons (Current) in the wire connecting them 
This problem can be solved very simply. The solutions in the two compartments may be 
connected, say, by a salt bridge. The salt bridge is a U-tube filled with an electrolyte such as NaCl, 
KCl, or K
2
SO
4
. It provides a passage to ions from one compartment to the other compartment without 
extensive mixing of the two solutions. With this ion flow, the circuit is complete and electrons pass 
freely through the wire to keep the net charge zero in the two compartments. 
 
3 
 
Voltaic Cells 
A Voltaic cell, also known as a galvanic cell is one in which electrical current is generated by a 
spontaneous redox reaction. A simple voltaic cell is shown in Fig. 3. Here the spontaneous reaction of 
zinc metal with an aqueous solution of copper sulphate is used. 
Zn + Cu
2+
 ??? Zn
2+
 + Cu 
A bar of zinc metal (anode) is placed in zinc sulphate solution in the left container. A bar of 
copper (cathode) is immersed in copper sulphate solution in the right container. The zinc and copper 
electrodes are joined by a copper wire. A salt bridge containing potassium sulphate solution 
interconnects the solutions in the anode compartment and the cathode compartment. 
The oxidation half-reaction occurs in the anode compartment.  Zn ??? Zn
2+
 + 2e
–
 
The reduction half-reaction takes place in the cathode compartment.  Cu
2+
 + 2e
–
 ??? Cu 
 
 
Fig. 3:  A simple voltaic (galvanic) cell 
When the cell is set up, electrons flow from zinc electrode through the wire to the copper 
cathode. As a result, zinc dissolves in the anode solution to form Zn
2+
 ions. The Cu
2+
 ions in the 
cathode half-cell pick up electrons and are converted to Cu atoms on the cathode. At the same time, 
SO
4
2–
 ions from the cathode half-cell migrate to the anode half-cell through the salt bridge.                    
Likewise, Zn
2+
 ions from the anode half-cell move into the cathode half-cell. This flow of ions from 
one half-cell to the other completes the electrical circuit which ensures continuous supply of current. 
The cell will operate till either the zinc metal or copper ion is completely used up. 
Page 4


1 
 
Electrochemistry Electrochemistry Electrochemistry Electrochemistry    
What are half reactions? 
Let us consider the reaction 2Na + Cl2 ? 2Na
+
 + 2Cl
–
. It occurs by the transfer of electrons from 
Na to Cl. Na loses an electron and is said to be oxidized to Na
+
 ion. At the same time,                                     
Cl gains an electron and is reduced to Cl
–
 ion. Such a reaction which is brought about by loss of 
electrons (oxidation) and gain of electrons (reduction) simultaneously is called an                                       
Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox 
reaction no free electrons are generated. 
The redox reaction can be considered as made up of two reactions. For example, the redox 
reaction 2Na + Cl 2 ? Na
+
 + 2Cl
–
 is composed of two half-reactions: 
2Na ? 2Na
+
 + 2e
–
 (oxidation) 
Cl 2 + 2e
– 
? 2Cl– (reduction) 
Each of the two reactions shows just its oxidation or just the reduction portion of the overall 
redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.                
The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction. 
The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.                  
When the two half-reactions are added together, the sum is the net redox reaction. 
Electrochemical cells 
A device for producing an electrical current from a chemical reaction (redox reaction) is called 
an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is 
dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).                      
The net reaction is  Zn + Cu
2+
 ??? Zn
2+
 + Cu 
 
 
Fig. 1:  Redox reaction taking place on zinc bar itself 
2 
 
This is a redox reaction and the two half-reactions are: 
Zn ??? Zn
2+
 + 2e
–
 
Cu
2+
 + 2e
–
 ??? Cu 
In this change, Zn is oxidized to give Zn
2+
 ions and Cu
2+
 ions are reduced to Cu atoms. The 
electrons released in the first half-reaction are used up by the second half-reaction. Both the half 
reactions occur on the zinc bar itself and there is no net charge. 
Now, let the two half-reactions occur in separate compartments which are connected by a wire 
(See Fig. 2) The electrons produced in the left compartment flow through the wire to the other 
compartment. However the current will flow for an instant and then stop. The current stops flowing 
because of the charge build up in the two compartments. The electrons leave the left compartment and 
it would become positively charged. The right compartment receives electrons and becomes negatively 
charged. Both these factors oppose the flow of electrons (electrical current) which eventually stops. 
 
Fig. 2:  Separate half-reactions cause flow of 
electrons (Current) in the wire connecting them 
This problem can be solved very simply. The solutions in the two compartments may be 
connected, say, by a salt bridge. The salt bridge is a U-tube filled with an electrolyte such as NaCl, 
KCl, or K
2
SO
4
. It provides a passage to ions from one compartment to the other compartment without 
extensive mixing of the two solutions. With this ion flow, the circuit is complete and electrons pass 
freely through the wire to keep the net charge zero in the two compartments. 
 
3 
 
Voltaic Cells 
A Voltaic cell, also known as a galvanic cell is one in which electrical current is generated by a 
spontaneous redox reaction. A simple voltaic cell is shown in Fig. 3. Here the spontaneous reaction of 
zinc metal with an aqueous solution of copper sulphate is used. 
Zn + Cu
2+
 ??? Zn
2+
 + Cu 
A bar of zinc metal (anode) is placed in zinc sulphate solution in the left container. A bar of 
copper (cathode) is immersed in copper sulphate solution in the right container. The zinc and copper 
electrodes are joined by a copper wire. A salt bridge containing potassium sulphate solution 
interconnects the solutions in the anode compartment and the cathode compartment. 
The oxidation half-reaction occurs in the anode compartment.  Zn ??? Zn
2+
 + 2e
–
 
The reduction half-reaction takes place in the cathode compartment.  Cu
2+
 + 2e
–
 ??? Cu 
 
 
Fig. 3:  A simple voltaic (galvanic) cell 
When the cell is set up, electrons flow from zinc electrode through the wire to the copper 
cathode. As a result, zinc dissolves in the anode solution to form Zn
2+
 ions. The Cu
2+
 ions in the 
cathode half-cell pick up electrons and are converted to Cu atoms on the cathode. At the same time, 
SO
4
2–
 ions from the cathode half-cell migrate to the anode half-cell through the salt bridge.                    
Likewise, Zn
2+
 ions from the anode half-cell move into the cathode half-cell. This flow of ions from 
one half-cell to the other completes the electrical circuit which ensures continuous supply of current. 
The cell will operate till either the zinc metal or copper ion is completely used up. 
4 
 
 
  
 
 Current is the flow of electrons through a wire or any conductor. 
 Electrode is the material: a metallic rod/bar/strip which conducts electrons into and out of a 
solution. 
 Anode is the electrode at which oxidation occurs. It sends electrons into the outer circuit. It has 
negative charge and is shown as (–) in cell diagrams. 
 Cathode is the electrode at which electrons are received from the outer circuit. It has a positive 
charge and is shown as (+) in cell diagrams. 
 Electrolyte is the salt solutions in a cell. 
 Anode compartment is the compartment of the cell in which oxidation half-reaction occurs.             
It contains the anode. 
 Cathode compartment is the compartment of the cell in which reduction half-reaction occurs.                        
It contains the cathode. 
 Half-cell. Each half of an electrochemical cell, where oxidation occurs and the half where 
reduction occurs, is called the half cell. 
Page 5


1 
 
Electrochemistry Electrochemistry Electrochemistry Electrochemistry    
What are half reactions? 
Let us consider the reaction 2Na + Cl2 ? 2Na
+
 + 2Cl
–
. It occurs by the transfer of electrons from 
Na to Cl. Na loses an electron and is said to be oxidized to Na
+
 ion. At the same time,                                     
Cl gains an electron and is reduced to Cl
–
 ion. Such a reaction which is brought about by loss of 
electrons (oxidation) and gain of electrons (reduction) simultaneously is called an                                       
Oxidation-Reduction reaction or Redox reaction in brief. It may be noted that in the overall redox 
reaction no free electrons are generated. 
The redox reaction can be considered as made up of two reactions. For example, the redox 
reaction 2Na + Cl 2 ? Na
+
 + 2Cl
–
 is composed of two half-reactions: 
2Na ? 2Na
+
 + 2e
–
 (oxidation) 
Cl 2 + 2e
– 
? 2Cl– (reduction) 
Each of the two reactions shows just its oxidation or just the reduction portion of the overall 
redox reaction. Being half components of the redox reaction, these reactions are called Half-reactions.                
The first half-reaction that proceeds by oxidation is often referred to as the Oxidation half-reaction. 
The second half-reaction that occurs by reduction, is referred to as the Reduction half-reaction.                  
When the two half-reactions are added together, the sum is the net redox reaction. 
Electrochemical cells 
A device for producing an electrical current from a chemical reaction (redox reaction) is called 
an electrochemical cell. How a Redox reaction can produce an electrical current? When a bar of zinc is 
dipped in a solution of copper sulphate, copper metal is deposited on the bar (See Figure 1).                      
The net reaction is  Zn + Cu
2+
 ??? Zn
2+
 + Cu 
 
 
Fig. 1:  Redox reaction taking place on zinc bar itself 
2 
 
This is a redox reaction and the two half-reactions are: 
Zn ??? Zn
2+
 + 2e
–
 
Cu
2+
 + 2e
–
 ??? Cu 
In this change, Zn is oxidized to give Zn
2+
 ions and Cu
2+
 ions are reduced to Cu atoms. The 
electrons released in the first half-reaction are used up by the second half-reaction. Both the half 
reactions occur on the zinc bar itself and there is no net charge. 
Now, let the two half-reactions occur in separate compartments which are connected by a wire 
(See Fig. 2) The electrons produced in the left compartment flow through the wire to the other 
compartment. However the current will flow for an instant and then stop. The current stops flowing 
because of the charge build up in the two compartments. The electrons leave the left compartment and 
it would become positively charged. The right compartment receives electrons and becomes negatively 
charged. Both these factors oppose the flow of electrons (electrical current) which eventually stops. 
 
Fig. 2:  Separate half-reactions cause flow of 
electrons (Current) in the wire connecting them 
This problem can be solved very simply. The solutions in the two compartments may be 
connected, say, by a salt bridge. The salt bridge is a U-tube filled with an electrolyte such as NaCl, 
KCl, or K
2
SO
4
. It provides a passage to ions from one compartment to the other compartment without 
extensive mixing of the two solutions. With this ion flow, the circuit is complete and electrons pass 
freely through the wire to keep the net charge zero in the two compartments. 
 
3 
 
Voltaic Cells 
A Voltaic cell, also known as a galvanic cell is one in which electrical current is generated by a 
spontaneous redox reaction. A simple voltaic cell is shown in Fig. 3. Here the spontaneous reaction of 
zinc metal with an aqueous solution of copper sulphate is used. 
Zn + Cu
2+
 ??? Zn
2+
 + Cu 
A bar of zinc metal (anode) is placed in zinc sulphate solution in the left container. A bar of 
copper (cathode) is immersed in copper sulphate solution in the right container. The zinc and copper 
electrodes are joined by a copper wire. A salt bridge containing potassium sulphate solution 
interconnects the solutions in the anode compartment and the cathode compartment. 
The oxidation half-reaction occurs in the anode compartment.  Zn ??? Zn
2+
 + 2e
–
 
The reduction half-reaction takes place in the cathode compartment.  Cu
2+
 + 2e
–
 ??? Cu 
 
 
Fig. 3:  A simple voltaic (galvanic) cell 
When the cell is set up, electrons flow from zinc electrode through the wire to the copper 
cathode. As a result, zinc dissolves in the anode solution to form Zn
2+
 ions. The Cu
2+
 ions in the 
cathode half-cell pick up electrons and are converted to Cu atoms on the cathode. At the same time, 
SO
4
2–
 ions from the cathode half-cell migrate to the anode half-cell through the salt bridge.                    
Likewise, Zn
2+
 ions from the anode half-cell move into the cathode half-cell. This flow of ions from 
one half-cell to the other completes the electrical circuit which ensures continuous supply of current. 
The cell will operate till either the zinc metal or copper ion is completely used up. 
4 
 
 
  
 
 Current is the flow of electrons through a wire or any conductor. 
 Electrode is the material: a metallic rod/bar/strip which conducts electrons into and out of a 
solution. 
 Anode is the electrode at which oxidation occurs. It sends electrons into the outer circuit. It has 
negative charge and is shown as (–) in cell diagrams. 
 Cathode is the electrode at which electrons are received from the outer circuit. It has a positive 
charge and is shown as (+) in cell diagrams. 
 Electrolyte is the salt solutions in a cell. 
 Anode compartment is the compartment of the cell in which oxidation half-reaction occurs.             
It contains the anode. 
 Cathode compartment is the compartment of the cell in which reduction half-reaction occurs.                        
It contains the cathode. 
 Half-cell. Each half of an electrochemical cell, where oxidation occurs and the half where 
reduction occurs, is called the half cell. 
5 
 
Daniel Cell 
It is a typical voltaic cell. It was named after the British chemist John Daniel. It is a simple zinc 
copper cell like the one described above. In this cell the salt-bridge has been replaced by a porous pot. 
Daniel cell resembles the above voltaic cell in all details except that Zn
2+
 ions and SO
4
2–
 ions flow to 
the cathode and the anode respectively through the porous pot instead of through the salt-bridge.   
Inspite of this difference, the cell diagram remains the same. 
 
 
Cell reaction 
The flow of electrons from one electrode to the other in an electrochemical cell is caused by the 
half-reactions taking place in the anode and cathode compartments. The net chemical change obtained 
by adding the two half-reactions is called the cell reaction. Thus, for a simple voltaic cell described 
above, we have 
(a) Half-reactions: 
Zn(s) ??? Zn
2+
 (aq) + 2e
–
 
Cu
2+
 (aq) + 2e
–
 ??? Cu(s) 
(b) Cell reaction by adding up the half-reactions:  
Zn(s) + Cu
2+
 (aq) ??? Zn2+ (aq) + Cu(s) 
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