In the previous section, we saw a galvanic cell made using zinc and copper. In this section, we will see another galvanic cell made using copper and silver.
1. Take two beakers. Add 100 mL of 1 M copper sulphate solution into one beaker.
2. Add 100 mL of silver nitrate (AgNO3) solution into the other. See fig.12.7 below:
[We have seen the method to prepare 1 M solution of any given salt. See details here]
3. Immerse a copper rod in the copper sulphate solution.
4. Immerse a silver rod in the silver nitrate solution.
5. Connect the negative terminal of a voltmeter to the copper rod. Connect the positive terminal to the silver rod.
6. Connect the solutions in the two beakers by a salt bridge (For details about salt bridge, see the previous section,). A long strip of filter paper soaked in KCl solution can be used instead of the salt bridge.
7. Now observe the change in the voltmeter reading.
From the voltmeter, it is clear that electricity is produced in the experiment. Let us see the reason:
8. In the previous experiments we have seen that copper is more reactive than silver.
So Cu in the copper rod, loses two electrons and become. Cu2+. The equation can be written as:
Cu0 (s) ⟶ Cu+2 (aq) + 2e-1
• This is a oxidation reaction. Because the oxidation number of Cu increases from zero to +2.
• In other words:
Losing electrons is oxidation. So an oxidation reaction is taking place here.
■ The electrode at which oxidation takes place is called the anode.
9. The newly formed Cu+2 ions gets detached from the copper rod and goes into the solution. But the released electrons stick to the Cu rod
• Note that many Cu+2 ions are already present in the copper sulphate solution because, the aqueous copper sulphate solution exists as a mixture of Cu+2 ions and (SO4)-2 ions.
10. Because of the released electrons, the Cu rod becomes negatively charged. These free electrons reach the silver rod through the external circuit
11. These electrons which reach the silver rod flows through the silver rod and reaches the silver nitrate solution.
• In the silver nitrate solution, Ag+1 ions are present. These ions receive the electrons and become Ag atoms. The equation can be written as:
2Ag+1 (aq) + 2e-1 ⟶ 2Ag0 (s)
• This is a reduction reaction. Because the oxidation number of Ag+1 decreases from +1 to zero.
• In other words:
Gaining electrons is reduction. So a reduction reaction is taking place here.
• Note the coefficient '2' in front of Ag. This is because, each Cu atom will donate 2 electrons while each Ag atom will donate only one electron. So two Ag+1 ions can benefit from one Cu+2 ion
■ The electrode at which reduction takes place is called the cathode.
12. So both oxidation and reduction takes place in this reaction. Thus it is a redox reaction.
• The transfer of electrons produced by the redox reaction causes the flow of electric current.
• If a flow of electric current is obtained from a device, we can call it a cell.
• The cell which converts chemical energy to electrical energy through redox reaction is called Galvanic cell or voltaic cell.
• So we have made a galvanic cell using copper and silver electrodes. In the previous section, we made one with zinc and copper electrodes.
• Out of the two metals copper and silver, copper is more ready to donate electrons.
• So oxidation takes place at the copper rod.
• We have seen that, the electrode at which oxidation takes place is called the anode.
So we can say this:
■ In the galvanic cell, the more reactive metal will become the anode
The opposite can also be written:
■ In a galvanic cell, the less reactive metal will become the cathode
Fact 2:
• We have seen that the free electrons are first formed when the Cu atoms become Cu+2 ions.
• This happens at the anode.
■ Thus the electrons flow is from the anode to cathode
The above facts are shown in the fig.12.8 below:
■ We have seen two galvanic cells. At this stage, if we are given two metal rods, we are in a position to determine which will be the anode and which will be the cathode. Let us see some examples:
• We are given 3 metals: A, B and C
• With the 3 metals, 3 combinations are possible: AB, BC and CA. In each of the 3 combinations, the metal which is more reactive will be the anode and the other will be the cathode.
■ Let the 3 metals be Zn, Cu and Ag. Which are the possible combinations? Write the anode and cathode in each?
Solution:
■ There are 3 possible combinations. That is., we can make three different galvanic cells. They are:
(i) Zn-Cu (ii) Cu-Ag (iii) Zn-Ag
• Consider the first combination:
♦ Zn is more reactive. So it will be the anode
♦ Cu will be the cathode
Consider the second combination:
♦ Cu is more reactive. So it will be the anode
♦ Ag will be the cathode
Consider the third combination:
♦ Zn is more reactive. So it will be the anode
♦ Ag will be the cathode
♦ In a galvanic cell, anode is the negative electrode because the free electrons are first formed there. It is the source of electrons.
♦ In a galvanic cell, cathode is the positive electrode cell because the negatively charged electrons flows towards it
■ So we can write:
The flow of electrons is from the negative electrode (anode) to the positive electrode (cathode)
• But the flow of electric current is considered to be from the positive electrode to the negative electrode.
Why is that so?
• In earlier days when electricity was discovered, the the flow of current was assumed to be from the positive to the negative electrode. On the basis of this assumption, many rules were written down, and many equations were formulated. Later it was discovered that, electrons flow from negative electrode to positive electrode. But then, all rules and equations would have to be rewritten. To avoid such a difficulty, the direction of current is still assumed to be from positive to negative electrodes.
• We will get correct answers if we follow the 'same convention' in all the calculations
• That is., if we consider the convention: 'current flow to be from positive to negative'. We must apply it in all the calculations
• So the current flow which is from positive to negative is considered as 'conventional current'
• The electron flow from negative to positive is considered as 'electron current'
These details are shown in the fig.12.9 below:
• We use Dry cells in our day to day life as a source of electricity.
♦ The name 'Dry cell' indicates that it is dry. So there is no liquid components in it.
♦ Indeed, the electrolyte in it, is in the form of a paste. This will help to avoid spillage.
♦ Some images can be seen here.
• Mercury cells are also of this type. Images can be seen here.
♦ They are used in small appliances like watches, cameras etc.,
■ Dry cells and Mercury cells cannot be recharged. Cells which cannot be recharged are called primary cells
• Another type available in the market of is the nickel-cadmium cell. Images can be seen here.
♦ It can be recharged.
■ Cells which can be recharged are called secondary cells.
• Lithium ion cells (images here) used in mobile phones are also of this type
So we have completed this discussion on Galvanic cells. In the next section, we will see Electrolysis.
1. Take two beakers. Add 100 mL of 1 M copper sulphate solution into one beaker.
2. Add 100 mL of silver nitrate (AgNO3) solution into the other. See fig.12.7 below:
[We have seen the method to prepare 1 M solution of any given salt. See details here]
 |
| Fig.12.7 |
4. Immerse a silver rod in the silver nitrate solution.
5. Connect the negative terminal of a voltmeter to the copper rod. Connect the positive terminal to the silver rod.
6. Connect the solutions in the two beakers by a salt bridge (For details about salt bridge, see the previous section,). A long strip of filter paper soaked in KCl solution can be used instead of the salt bridge.
7. Now observe the change in the voltmeter reading.
From the voltmeter, it is clear that electricity is produced in the experiment. Let us see the reason:
8. In the previous experiments we have seen that copper is more reactive than silver.
So Cu in the copper rod, loses two electrons and become. Cu2+. The equation can be written as:
Cu0 (s) ⟶ Cu+2 (aq) + 2e-1
• This is a oxidation reaction. Because the oxidation number of Cu increases from zero to +2.
• In other words:
Losing electrons is oxidation. So an oxidation reaction is taking place here.
■ The electrode at which oxidation takes place is called the anode.
9. The newly formed Cu+2 ions gets detached from the copper rod and goes into the solution. But the released electrons stick to the Cu rod
• Note that many Cu+2 ions are already present in the copper sulphate solution because, the aqueous copper sulphate solution exists as a mixture of Cu+2 ions and (SO4)-2 ions.
10. Because of the released electrons, the Cu rod becomes negatively charged. These free electrons reach the silver rod through the external circuit
11. These electrons which reach the silver rod flows through the silver rod and reaches the silver nitrate solution.
• In the silver nitrate solution, Ag+1 ions are present. These ions receive the electrons and become Ag atoms. The equation can be written as:
2Ag+1 (aq) + 2e-1 ⟶ 2Ag0 (s)
• This is a reduction reaction. Because the oxidation number of Ag+1 decreases from +1 to zero.
• In other words:
Gaining electrons is reduction. So a reduction reaction is taking place here.
• Note the coefficient '2' in front of Ag. This is because, each Cu atom will donate 2 electrons while each Ag atom will donate only one electron. So two Ag+1 ions can benefit from one Cu+2 ion
■ The electrode at which reduction takes place is called the cathode.
12. So both oxidation and reduction takes place in this reaction. Thus it is a redox reaction.
• The transfer of electrons produced by the redox reaction causes the flow of electric current.
• If a flow of electric current is obtained from a device, we can call it a cell.
• The cell which converts chemical energy to electrical energy through redox reaction is called Galvanic cell or voltaic cell.
• So we have made a galvanic cell using copper and silver electrodes. In the previous section, we made one with zinc and copper electrodes.
In this cell also, we can see two interesting facts:
Fact 1:• Out of the two metals copper and silver, copper is more ready to donate electrons.
• So oxidation takes place at the copper rod.
• We have seen that, the electrode at which oxidation takes place is called the anode.
So we can say this:
■ In the galvanic cell, the more reactive metal will become the anode
The opposite can also be written:
■ In a galvanic cell, the less reactive metal will become the cathode
Fact 2:
• We have seen that the free electrons are first formed when the Cu atoms become Cu+2 ions.
• This happens at the anode.
■ Thus the electrons flow is from the anode to cathode
The above facts are shown in the fig.12.8 below:
• We are given 3 metals: A, B and C
• With the 3 metals, 3 combinations are possible: AB, BC and CA. In each of the 3 combinations, the metal which is more reactive will be the anode and the other will be the cathode.
■ Let the 3 metals be Zn, Cu and Ag. Which are the possible combinations? Write the anode and cathode in each?
Solution:
■ There are 3 possible combinations. That is., we can make three different galvanic cells. They are:
(i) Zn-Cu (ii) Cu-Ag (iii) Zn-Ag
• Consider the first combination:
♦ Zn is more reactive. So it will be the anode
♦ Cu will be the cathode
Consider the second combination:
♦ Cu is more reactive. So it will be the anode
♦ Ag will be the cathode
Consider the third combination:
♦ Zn is more reactive. So it will be the anode
♦ Ag will be the cathode
We have seen different galvanic cells. Now we will have a short discussion about the 'direction of current'
• In the discussions, we have seen that, the flow of electrons is from the anode to cathode.♦ In a galvanic cell, anode is the negative electrode because the free electrons are first formed there. It is the source of electrons.
♦ In a galvanic cell, cathode is the positive electrode cell because the negatively charged electrons flows towards it
■ So we can write:
The flow of electrons is from the negative electrode (anode) to the positive electrode (cathode)
• But the flow of electric current is considered to be from the positive electrode to the negative electrode.
Why is that so?
• In earlier days when electricity was discovered, the the flow of current was assumed to be from the positive to the negative electrode. On the basis of this assumption, many rules were written down, and many equations were formulated. Later it was discovered that, electrons flow from negative electrode to positive electrode. But then, all rules and equations would have to be rewritten. To avoid such a difficulty, the direction of current is still assumed to be from positive to negative electrodes.
• We will get correct answers if we follow the 'same convention' in all the calculations
• That is., if we consider the convention: 'current flow to be from positive to negative'. We must apply it in all the calculations
• So the current flow which is from positive to negative is considered as 'conventional current'
• The electron flow from negative to positive is considered as 'electron current'
These details are shown in the fig.12.9 below:
 |
| Fig.12.9 |
♦ The name 'Dry cell' indicates that it is dry. So there is no liquid components in it.
♦ Indeed, the electrolyte in it, is in the form of a paste. This will help to avoid spillage.
♦ Some images can be seen here.
• Mercury cells are also of this type. Images can be seen here.
♦ They are used in small appliances like watches, cameras etc.,
■ Dry cells and Mercury cells cannot be recharged. Cells which cannot be recharged are called primary cells
• Another type available in the market of is the nickel-cadmium cell. Images can be seen here.
♦ It can be recharged.
■ Cells which can be recharged are called secondary cells.
• Lithium ion cells (images here) used in mobile phones are also of this type
So we have completed this discussion on Galvanic cells. In the next section, we will see Electrolysis.
