In the previous section, we saw reason for the displacement of copper from a solution of copper sulphate. In this section, we will see a similar case.
• In a solution of silver nitrate (AgNO3),we introduce copper.
♦ It is convenient to use copper wire available in the form of a coil. In this way, there will be more copper within a less space.
• We know that copper occupies a higher position than silver in the reactivity series. Then what will be the result?
Ans: The copper has a greater tendency to give off electrons. So it will become Cu2+ ions.
• The silver, which is present as Ag1+ ions will receive these electrons and will become Ag atoms. Thus we will get pure silver.
♦ Note that silver ions (Ag1+) have only one positive charge.
♦ So two silver ions can benefit from one copper atom.
♦ Because each copper atom will give donate two elecrons and become Cu2+.
• This is a displacement reaction. Because, silver has been displaced from it's solution.
Note down the observations:
1. Initially the copper wire has it's own reddish brown appearance.
• But after the experiment, the wire is covered with deposits of silver
2. Initially, the solution is colourless.
• But after the experiment, it's colour changes to blue
■ Let us see the explanation for the above two observations:
• When the copper wire is dipped in the solution, a reaction takes place. The equation for this reaction is:
Cu (s) + 2AgNO3 (aq) ⟶ Cu(NO3)2 (aq) + 2Ag (s)
• We can see that pure silver (Ag) is formed as one of the products.
• This is because, the Cu atoms, which is more reactive than Ag, donated electrons and became Cu2+ ions. These electrons were received by the Ag1+ ions, and they became Ag atoms.
• The newly formed Ag atoms stick to the surface of copper wire. This is shown in the enlarged view inside the yellow square in fig.12.4 above. The enlarged view shows what is actually happening in a small square portion on the surface of the copper wire
• The formation of the blue colour of the solution after the experiment is due to the presence of Cu2+ ions.
• A video of the experiment can be seen here.
• It is interesting to note that, the deposited silver can be separated from the copper wire, just by shaking the wire
♦ oxidation number of Cu increased from 0 to +2
♦ oxidation number of Ag decreased from +1 to 0
• So we can write:
♦ Cu is oxidized
★ In other words, Cu which was in it's pure form, is oxidized to a positive ion
♦ Ag is reduced
★ In other words, positive Ag ion is reduced to pure silver
■ In this reaction, oxidation and reduction take place simultaneously. It is a redox reaction.
■ Note the following points:
• Copper is more reactive than silver. Copper is situated above silver in the reactivity series
• As a result of the reaction:
♦ Cu donated electrons and got oxidized
♦ Ag received electrons and got reduced
• Magnesium sulphate • Copper sulphate • Zinc sulphate • Ferrous sulphate • Silver nitrate
■ Also we are given the following metals in their pure forms:
• Magnesium • Copper • Zinc • Iron • Silver
■ Let us now try all the possible combinations:
A. Magnesium sulphate solution
1. Magnesium:
If we dip pure magnesium in magnesium sulphate solution, no reaction will take place. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to magnesium
2. Copper:
If we dip pure copper in magnesium sulphate solution, displacement reaction will not take place. This is because, copper is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to copper
3. Zinc:
If we dip pure zinc in magnesium sulphate solution, displacement reaction will not take place. This is because, zinc is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to zinc
4. Iron:
If we dip pure iron in magnesium sulphate solution, displacement reaction will not take place. This is because, iron is is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to iron
5. Silver: If we dip pure silver in magnesium sulphate solution, displacement reaction will not take place. This is because, silver is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to silver
B. Copper sulphate solution
1. Magnesium:
If we dip pure magnesium in copper sulphate solution, displacement reaction will take place. Magnesium will displace copper from the solution. This displaced copper will get deposited. This is because, magnesium is more reactive than copper. Since the displacement reaction is possible, we put a '✔' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to magnesium
2. Copper:
If we dip pure copper in copper sulphate solution, no reaction will take place. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to copper
3. Zinc:
If we dip pure zinc in copper sulphate solution, displacement reaction will take place. Zinc will displace copper from the solution. This displaced copper will get deposited. This is because, zinc is more reactive than copper. Since the displacement reaction is possible, we put a '✔' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to zinc
In the above table, we find that:
• Mg has 4 '✔' marks
• Zn has 3 '✔' marks
• Iron has 2 '✔' marks
• Copper has 1 '✔' mark
• Silver has 0 '✔' mark
■ So we can arrange them in the decreasing order of reactivity:
Mg > Zn > Fe > Cu > Ag
■ This agrees with the order in the actual reactivity series that we saw in the previous section
In the next section, we will see how metals can be used to produce electricity.
• In a solution of silver nitrate (AgNO3),we introduce copper.
♦ It is convenient to use copper wire available in the form of a coil. In this way, there will be more copper within a less space.
• We know that copper occupies a higher position than silver in the reactivity series. Then what will be the result?
Ans: The copper has a greater tendency to give off electrons. So it will become Cu2+ ions.
• The silver, which is present as Ag1+ ions will receive these electrons and will become Ag atoms. Thus we will get pure silver.
♦ Note that silver ions (Ag1+) have only one positive charge.
♦ So two silver ions can benefit from one copper atom.
♦ Because each copper atom will give donate two elecrons and become Cu2+.
• This is a displacement reaction. Because, silver has been displaced from it's solution.
Now we will see an experiment to demonstrate the above findings.
• Prepare some AgNO3 solution in a beaker. Dip a copper wire in it. See fig.12.4 below: |
| Fig.12.4 |
1. Initially the copper wire has it's own reddish brown appearance.
• But after the experiment, the wire is covered with deposits of silver
2. Initially, the solution is colourless.
• But after the experiment, it's colour changes to blue
■ Let us see the explanation for the above two observations:
• When the copper wire is dipped in the solution, a reaction takes place. The equation for this reaction is:
Cu (s) + 2AgNO3 (aq) ⟶ Cu(NO3)2 (aq) + 2Ag (s)
• We can see that pure silver (Ag) is formed as one of the products.
• This is because, the Cu atoms, which is more reactive than Ag, donated electrons and became Cu2+ ions. These electrons were received by the Ag1+ ions, and they became Ag atoms.
• The newly formed Ag atoms stick to the surface of copper wire. This is shown in the enlarged view inside the yellow square in fig.12.4 above. The enlarged view shows what is actually happening in a small square portion on the surface of the copper wire
• The formation of the blue colour of the solution after the experiment is due to the presence of Cu2+ ions.
• A video of the experiment can be seen here.
• It is interesting to note that, the deposited silver can be separated from the copper wire, just by shaking the wire
• We must analyse the above reaction in terms of Oxidation and Reduction
• We have learned about oxidation and reduction in a previous lesson. Details here.
• Consider the equation again:
Cu (s) + 2AgNO3 (aq) ⟶ Cu(NO3)2 (aq) + 2Ag (s)
Cu (s) + 2AgNO3 (aq) ⟶ Cu(NO3)2 (aq) + 2Ag (s)
We will write the oxidation state of each component on the reactants side as well as the products side.
Reactants:
1. Cu is an atom of the element. So it's oxidation number will be obviously zero. We can write: Cu0
2. AgNO3 is an ionic compound. Let us write it as a combination: (Ag)(NO3)
• The nitrate ion (NO3)-1 has a known oxidation number of -1
• So the oxidation number of Ag in AgNO3 must be +1
• Thus we can write: Ag+1NO3-1
3. Cu(NO3)2 is similar to AgNO3
• Cu(NO3)2 is an ionic compound. Let us write it as a combination: (Cu)[(NO3)2]
• The nitrate ion (NO3)-1 has a known oxidation number of -1
• So there must be two (NO3)-1 groups to compensate for the +2 oxidation number of Cu
• Thus we can write: Cu+2(NO3)2-1
4. Ag is an atom of the element. So it's oxidation number will be obviously zero. We can write: Ag0
• So the equation can be written as:
Cu0 (s) + 2(Ag+1NO3-1) ⟶ Cu+2(NO3)2-1 + Ag0
• In the above equation, we can see that:♦ oxidation number of Cu increased from 0 to +2
♦ oxidation number of Ag decreased from +1 to 0
• So we can write:
♦ Cu is oxidized
★ In other words, Cu which was in it's pure form, is oxidized to a positive ion
♦ Ag is reduced
★ In other words, positive Ag ion is reduced to pure silver
■ In this reaction, oxidation and reduction take place simultaneously. It is a redox reaction.
■ Note the following points:
• Copper is more reactive than silver. Copper is situated above silver in the reactivity series
• As a result of the reaction:
♦ Cu donated electrons and got oxidized
♦ Ag received electrons and got reduced
Now we are in a position to tell whether a displacement reaction will take place or not. See the example below:
■ We are given the following solutions:• Magnesium sulphate • Copper sulphate • Zinc sulphate • Ferrous sulphate • Silver nitrate
■ Also we are given the following metals in their pure forms:
• Magnesium • Copper • Zinc • Iron • Silver
■ Let us now try all the possible combinations:
A. Magnesium sulphate solution
1. Magnesium:
If we dip pure magnesium in magnesium sulphate solution, no reaction will take place. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to magnesium
2. Copper:
If we dip pure copper in magnesium sulphate solution, displacement reaction will not take place. This is because, copper is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to copper
3. Zinc:
If we dip pure zinc in magnesium sulphate solution, displacement reaction will not take place. This is because, zinc is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to zinc
4. Iron:
If we dip pure iron in magnesium sulphate solution, displacement reaction will not take place. This is because, iron is is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to iron
5. Silver: If we dip pure silver in magnesium sulphate solution, displacement reaction will not take place. This is because, silver is less reactive than magnesium. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to magnesium sulphate
• Column corresponding to silver
B. Copper sulphate solution
1. Magnesium:
If we dip pure magnesium in copper sulphate solution, displacement reaction will take place. Magnesium will displace copper from the solution. This displaced copper will get deposited. This is because, magnesium is more reactive than copper. Since the displacement reaction is possible, we put a '✔' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to magnesium
2. Copper:
If we dip pure copper in copper sulphate solution, no reaction will take place. Since the displacement reaction is not possible, we put a '✘' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to copper
3. Zinc:
If we dip pure zinc in copper sulphate solution, displacement reaction will take place. Zinc will displace copper from the solution. This displaced copper will get deposited. This is because, zinc is more reactive than copper. Since the displacement reaction is possible, we put a '✔' mark in the table 12.1 given below. This mark is put at the intersection of:
• Row corresponding to copper sulphate
• Column corresponding to zinc
In this way appropriate marks can be put filled up in the table as shown below:
Table 12.1| Mg | Cu | Zn | Fe | Ag | |
|---|---|---|---|---|---|
| Magnesium sulphate |
✘
|
✘ | ✘ | ✘ | ✘ |
| Copper sulphate | ✔ | ✘ | ✔ | ✔ | ✘ |
| Zinc sulphate | ✔ | ✘ | ✘ | ✘ | ✘ |
| Ferrous sulphate | ✔ | ✘ | ✔ | ✘ | ✘ |
| Silver nitrate | ✔ | ✔ | ✔ | ✔ | ✘ |
• Mg has 4 '✔' marks
• Zn has 3 '✔' marks
• Iron has 2 '✔' marks
• Copper has 1 '✔' mark
• Silver has 0 '✔' mark
■ So we can arrange them in the decreasing order of reactivity:
Mg > Zn > Fe > Cu > Ag
■ This agrees with the order in the actual reactivity series that we saw in the previous section
In the next section, we will see how metals can be used to produce electricity.
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