In the previous section, we completed a discussion on Equilibrium of chemical reactions. In this chapter we will discuss about Reactivity series and Electrochemistry.
A few metals are given below:
Sodium, magnesium, iron and copper.
Let us consider the reaction of each of the above metals with water:
First we take sodium.
1. Take a little water in a test tube and two drops of phenolphthalein to it
2. Pour equal volume of kerosene to this mixture. See fig.12.1 below:
3. Put a small piece of sodium metal into it.
■ A reaction will occur. Note down the observations:
• Bubbles are formed on the surface of sodium
• When more bubbles are formed, the sodium piece goes up the kerosene and when all the bubbles break, it comes back to the water surface
• The water becomes pink in colour
■ Now we will see the explanation for each of the above observations:
1. Sodium and water reacts together according to the balanced equation given below:
2Na (s) + 2H2O(l) ⟶ 2NaOH (aq) + H2 (g)
• We can see that hydrogen gas is formed as one of the products. This hydrogen gas is the reason for the formation of bubbles.
2. When more bubbles are formed, they rise to the top surface of kerosene. At the top surface of kerosene, the bubbles come into contact with air. Then the bubbles break and the sodium falls back into the top surface of water.
3. We can see that NaOH is also formed as a product. This makes the water in the test tube alkaline. We have already added phenolphthalein as the indicator. So the water turns pink in colour.
• Sodium does not react with kerosene. In the above experiment, kerosene was used so that we can see the formation of gas bubbles clearly.
2. Take a little hot water in another test tube
3. Add one drop of phenolphthalein to each test tube
4. Put some magnesium in each test tube
Note down the observations:
• Bubbles come out of the test tube containing hot water. The water in this test tube becomes pink in colour
• There is no notable change in the test tube containing cold water
♦ But if we observe this test tube carefully for a long time, we will be able to see some bubbles come out slowly
■ Now we will see the explanation for the above observations:
1. Magnesium and water reacts according to the balanced equation given below:
Mg (s) + 2H2O(l) ⟶ Mg(OH)2 (aq) + H2 (g)
2. But for this reaction, a higher temperature is required. So the magnesium reacts readily with hot water. It reacts with cold water only slowly.
• So we see quicker formation of hydrogen gas bubbles in the test tube containing hot water
3. We can see that magnesium hydroxide is also a product. This makes the water in the test tube alkaline. We have already added phenolphthalein as the indicator. So the water turns pink in colour.
• Again note that, the pink colour for water is observed only in the test tube containing hot water.
• The answer is 'yes'. But we will need a very high temperature for this reaction. But at such high temperature, water will become steam. In fact, we need 'steam heated to high temperature'. It is called super heated steam. So we can say:
• Iron reacts with super heated steam. The balanced equation is:
3Fe (s) + 4H2O(g) ⟶ Fe3O4 (s) + 4H2 (g)
♦ Note that '(g)' is written after H2O. This indicates that water is in the gaseous form.
• This is because copper will never react with water. Not even with super heated steam.
• The given list is: Copper, Magnesium, Iron, Sodium
• The arrangement in the order of decreasing reactivity is:
Sodium, Magnesium, Iron, Copper
1. Cut a piece of sodium using a knife.
2. Observe the freshly cut portion.
• Within a short span of time, the shining lustre fades away.
Let us see the reason:
• Sodium reacts with oxygen in the atmospheric air to form sodium oxide. The balanced equation is:
4Na (s) + O2 (g) ⟶ 2Na2O (s)
• Sodium also reacts with water vapour in the atmospheric air and form sodium hydroxide. The balanced equation is:
2Na (s) + 2H2O(l) ⟶ 2NaOH (s) + H2 (g)
• The sodium hydroxide thus formed reacts with carbon dioxide and form sodium carbonate. The balanced equation is:
2NaOH (s) + CO2 (g) ⟶ Na2CO3 (aq) + H2 (g)
• So we see some new products: 2Na2O, 2NaOH, Na2CO3
• These new products form a layer over the freshly cut portion and thus the shining lustre fades away.
• It also loses it's lustre.
♦ But it will take a few days for this loss in lustre.
• That means, this reaction is slow.
• The given list is: Magnesium, Copper, Gold, Sodium and Aluminum
• The arrangement in the order of decreasing reactivity is:
Sodium, Magnesium, Aluminum, Copper, Gold
♦ Equal quantities of the reactants should be taken in all the test tubes
■ If the metals taken in the test tubes are:
Fe, Cu, Pb, Mg and zn, we will get the following result:
Mg> Zn> Fe> Pb> Cu
• That is., Mg will show the greatest reactivity with HCl and Cu will show the least reactivity
In the next section, we will see Reactivity Series.
A few metals are given below:
Sodium, magnesium, iron and copper.
Let us consider the reaction of each of the above metals with water:
First we take sodium.
1. Take a little water in a test tube and two drops of phenolphthalein to it
2. Pour equal volume of kerosene to this mixture. See fig.12.1 below:
 |
| Fig.12.1 |
■ A reaction will occur. Note down the observations:
• Bubbles are formed on the surface of sodium
• When more bubbles are formed, the sodium piece goes up the kerosene and when all the bubbles break, it comes back to the water surface
• The water becomes pink in colour
■ Now we will see the explanation for each of the above observations:
1. Sodium and water reacts together according to the balanced equation given below:
2Na (s) + 2H2O(l) ⟶ 2NaOH (aq) + H2 (g)
• We can see that hydrogen gas is formed as one of the products. This hydrogen gas is the reason for the formation of bubbles.
2. When more bubbles are formed, they rise to the top surface of kerosene. At the top surface of kerosene, the bubbles come into contact with air. Then the bubbles break and the sodium falls back into the top surface of water.
3. We can see that NaOH is also formed as a product. This makes the water in the test tube alkaline. We have already added phenolphthalein as the indicator. So the water turns pink in colour.
• Sodium does not react with kerosene. In the above experiment, kerosene was used so that we can see the formation of gas bubbles clearly.
■ From the experiment, it is clear that sodium reacts with water even if the water is at ordinary temperature.
Next we take magnesium.
1. Take a little cold water in one test tube 2. Take a little hot water in another test tube
3. Add one drop of phenolphthalein to each test tube
4. Put some magnesium in each test tube
Note down the observations:
• Bubbles come out of the test tube containing hot water. The water in this test tube becomes pink in colour
• There is no notable change in the test tube containing cold water
♦ But if we observe this test tube carefully for a long time, we will be able to see some bubbles come out slowly
■ Now we will see the explanation for the above observations:
1. Magnesium and water reacts according to the balanced equation given below:
Mg (s) + 2H2O(l) ⟶ Mg(OH)2 (aq) + H2 (g)
2. But for this reaction, a higher temperature is required. So the magnesium reacts readily with hot water. It reacts with cold water only slowly.
• So we see quicker formation of hydrogen gas bubbles in the test tube containing hot water
3. We can see that magnesium hydroxide is also a product. This makes the water in the test tube alkaline. We have already added phenolphthalein as the indicator. So the water turns pink in colour.
• Again note that, the pink colour for water is observed only in the test tube containing hot water.
Next we take iron.
• Reaction of iron with water is some what familiar to us. We know that rust is formed as a result. But this reaction is slow. Will a 'higher temperature' increase the speed of this reaction?• The answer is 'yes'. But we will need a very high temperature for this reaction. But at such high temperature, water will become steam. In fact, we need 'steam heated to high temperature'. It is called super heated steam. So we can say:
• Iron reacts with super heated steam. The balanced equation is:
3Fe (s) + 4H2O(g) ⟶ Fe3O4 (s) + 4H2 (g)
♦ Note that '(g)' is written after H2O. This indicates that water is in the gaseous form.
Finally we take copper from our list.
• But there is not much to write about the 'reaction between copper and water'.• This is because copper will never react with water. Not even with super heated steam.
■ So we see that some metals react quickly with water. While some others react slowly. And some others does not react at all.
• If we are given a list of metals, we can arrange them in a 'decreasing order of reactivity' with water. Let us see an example:• The given list is: Copper, Magnesium, Iron, Sodium
• The arrangement in the order of decreasing reactivity is:
Sodium, Magnesium, Iron, Copper
Now we will consider the reaction between metals and atmospheric air
First we take sodium. 1. Cut a piece of sodium using a knife.
2. Observe the freshly cut portion.
• Within a short span of time, the shining lustre fades away.
Let us see the reason:
• Sodium reacts with oxygen in the atmospheric air to form sodium oxide. The balanced equation is:
4Na (s) + O2 (g) ⟶ 2Na2O (s)
• Sodium also reacts with water vapour in the atmospheric air and form sodium hydroxide. The balanced equation is:
2Na (s) + 2H2O(l) ⟶ 2NaOH (s) + H2 (g)
• The sodium hydroxide thus formed reacts with carbon dioxide and form sodium carbonate. The balanced equation is:
2NaOH (s) + CO2 (g) ⟶ Na2CO3 (aq) + H2 (g)
• So we see some new products: 2Na2O, 2NaOH, Na2CO3
• These new products form a layer over the freshly cut portion and thus the shining lustre fades away.
Next we take magnesium
• Magnesium also reacts with atmospheric air. • It also loses it's lustre.
♦ But it will take a few days for this loss in lustre.
• That means, this reaction is slow.
Next we take aluminum.
• It too loses it's lustre.
• But it takes even more time than magnesium.
• Next we take copper.
• It too loses it's lustre.
• But it takes even more time than aluminum.
♦ In fact, it will takes months for the formation of verdigris on the surface of copper.
Finally we take gold. Gold does not react with atmospheric air at all.
• So it is clear that, like the reaction between 'metals and water', reaction between 'metals and atmospheric air' also varies.
■ Some metals react quickly with atmospheric air. While some others react slowly. And some others does not react at all.
• If we are given a list of metals, we can arrange them in a 'decreasing order of reactivity' with air. Let us see an example:• The given list is: Magnesium, Copper, Gold, Sodium and Aluminum
• The arrangement in the order of decreasing reactivity is:
Sodium, Magnesium, Aluminum, Copper, Gold
Now we will consider the reaction between metals and hydrochloric acid (HCl).
• We know that some metals react with acids like HCl and H2SO4 and produce hydrogen gas. We have seen the reaction between zinc and hydrochloric acid here. The balanced equation is:
Zn + 2HCl ⟶ ZnCl2 + H2
• In our present discussion we want to know the 'reactivity of various metals with HCl'.
• We can get a general idea about the reactivity by observing the 'quantity of bubbles' formed. These bubbles are formed due to the production of hydrogen.
♦ If more bubbles are formed, we can say that that metal has a greater reactivity with HCl.
• The fig.12.2 below shows such an experiment. In each test tube, a small metal piece is put in HCl. Different metals are put in different test tubes.
 |
| Fig.12.2 |
• We can say that:
♦ The metal in test tube no.2 has a greater reactivity with HCl
♦ The metal in test tube no.4 has very low reactivity with HCl
♦ The metal in test tube no.3 has no reactivity at all with HCl
• It is important to ensure that the following two conditions are satisfied while doing the experiment in fig.12.2:
♦ The 'quantity of bubbles' formed should be observed within the 'same time span' in all the test tubes♦ Equal quantities of the reactants should be taken in all the test tubes
■ If the metals taken in the test tubes are:
Fe, Cu, Pb, Mg and zn, we will get the following result:
Mg> Zn> Fe> Pb> Cu
• That is., Mg will show the greatest reactivity with HCl and Cu will show the least reactivity
In the next section, we will see Reactivity Series.
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