Chapter 12.3 - The Zinc - Copper Galvanic cell

In the previous section, we saw reason for the displacement of silver from a solution of silver nitrate. In this section, we will see how metals can be used to make electricity.

1. Take two beakers. Add 100 mL of 1 M zinc sulphate solution into one beaker. 
2. Add 100 mL of copper sulphate solution into the other. See fig.12.5 below:
[We have seen the method to prepare 1 M solution of any given salt. See details here]

Fig.12.5

3. Immerse a zinc rod in the zinc sulphate solution. 
4. Immerse a copper rod in the copper sulphate solution. 
5. Connect the negative terminal of a voltmeter to the zinc rod. Connect the positive terminal to the copper rod. 
6. Connect the solutions in the two beakers by a salt bridge. A long strip of filter paper soaked in KCl solution can be used instead of the salt bridge.

■ Method of making a salt bridge: • Make a paste by mixing two items: (i) gelatin (ii) Potassium chloride (KCl)     ♦ agar agar gel can be used instead of gelatin     ♦ Potassium nitrate (KNO3) or Ammonium chloride (NH4Cl) can be used instead of KCl • Fill this paste into an U-tube • Close the ends of the tube with cotton balls • The salt bridge is ready to use. We will see it's function later in this section

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 zinc is more reactive than copper. 
So Zn in the zinc rod, loses two electrons and become. Zn²⁺. The equation can be written as:
Zn⁰ (s) ⟶ Zn⁺² (aq) + 2e^-1
• This is a oxidation reaction. Because the oxidation number of Zn 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 Zn⁺² ions gets detached from the zinc rod and goes into the solution. But the released electrons stick to the Zn rod
• Note that many Zn⁺² ions are already present in the zinc sulphate solution because, the aqueous zinc sulphate solution exists as a mixture of Zn⁺² ions and (SO₄)^-2 ions.
10. Because of the released electrons, the Zn rod becomes negatively charged. These free electrons reach the copper rod through the external circuit
11. These electrons which reach the copper rod flows through the copper rod and reaches the copper sulphate solution. 
• In the copper solution, Cu⁺² ions are present. These ions receive the electrons and become Cu atoms. The equation can be written as:
Cu⁺² (aq) + 2e^-1 ⟶ Cu⁰ (s)
• This is a reduction reaction. Because the oxidation number of Cu⁺² decreases from +2 to zero.
• In other words:
Gaining electrons is reduction. So a reduction reaction is taking place here.
■ 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.
• They are named after the scientists Luigi Galvani and Alessandro Volta who achieved early developments in such cells.

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In such cells, we can see two interesting facts:

Fact 1:
• Out of the two metals zinc and copper, zinc is more ready to donate electrons. 
• So oxidation takes place at the zinc 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 Zn atoms become Zn⁺² ions. 
• This happens at the anode. 
■ Thus in a galvanic cell, the electrons flow is from the anode to cathode
The above facts are shown in the fig.12.6 below: 

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Corrosion of metals

At this stage, we can have a short discussion about the basics of 'corrosion of metals'. 

• We have seen that the zinc atoms become Zn⁺² ions and leave the zinc rod. If the process continues, the zinc rod will soon become useless.

• The zinc atoms left the rod because favorable conditions were available on the surface of the zinc rod.

    ♦ We provided those favorable conditions by connecting it with a copper rod, providing zinc sulphate and copper sulphate solutions etc.,

• If such favorable conditions occur naturally around the surface of any metal, it's atoms would surely leave. 

• For example, the oxygen and water vapour present in the atmospheric air can provide favorable conditions for certain metals. 

• The atoms of those metals will then become ions and will leave the original metal surface. 
• The released electrons will be received by oxygen. The oxygen become negative ions. 
• The 'positive metal ions' and 'negative oxygen ions' together will form a new compound. The rust which we see on the surface of iron is such a compound.           

■ The process of conversion of a metal into it's compounds by continuous interaction with atmospheric air and water vapour is termed corrosion of metals. This is an electrochemical reaction. 
• Metals like potassium, sodium, zinc, copper etc., also undergo corrosion.

Cathodic protection

• In the above discussion, we have seen that the zinc which is more reactive than copper has become the anode.
    ♦ Atoms of the anode loses electrons and become ions. 
• If in the place of copper, we use a metal which is more reactive than zinc, then that metal will become anode. 
    ♦ Zinc will become the cathode and will be saved. 
• So we must make the metal to be protected, the cathode
• This is the basis of cathodic protection.
■ Cathodic protection is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell. In the simplest form, the metal to be protected is connected to a more easily corroded 'sacrificial metal' to act as the anode
• We will see more technical details of this method in higher classes

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Now we will discuss about the salt bridge
• Consider the cathode compartment in the above galvanic cell. 
    ♦ We have a copper rod in a copper sulphate solution. 
• The copper sulphate solution always exists as a mixture of Cu⁺² and (SO₄)^-2 ions. 
• We have seen that, the electrons reaching the copper rod will be received by the Cu⁺² ions, and they will become Cu atoms. 
• So the positive charges in the solution will continuously decrease. In other words, there will be an excess negative charge (due to the (SO₄)^-2 ions) in the solution. 
• So these ions flow from the copper compartment to the zinc compartment through the salt bridge.
• This will help to maintain the electrical neutrality of the solution

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So we have completed this discussion on Galvanic cell using Zinc and copper. In the next section, we will see another Galvanic cell. 

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Chapter 5.1 - Reaction of Oxygen with Metals

In the previous section, we saw some reactions of oxygen with non-metals. In this section we will see some reactions of oxygen with metals.

A freshly cut surface of metals like aluminium and iron will have a metallic lustre. But this lustre will fade away gradually. This is due to the formation of a new substance on the surface. This new substance is formed because of the reaction of the metal with oxygen.

Reaction with Aluminium:
Aluminium reacts with the oxygen in the atmosphere, and forms aluminium oxide. Let us write the equation:

• One molecule of aluminium is Al. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is Al₂O₃. We will write it on the right side

• So the skeletal equation is Al + O₂ → Al₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

So the balanced equation is: 4Al + 3O₂ → 2Al₂O₃

Reaction with Iron:

Iron reacts with the oxygen in the atmosphere, and forms iron oxide. Let us write the equation:

• One molecule of iron is Fe. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of iron oxide is Fe₂O₃. We will write it on the right side

• So the skeletal equation is Fe + O₂ → Fe₂O₃ . This is not a balanced equation

• The steps for balancing the equation are given below:

• So the balanced equation is: 4Fe + 3O₂ → 2Fe₂O₃

• The iron oxide is commonly known as 'rust'

Reaction with magnesium:

When magnesium burns, it reacts with oxygen. We can do a simple experiment to demonstrate this. [Note that all experiments should be done only under the supervision of teachers. All safety precautions should be taken. In the following experiment, avoid looking directly at the flash]

1. Take a magnesium ribbon about 2 cm long. Clean it's surface using sand paper. This is to remove any impurities. Because we want the reaction to take place directly between magnesium and oxygen. We do not want any impurities in between.

2. Hold the ribbon with a pair of tongs. Burn it using a spirit lamp or burner. Note that it should be burnt while keeping it as far away as possible from the eyes.

3. The ribbon will burn with bright white light. Do not look directly at the light. When the burning is complete, we will see that the ribbon has changed into a white powder. This powder is magnesium oxide.

• One molecule of magnesium is Mg. One molecule of oxygen is O₂. We will write each of them on the left side

• One molecule of aluminium oxide is MgO. We will write it on the right side

• So the skeletal equation is Mg + O₂ → MgO . This is not a balanced equation.

• The steps for balancing the equation are given below:

So the balanced equation is: 2Mg + O₂ → 2MgO

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We have seen some of the reactions of oxygen with metals as well as non-metals. Next we will discuss about Ozone layer.

■ Oxygen (O₂) is found as a diatomic molecule. That is., each molecule of oxygen will have two atoms of oxygen. 

■ But some times, three atoms of oxygen will combine together to form a single molecule. That molecule is entirely different from oxygen in physical and chemical properties. It is called Ozone.

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■ Some elements are found in two or more physical forms. This is called allotropy. The different physical forms are called allotropes of the element. Example:

• Diamond and graphite are two allotropes of carbon.

• Oxygen (O₂) and Ozone (O₃) are two allotropes of oxygen

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Ozone is present mostly in the stratosphere of the atmosphere. Let us see the steps in the formation of Ozone:

1. The ordinary oxygen (O₂) molecules absorbs high energy ultraviolet radiation, and dissociates into individual oxygen atoms.

2. These individual oxygen atoms combine together to form molecules of ozone (O₃).

3. Ozone molecules so formed, absorb low energy ultraviolet radiations and decompose into oxygen molecules (O₂).

4. The O₂ molecules so formed will again undergo the process in step (1)

■ So we see that it is a cyclic process. The cycle is shown in the fig.5.2 below:

Fig.5.2

Let us see the important points to be noted in the cycle:

• In (1), the O₂ absorbs high energy ultraviolet radiation. But in (3), the O₃ absorbs low energy ultraviolet radiation.

• In (1), the O₂ dissociates into individual atoms. But in (3), the O₃ decomposes into O₂.

Importance of the ozone cycle:

We see that ultraviolet radiations are absorbed during the cyclic process. As a result, those harmful radiations do not reach the surface of the earth. Such radiations can cause diseases like cancer. So the ozone layer is very important for sustaining life on earth.

Ozone layer depletion

The proper functioning of Ozone layer is under threat. Let us see why:

• Compounds known as CFC or chlorofluorocarbons, are special compounds that contain chlorine, fluorine and carbon. 

• They have a special property: They are normally in the gaseous state. But they can be easily brought into liquid state by applying pressure. 

• Once they are in the liquid state, they absorb heat from the surroundings. Using this heat, they evaporate and gets converted back into gaseous state. 

• When they absorb heat, the surroundings are cooled. So they are used in refrigerators and air conditioners.

• After long periods of usage, the appliances like refrigerators, air conditioners etc., become worn out. They are then abandoned. 

• The CFC in them will escape into the atmosphere. They reach the stratosphere. 

• The molecules of CFC will break down under the action of ultraviolet radiations. When they break down, individual chlorine atoms become available. 

• These chlorine atoms react with ozone molecules. This reaction will cause the decomposition of ozone molecules. The ozone molecules thus decomposed will be out of the ozone cycle. 

• So if more CFC escapes into the atmosphere, more chlorine will become available, and so more molecules of ozone will be decomposed. Gaps will begin to appear in the ozone layer. This is known as ozone layer depletion.

•When more molecules of ozone is decomposed, only a lesser quantity of ozone will remain to take part in the cycle. This remaining ozone will not be able to absorb all the harmful radiations coming from the sun. As a result, much of those radiations will reach the surface of the earth.

■ In order to create awareness about the need to preserve the ozone layer, September 16^th is celebrated as International Ozone Day. 

■ Now a days many countries have introduced strict rules to avoid using harmful CFC. Safer substances should be used instead. We must all abide by the rules so that, the ozone layer can be restored back to it's original state.

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We have completed our present discussion on oxygen. In the next section, we will discuss about Nitrogen. 

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