Chapter 2 Element Compound And Chemical Reaction Chemical Effects Of Electricity Long Answer Type Questions
Question 1. Describe briefly the process of electrolysis of molten sodium chloride, using suitable electrodes.
Answer:
The process of electrolysis of molten sodium chloride, using suitable electrodes:
When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode, NaCI is electrolyzed.
Na+ ions move towards the cathode and negatively charged Cl– ions move towards the anode. At the cathode, Na+ ions take up electrons and are converted to sodium metal.
At the anode, Cl– ions give up electrons and are ultimately converted to chlorine gas (Cl2) Electrons are taken up by Na+ ions at the cathode.
So reduction occurs at the cathode. Oxidation occurs at the anode because the electron is given up by the Cl2 ion. So sodium metal is produced at the cathode and chlorine gas is produced at the anode.
Electrode reaction: At cathode (Reduction) : \(\mathrm{Na}^{+}+e \rightarrow \mathrm{Na}\)
At anode (Oxidation): \(2 \mathrm{Cl}^{-}-2 e \rightarrow \mathrm{Cl}_2 (gas)\)
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Question 2. Describe briefly the process of electrolysis of acidified water, using suitable electrodes.
Answer:
Electrolysis Of Sodium Chloride:
Let us now discuss what happens when fused sodium chloride is electrolyzed using the above setup,
We know that Aien NaCI is dissolved in water, it is completely dissociated into Na+ and Cl– ions and they move freely within the solution. In molten or fused states also Na+ and ions are produced which move freely within the molten state.
⇒ \(\mathrm{NaCl} \rightleftharpoons \mathrm{Na}^{+}+\mathrm{Cl}^{-}\)
When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode,
Na+ ions (which are positively charged) move towards the cathode (connected to the negative terminal of the battery) and negatively charged Cl– ions move towards the anode (connected to the positive terminal of the battery).
At the cathode, Na+ ions take up electrons and are converted to sodium metal. At the anode, Cl– ions give up their “extra” electrons and are ultimately converted to chlorine gas (Cl2).
Electrons are taken up by Na+ ions at the cathode. So reduction occurs at the cathode while oxidation occurs at the anode because the electron is given up by the Cl– ion. Hence sodium metal is produced at the cathode and chlorine gas is produced at the anode.
The electrolysis of molten or fused sodium chloride can be summarized in the following table.
| Electrolysis of molten or fused sodium chloride | |
| Electrolyte | Molten or fused sodium chloride (NaCI) |
| Electrode | Anode: graphite Cathode: Iron |
| Electrode reaction : | |
| At the cathode (Reduction): | \(\mathrm{Na}^{+}+\mathrm{e} \rightarrow \mathrm{Na}\) |
| At anode (Oxidation): | \(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \text { (gas }\) |
Question 3. The phenomenon of electrolysis is actually a phenomenon of oxidation reduction-explain with a suitable example. Or “During electrolysis, reduction occurs at the cathode and oxidation occurs at anode”. Explain with a suitable example.
Answer:
Let us describe it with the help of electrolysis of molten sodium chloride.
When electricity is passed through the molten or fused sodium chloride through graphite anode and iron cathode, positively Na+ ions solution of the chloride salts of No, Mg or Ca is electrolyzed, and the metals cannot be extracted.
moves towards the cathode and negatively charged Cl– ions move towards the anode. At the cathode, Na+ ions take up electrons and are converted to sodium metal.
At the anode, Cl– ions give up the electron and are ultimately converted to chlorine gas (Cl2). So sodium metal is produced at the cathode and chlorine gas is produced at the anode.
Electrons are taken up by Na+ ions at the cathode. So reduction occurs at the cathode. Oxidation occurs at the anode because the electron is given up by the Cl– ion.
Electrode reaction:At cathode (Reduction) : \(\mathrm{Na}^{+}+e \rightarrow \mathrm{Na}\)
At anode (Oxidation) : \(2 \mathrm{Cl}-2 e \rightarrow \mathrm{Cl}, (gas)\)
Question 4. Discuss the electro-refining of metal with a suitable example.
Answer:
Purification or electro-refining of metals:
Electrolysis is used in the purification of impure metals that are extracted from their ores. In this process:
A thick rod of impure metal is made the anode. It is connected to the positive terminal of the battery.
A thin strip of pure metal is made of the cathode. It is connected to the negative terminal of the battery.
A water-soluble salt of the metal to be purified is taken as an electrolyte.
On passing an electric current, the metal dissolves from the impure anode and goes into the electrolyte solution. The metal present in dissolved form in the electrolyte gets deposited on the cathode in the pure form.
The impurities are left behind in the electrolyte solution. Metals like copper, zinc and aluminium etc are purified by electrolysis.
Purification of copper may be taken up as a typical example to discuss electro-refining. Copper is obtained from copper ores which contain various impurities.
Hence when copper is extracted from the ores, it contains various impurities. The metallic copper with impurities is not suitable for various uses such as in electrical appliances such as copper wire.
So, impure copper is to be purified by electrolysis. The impure metal in the form of thick blocks is used as an anode and the pure metal in the form of a thin sheet is used as a cathode.
An acidified copper sulphate solution is taken as an electrolyte. On carrying out electrolysis, copper atoms dissolve out as Cu2+ ions from the anode and are deposited at the cathode as metallic copper.
So cathode now consists of pure copper. As the process continues more and more copper dissolves from the anode and is deposited as pure copper on the anode. This process is known as the electro-refining of metals.
| Electro-refining of copper | |
| Electrolyte | Acidified copper sulphate solution (CuSO4)\(\left[15 \% \mathrm{CuSO}_4+5 \% \mathrm{H}_2 \mathrm{SO}_4\right]\) |
| Electrode | Anode: copper bar containing impurities Cathode: a thin sheet of pure copper |
| Electrode reaction : | |
| At anode (oxidation) : | \(\mathrm{Cu} \rightarrow \mathrm{Cu}^{2+}+2 e (pure copper comes into) solution as \left(\mathrm{Cu}_2+\right. ion )\) |
| At the cathode (reduction): | \(\mathrm{Cu}^{2+}+2 e \rightarrow \mathrm{Cu} (copper metal is deposited on the cathode)\) |
Question 5. Describe with a suitable example, the extraction of a metal from its halide salt.
Answer:
The extraction of a metal from its halide salt:
This can be illustrated by the extraction of calcium metal from calcium chloride. In this process, molten calcium chloride is taken in a graphite container.
The walls of the graphite container act as an anode while an iron electrode is immersed partially in the molten salt and acts as a cathode. Calcium metal is formed at the cathode and chlorine gas is liberated at the anode.
| Extraction of Calcium by Electrolysis of molten or fused calcium chloride | |
| Electrolyte | Molten or fused magnesium chloride (CaCl2) |
| Electrode | Anode: graphite Cathode: iron |
| Electrode reaction: | |
| At the cathode (Reduction) : | \(\mathrm{Ca}^{2+}+2 e \rightarrow \mathrm{Ca}\) |
| At anode (Oxidation) : | \(2 \mathrm{Cl}-2 e \rightarrow \mathrm{Cl}_2 \text { (gas) }\) |
(In the industrial process, calcium chloride is mixed with calcium fluoride. This lowers the melting point of calcium chloride. The molten electrolyte is heated at 700°C and at this temperature electrolysis is carried out.)
Question 6. Describe briefly the process of electroplating copper on iron nails.
Answer:
The process of electroplating copper on iron nails:
During the electroplating of copper on iron nails, the pure copper metal is used as anode and the iron nails (i.e., the objects which are to be electroplated) are used as cathode.
An acidified copper sulphate solution is taken as an electrolyte. On carrying out electrolysis, copper atoms gradually dissolve out as Cu2+ ions from the anode and are deposited at the cathode as metallic copper.
As the process continues more and more copper dissolves from the anode and is deposited as pure copper on the iron nails (which are used as the anode). So after some time, the iron nails are coated with a thin layer of copper.
| Electro-refining of copper on iron nails | |
| Electrolyte | Acidified copper sulphate solution (CuS04) |
| Electrode | Anode: pure copper metal Cathode: iron nails |
| Electrode reaction: | |
| At anode (oxidation) : | \(\mathrm{Cu} \rightarrow \mathrm{Cu}^{2+}+2 e (pure copper comes into) solution as \left(\mathrm{Cu}_2+\right. ion )\) |
| At the cathode (reduction): | \(\mathrm{Cu}^{2+}+2 e \rightarrow \mathrm{Cu} (copper metal is deposited on the cathode)\) |
Question 7. Describe briefly the electroplating of different metals on various objects familiar to us.
Answer:
The electroplating of different metals on various objects familiar to us:
The various objects familiar to us can be electroplated by various metals for various purposes such as to protect from rusting, to improve the external appearance, to enhance commercial value, etc.
In the case of electroplating, the metal which is to be electroplated is used as an anode and the article on which the coating is given is used as the cathode, and a suitable salt solution is used as an electrolyte.
During electroplating, the pure metal from the anode is gradually dissolved as metal ions into the solution and the metal ions are deposited on the cathode material in the form of a thin film.
Various instances have been presented in tabular form below:
| Objects which is to electroplated | The metal which has to be electroplated on the objects | The material used as cathode | The material used as anode | Electrolyte |
| Iron pipe | Zinc | Iron pipe | zinc | An aqueous solution of zinc chloride |
| The handle of the bicycle made of iron | Chromium | The handle of the bicycle made of iron | Pure sheet of chromium | An aqueous solution of chromic sulphate and chromic acid |
| Tap made of brass | Chromium | Tap made of brass | Pure sheet of chromium | The aqueous solution of chromic sulphate and chromic acid |
Question 8. What are the chemical changes associated with electrolysis?
Answer:
The chemical changes associated with electrolysis:
The products obtained at electrodes vary as the electrodes used in electrolysis support. The chemical changes brought about by electrolysis are-
- A bubble of gases may be formed on the electrodes
- deposits of metals may occur on electrodes
changes in the colour of the solution may occur Electrolysis of the same electrolyte with different electrodes produce different products.
When an aqueous solution of CuSO4 is electrolysed with Cu electrodes, SO2- ions, in preference to OH– ions accumulate at the anode and Cu2+ ions will be discharged at the cathode.
But, if platinum electrodes are used in this electrolysis, OH– ions, in preference to SO4 ions will be discharged at the anode and Cu2+ ions will be discharged at the cathode.
Similarly, if an aqueous solution of NaCI is electrolysed with platinum electrodes, H+ ions, in preference to Na+ ions are discharged at the cathode.
But, if a mercury cathode is used in this electrolysis, Na+ ions, in preference to H+ ions will be discharged at the cathode.