Chapter 2 Element Compound And Chemical Reaction Section 4 Chemical Effects Of Electricity
Conductors Of Electricity
Some solid objects are known to conduct electricity. For example, some metals like copper, aluminium etc., conduct electricity. But what about a liquid? Can liquid conduct electricity? To experimentally verify this, an experimental set-up can be built up as follows.
A battery is connected on both sides by two wires. The open ends of the two wires are connected to two metallic rods. The rods act as electrodes.
These two metallic rods are partially immersed into a liquid or solution taken in a non-conducting vessel. One LED bulb (used in torch) is connected in the circuit between the neg
ative end of the battery and the electrode.
(The battery is then switched on. If the liquid conducts electricity, the LED bulb will glow. If the liquid or the Solution is a good conductor of electricity, the LED bulb will glow brightly. If the liquid or the solution is a bad conductor of electricity, the LED bulb does not glow at all.
[Sometimes to detect the passage of a very feeble current through the circuit, the circuit wire is placed over a magnetic needle, pointing North-South. Passage of even a very low current will cause deflection of the needle].
Using this experimental setup, the electrical conductance of some liquids can be tested. The results for some of the liquids tested are given below.
Read And Learn More WBBSE Notes For Class 8 School Science
| Name of the liquid | Result | Inference |
| 1. Distilled water | Bulb does not glow | Does not conduct electricity |
| 2. Coconut oil | Bulb does not glow | Does not conduct electricity |
| 3. Aqueous solution of ammonia | Bulb glows dimly | The solution is a poor conductor of electricity |
| 4. Lime juice | Bulb glows | Solution conducts electricity |
| 5. Vinegar solution | Bulb glows | Solution conducts electricity |
| 6. Dilute HCI solution | Bulb glows brightly | The solution is a good conductor of electricity |
| 7. Distilled water mixed with sodium chloride | Bulb glows brightly | The solution is a good conductor of electricity |
From the experimental results, we come to know that some substances (compounds) conduct electric current in solution and some other compounds do not conduct electric current in the solution.
This fact gives us an idea about electrolytes and non-electrolytes. Electrolytes are substances which conduct electricity in solution (or in their molten state) and at the same time decompose chemically.
These exist as ions in solution. The charged ions migrate in an electric field set up by the two electrodes and they conduct electricity.
Non-electrolytes are substances which do not conduct electricity in solution (or in their molten state). These substances dissolve as molecules.
Since molecules are neutral, they do not migrate in an electric field, i.e. they do not conduct electric current.
Acids, bases, salts etc are electrolytes, but pure water, sugar, urea, benzene etc are non-electrolytes. Electrolytes can be further subdivided based on their ability to conduct electric current as shown below:
It is found that solutions of weak organic acids like acetic acid (vinegar) and weak bases like ammonium hydroxide are very feeble conductors in comparison to solutions of strong acids, strong bases and salts like sulphuric acid, hydrochloric acid, sodium hydroxide, sodium chloride etc. The former are called weak electrolytes and the latter, are strong electrolytes.
Distilled water (or deionized water) does not conduct electricity. But when sodium chloride is dissolved in water it becomes a good conductor of electricity.
When dissolved in water, sodium chloride dissociates to form Na+ and Cl ions. It is these ions which conduct electricity through the solution.
Distilled water lacks any ions and hence does not conduct any electricity. Hence, distilled water is a nonelectrolyte but when mixed with sodium chloride, the aqueous solution becomes a strong electrolyte.
Our blood, a strong electrolyte, also contains a number of ions and so our body is a good conductor of electricity. Hence, whenever it is required to touch electrical appliances it is strictly advised not to touch those appliances with wet hands and on bare feet.
Electrolysis
Where electricity is passed through an aqueous solution of acid (say sulphuric acid), it undergoes dissociation. Electricity can be passed through molten sodium chloride also and dissociation can occur there too. So we find that when electricity is passed through a solution, a chemical change may occur.
This process where certain chemical compounds in a molten state or aqueous solution state dissociate during the passage of electricity through them is called electrolysis.
During electrolysis, Direct Current is used. Electrolysis is a chemical change brought about by electricity. In this process, an electrolyte decomposes into some components whose properties are different from those of the parent substance.
For example, sodium chloride (NaCI) produces Na+ and Cl ions in an aqueous state. Potassium hydroxide (KOH) produces K+ and OH ions in an aqueous solution.
Sulphuric acid dissociates in an aqueous solution to produce H+ and SO42- ions, while potassium nitrate (KN03) produces K+ and N03- ions when dissolved in water.
But sugar, urea, alcohol etc., do not produce any ions when dissolved in water hence they are not electrolytes and they do not conduct electricity through their aqueous solution.
The process of electrolysis is carried out in an electrolytic cell. The experimental arrangement is almost similar to the one. Here no LED bulb is attached.
Let us describe in brief the various components used in the electrolytic cell.
Electrolytic cell: This is a non-conducting vessel containing electrolyte solution and two electrodes partially immersed within it.
Electrolysis is generally carried out in a vessel called a voltameter. Usually, it is made up of glass or porcelain, which contains the electrolyte solution or molten mass of electrolyte. It is provided with two conducting metal rods or graphite rods, separated by a distance.
Electrolyte: This is the chemical compound which when dissolved in water produces ions and the solution becomes the conductor of electricity. It undergoes dissociation when electricity is passed through it.
Electrodes: There are two metal rods partially dipped within the electrolyte solution (or fused electrolyte). They are connected to two terminals of the battery. Through these two electrodes, electricity is passed through the electrolyte solution.
Anode: This is the electrode which is connected to the positive terminal of the battery.
Cathode: This is the electrode which is connected to the negative terminal of the battery. (We must mention here that different materials used as electrodes can produce different products due to the passage of electricity through the same aqueous solution of the electrolyte).
Battery: Acts as a source of electricity. It supplies Direct current (DC).
Explanation Of Electrolysis And Electrolytic Dissociation
Let us take the compound AB which is a strong electrolyte. In an aqueous solution (or in a molten state), AB dissociates into its ions as:
⇒ \(\mathrm{AB} \rightleftharpoons \mathrm{A}^{+}+\mathrm{B}^{-} (cation) (anion)\)
When electricity is passed through the electrolyte (in solution or molten state), the cation A+ moves towards the cathode (negative electrode). On reaching the cathode, it is discharged there by taking up an electron (e).
⇒ \(\mathrm{A}^{+}+\mathrm{e} \rightarrow \mathrm{A} \text { (atom) }\)
If neutral A is sufficiently stable to exist in an atomic state (e.g. if it is a metal), it will remain as such; otherwise, it will immediately form molecules through the union of two (or more) atoms:
⇒ \(A+A \rightarrow A_2\)
If molecule A2 be solid or liquid, it will remain within the electrolytic cell either as a deposit on the cathode or a precipitate at the bottom of the vessel; but if it is gas, it escapes.
In a similar mechanism, the anion B– reaches the anode (positive electrode) and is discharged thereby leaving an electron (e).
⇒ \(\mathrm{B}^{-}-\mathrm{e} \rightarrow \mathrm{B} \text { (atom) }\)
B may exist as such if it is stable in the atomic state, otherwise, it will immediately form the molecule such as,
⇒ \(\mathrm{B}+\mathrm{B} \rightarrow \mathrm{B}_2\)
Like A2, this species will remain in the vessel if it is a solid or a liquid, or it will escape, if gaseous.
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 Cl– 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 CT 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): | \(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \text { (gas }\) |
Electrolysis Of Acidified Water
Pure water is acidified before the electrolysis reduction occurs, whereas at the anode, oxidation of water. Such a mixture is called acidulated water.
This is because pure water is a very poor conductor of electricity. So, to make it a good conductor of electricity, the number of ions within it has to be increased.
This is done by adding some acids like sulphuric acid (which produces H+ and SO42- ions) or alkali like NaOH (producing Na+ and OH– ions). The two electrodes used in this case are made of platinum.
The electrolyte contains H+, OH– and SO42 ions formed through dissociations:
⇒ \(\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OH}^{-} ; \mathrm{H}_2 \mathrm{SO}_4 \rightleftharpoons 2 \mathrm{H}^{+}+\mathrm{SO}_4^{2-}\)
When electricity is passed through the acidified water, H+ ions (produced from the dissociation of H2O move towards the cathode and are discharged by accepting electrons and eventually hydrogen gas (H2) is produced at the cathode.
The hydroxyl ions (OH ) produced from the dissociation of water move towards the anode and lose electrons and ultimately oxygen gas (02) and water are produced at the anode. At cathode occurs.
Two types of anions are present in the electrolyte in this case. They are SO42 and OH–. When electricity is passed through the electrolyte solution,
both the anions move towards the anode, but OH– is preferentially discharged at the platinum electrode, producing water and oxygen gas is liberated.
| Electrolysis of acidified water | |
| Electrolyte | Acidified water (acidified with dilute H2S04) |
| Electrode | Anode: Platinum Cathode: Platinum |
| Electrode reaction : | |
| At cathode: (Reduction) | \(\mathrm{H}^{+}+\mathrm{e} \rightarrow \mathrm{H}\) \(\mathrm{H}+\mathrm{H} \rightarrow \mathrm{H}_2\) |
| At anode: (Oxidation) | \(4 \mathrm{OH}^{-}-4 \mathrm{e} \rightarrow 2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2 \text { (gas) }\) |
Electrolysis Of Calcium Chloride
Calcium metal is extracted by electrolysis of molten 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.
The electrolyte contains Ca2+ and Cl– ions formed through the dissociation of \(\mathrm{CaCl}_2: \mathrm{CaCl}_2 \rightleftharpoons \mathrm{Ca}^{2+}+2 \mathrm{Cl}^{-}\) Calcium metal is formed at the cathode and chlorine gas is liberated at the anode.
| Electrolyte Molten or fused magnesium chloride (CaCl2) | |
| Electrolyte | Anode: graphite |
| Electrode | Cathode: iron |
| Electirbde reaction : | |
| At the cathode (Reduction): | \(\mathrm{Ca}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Ca}\) |
| At anode (Oxidation): | \(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \text { (gas) }\) |
Electrolysis Of Magnesium Chloride
Magnesium metal is extracted by electrolysis of molten magnesium chloride. In this process, molten magnesium chloride is taken in a rectangular iron container.
The walls of the iron container act as a cathode while a graphite electrode is immersed partially in the molten salt and it acts as an anode. In molten conditions, Mg2+ and Cl– ions are produced through dissociation:
⇒ \(\mathrm{MgCl}_2 \rightleftharpoons \mathrm{Mg}^{2+}+2 \mathrm{Cl}^{-}\)
Magnesium metal is formed at the cathode and chlorine gas is liberated at the anode.
| Electrolysis of molten or fused magnesium chloride | |
| Electrolyte | Molten or fused magnesium chloride (MgCl2) |
| Electrode | Anode: graphite Cathode: iron |
| Electrode reaction: | |
| At the cathode (Reduction): | \(\mathrm{Mg}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Mg}\) |
| At anode (Oxidation): | \(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \text { (gas) }\) |
Conduction Of Electricity Through Metals And Electrolytes
The passage or conduction of electric current requires some material medium. Those substances which allow the passage of electricity through them are called the conductors of electricity.
Both metals and electrolytes are conductors. But, there are certain differences in the mode of conduction of electricity by these two classes of substances.
- The difference between metallic conduction and electrolytic conduction is:
- The conductivity of metals is much higher than that of electrolytes.
- In metals, the current is carried by electrons, whereas in electrolytes, the current is carried by the ions produced by the dissociation of the electrolytes.
- In metallic conduction, there is no transport of matter. Electrolytic conduction is accompanied by the transport of matter.
- When a current is passed through a metal, there is no material change; the only change is that the metal becomes heated. But chemical change takes place in electrolytic conduction (either in solution or in molten state).
- This chemical change is, in fact, called electrolysis.
- Metallic conduction decreases with the temperature rise, whereas electrolytic conduction increases with the rise in temperature.
- Metals can conduct electricity in a solid state, but electrolytic conduction occurs either in a molten state or in a solution.
Important Uses Of Electrolysis
Metal extraction
Some metals are extracted from their naturally occurring compounds (ores) by electrolysis. For example, sodium metal can be obtained from the electrolysis of fused or molten sodium chloride; calcium metal is obtained from the electrolysis of fused or molten calcium chloride, and magnesium metal is obtained from the electrolysis of molten magnesium chloride.
In all cases, metal is produced at the negatively charged electrode or cathode. This is because the positively charged ions of metals present in their molten compounds are attracted by the negatively charged electrode.
Anode reaction: \(2 \mathrm{Cl}^{-}-2 \mathrm{e} \rightarrow \mathrm{Cl}_2 \uparrow\)
Cathode reaction: \(\mathrm{M}^{+n}+n e \rightarrow M\) where M is metal.
One thing worth mentioning that is fused or molten salt is used for electrolysis instead of the aqueous solution of the electrolyte.
This is because electrolysis of the aqueous solution of these salts will not yield metals at the cathode.
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 a 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.
3. Electroplating
The surface of some metal sometimes needs a protective coating or needs shine for a longer time. For example, iron is coated with a thin layer of zinc to protect it from rusting.
Electroplating zinc on an iron surface is called galvanization. Ornaments of bronze or silver are coated with a thin layer of gold to make them shining and attractive.
Ornaments coated with thin layers of gold are known as “rolled gold” Sometimes iron tools are coated with a thin layer of nickel to protect them and to appear like stainless steel as well.
| Electroplating of copper | |
| Electrolyte | Aqueous copper sulphate solution
\(\left(\mathrm{CuSO}_4\right)\left[15 \% \mathrm{CuSO}_4+5 \% \mathrm{H}_2 \mathrm{SO}_4\right]\) |
| Electrode | Anode: Pure copper bar Cathode: Articles to be coated with copper |
| Electrode reaction: | |
| At anode: At Cathode: | \(\begin{aligned} & \mathrm{Cu} \rightarrow \mathrm{Cu}^{2+}+2 \mathrm{e} \\ & \mathrm{Cu}^{2+}+2 \mathrm{e} \rightarrow \mathrm{Cu} \end{aligned}\) |
The process by which a thin layer of one metal is coated on the surface of the other to protect the latter from rusting or to improve its appearance is called electroplating.
The metal which is to be electroplated is used as the 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.
Usually, articles of iron or brass are plated with nickel, copper, chromium, gold, etc. The salt or electrolyte solution must contain the metal ions like Ag, Au, and Ni since these ions travel towards the cathode and get deposited on the placed as a cathode.
Less amount of current is supplied for a longer time. It helps to reduce the uneven deposition of metal. Only direct current is to be used because alternating current causes ionization on the alternate electrode.
In copper electroplating, a pure copper bar is used as a node and the article to be coated (i.e., electroplated) by copper is used as a cathode.
Copper sulphate solution is used as the electrolyte solution. During electroplating, copper dissolves out from the anode as Cu2+ and is deposited as copper metal on the surface of the anode (i.e., the articles to be electroplated by copper).
In the case of nickel and silver electroplating, nickel salt (say, nickel sulphate) and silver salt (say, silver nitrate) are used as electrolytes, respectively;
the anode is a strip of pure nickel (for nickel plating) or pure silver (for silver plating) and the articles to be electroplated are used as cathode.
Here also, the pure metal dissolves out as a cation from the anode into the solution and is deposited on the cathode as a pure metal.
[in nickel plating, nickel sulphate solution containing some boric acid is electrolysed. In silver plating, a solution of potassium argento cyanide is electrolyzed. The electrolyte is prepared by adding excess potassium cyanide to the silver nitrate solution.
During gold plating, a solution of potassium aurocyanide or chloroauric acid (HAuCI4) is used as the electrolyte.]
Applications of electroplating
For anti-corrosion: Electroplating induces anti-corrosive properties by coating a metal with another metal. The metal which is deposited in the form of a thin layer (Chromium, tin etc) during electroplating is less reactive than the metal of the object and hence resists corrosion.
It has also a shiny appearance whereas the metal of the object has a dull appearance.
For decorative purposes: Electroplating gives a pleasing appearance and a coating of more expensive metal makes a cheap metal look more attractive.
For example, a coating of gold or silver over brass gives a shining or costly appearance to the jewellery made of brass.