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Acids, Bases and Salts

Acids

An ionic compound or electrovalent compound is formed when metallic atoms donate electrons and non-metallic ions accept electrons.

The word electrolysis is made up of two words electro, which means the flow of electrons, and lysis, which means pertaining to.

Electrolysis is the process by which a chemical compound in the fused state or in aqueous solution undergoes a chemical change on the passage of current.

During electrolysis, a chemical change is brought about by electric energy.

  • Electrolytic cell or voltameter is the vessel consisting of glass, containing two electrodes and an electrolyte.

 
  • The graphite or metal rods through which electric current enters or leaves from an electrolyte are called electrodes.

Therefore, a cathode is the electrode which is connected to the negative terminal of the battery. It has an excess of electrons. An anode is than electrode which is connected to positive terminal of battery. It has deficiency of electrons.

 

Difference between cathode and anode 

Anode Cathode
Connected to positive terminal of the battery Connected to negative terminal of the battery
Migration of anion occurs Migration of cations occurs
Oxidation occurs Reduction occurs


Differences between metallic and electrolytic conductors

Metallic Conductors Electrolytic Conductors
Flow electrons from negative pole to positive pole is responsible for conduction. Flow of ions in solution to the respective electrodes is responsible for conduction.
Shown by metals and alloys Shown by aqueous solution of ionic compounds
Present in both solid and liquid state of metals Present in molten or aqueous solution of the ionic compounds
Physical process Chemical process
  • Electrolyte refers to the compound, which in fused state or in aqueous state conducts electric current so that chemical decomposition of the compound takes place.

The electrolytes which allow a large amount of current to flow through them are called strong electrolytes. The strong electrolytes in fused state are completely dissociated and hence, their ions are free to move in any direction.

Examples include an aqueous solution of mineral acid and caustic alkalis.

The electrolytes which allow a very small amount of current to flow through them are called weak electrolytes. The strong electrolytes in fused state are completely dissociated. Hence, their ions are free to move in any direction.

Examples include aqueous solution of acid and bases.

  • Non-electrolyte is the chemical compound which does not conduct electric current in fused state or in aqueous solution and does not undergo any chemical decomposition.

Examples are carbon tetrachloride, alcohol, ether, pure water, etc.

Differences between strong and weak electrolytes

Strong electrolytes Weak electrolytes
Allows large amount of electricity to flow through them Allows small amount of electricity to flow through them
Good conductors Poor conductors
Undergo complete dissociation in molten or aqueous solution Undergo partial dissociation in molten or aqueous solution
Contain free mobile ions Contains ions and molecules
Examples are HCl, H2SO4, HNO3, NaOH, KOH, NaCl, CuSO4, etc. Examples are H2CO3, CH3COOH, HOOC-COOH, NH4OH, Ca(OH)2, (NH4)2CO3, Pb(CH3COO)2
 
  • Ions

When a chemical compound in fused state or in aqueous solution breaks up into electrically charged atoms or group of atoms, the charged particles are called ions. A positively charged ion is cation. They migrate towards cathode. A negatively charged ion is an anion. They migrate towards anode.

Differences between cation and anion

Cation Anion
Positively charged Negatively charged
Migrate to cathode Migrate to anode
Get reduced to form neutral atom Get oxidised to form neutral atom
Examples, Na+, Al3+ Examples, Cl , OH


Mechanism of electrolysis

It was put forward by Avante Arrhenius, a Swedish chemist. Following are the points given in his theory of electrolytic dissociation.

  • An electrolyte is a substance which in an aqueous solution breaks into positively charged cations and negatively charged anions.
  • The movement of ions within the electrolyte is responsible for the flow of electric current.
  • The degree of dissociation refers to the extent to which electrolyte dissociates into ions.
  • Cations and anions unite to form neutral ion, setting up equilibrium.
  • An electrolyte is electrically neutral.

Electrolysis of water:

Add a small amount of salt to water and pour this solution into a beaker. Connect two electrical wires to the terminal of the battery as shown in the figure. The two ends of the wires will serve as electrodes when the insulating cladding is removed from them. Take two test tubes and fill them with the solution of salt and water. Invert these test tubes on the electrodes and restrict the formation of air bubbles. Switch on the power supply. 


Observe the two test tubes when the electric current is passed through it. The volume of the gas formed in one test tube is double than that of the gas formed in the other test tube.
The gas formed at the cathode is hydrogen while the gas formed at the anode is oxygen. 
Cathode: 2H2O (l) + 2e- H2  (g) + 2OH- (aq)
Anode: 2H2O (l)  O2 (g) + 4H+ (aq) + 4e- 

Characteristics of electrolysis

  • On passing the electric current, cations migrate towards cathode while the anions migrate towards the anode.
  • The number of electrons accepted by an anode is equal to the number of electrons donated by the cathode.
  • The products of electrolysis are formed on the surface of the cathode where the exchange of electrons occurs.
  • The preferential discharge of ions depends upon their position in electrochemical series and the concentration of ions.
  • Neutral atoms of metals and hydrogen are liberated at the cathode. Therefore, they are called electropositive elements. Neutral atoms of non-metals are liberated at the anode. Therefore, they are called electronegative elements.
  • According to Faraday's law of electrolysis, the mass of a substance discharged at an electrode is directly proportional to the quantity of electricity passing through the electrolyte.
  • Direct current is suitable to carry out electrolysis.
  • Electrolysis is a redox reaction, where reduction occurs at cathode and oxidation occurs at anode.
Do you know solid ionic compounds do not conduct electricity while fused ionic compound and aqueous solution do?

This happens because the electric current passes through an ionic compound only if it has free ions for migration. In a solid ionic compound, ions are held together by strong electrostatic forces. Hence, the ions are not free to migrate. Therefore, solid ionic compounds do not conduct electricity. But fused ionic compound and an aqueous solution of a compound have free ions produced by electrolytic dissociation of the ionic compound. These ions are free to conduct electricity.

Let us study it with the help of some examples.

Example- Solid ionic lead bromide does not conduct electricity as lead ions and bromide ions are not free to migrate. This is because they are held together by strong electrostatic forces.

But when lead bromide is heated, ions gain kinetic energy. At one point, ions have more kinetic energy than the electrostatic bonds holding them and they start moving freely. Hence, the fused compound conducts electricity.

In an aqueous solution of compounds, say lead bromide, the slight positive charge on hydrogen atom pulls the bromine ions and slight negative charge on oxygen atom pulls the lead ions. This weakens the electrostatic attraction …

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