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Electrochemistry

Electrolysis of Some Common Electrolytes

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 flow of electrons, and lysis,which means pertaining to.

Electrolysis is the process by which a chemical compound in 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.

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

Therefore, cathode is the electrode which is connected to the negative terminal of battery. It has excess of electrons. Anode is the electrode which is connected to positive terminal of battery. It has deficiency of electrons.

  • 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 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 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.

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

Mechanism of electrolysis

It was put forwardby 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.
  • 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.

Characteristics of electrolysis

  • On passing the electric current, cations migrate towards cathode while the anions migrate towards anode.
  • The number of electrons accepted by an anode is equal to the number of electrons donated by cathode.
  • 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 cathode. Therefore, they are called electropositive elements. Neutral atoms of non-metals are liberated at anode. Therefore, they are called electronegative elements.

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 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 aqueous solution of a compound have free ions 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 aqueous solution of compounds, say lead bromide, the slight positive charge on hydrogen atom pulls the bromine ions and slight…

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