Chemistry, SS 2 Week: 6

Topic: Electrolysis

What is Electrolysis?

Ionic substances contain charged particles called ions. For example, lead bromide contains positively charged lead ions and negatively charged bromide ions.

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them. In other word, electrolysis is the decomposition of certain compounds in aqueous or molten state, when electric current is passed through them.

Electrolyte: These are solutions or molten compounds of inorganic acids, bases and salts that conduct electricity. They dissociate into their ions on the passage of electric voltage. There are two types of ions, these are:

  • Anion: It is a negatively charged ion that moves to the anode during electrolysis.
  • Cation: It is a positively charged ion that moves to the cathode during electrolysis.

Strong electrolytes: are electrolytes that give strongly conducting solution and dissociate readily into ions. Examples are HNO3, NaCl

Weak electrolytes: They are electrolytes that produce weakly conducting solutions and only partially dissociate into their ions on the passage of electric current and voltage. Examples include HgCl, CH3COOH acid.

Non-Electrolytes: These are solutions of organic substances, which produce molecules instead of ions and are therefore poor conductors of electricity. Examples include sugar, ethanol and glycerine.

Conductors: are solid substances except mercury that allow electric current to pass through them. For example metals like copper and aluminium, and non-metal like graphite (carbon)

Non-conductors: are solid or liquid substances that do not allow an electric current to pass through them. For example diamond (carbon), rubber/plastics, benzene, dry wood etc.

Electrodes: These are rods or plates that connect the wiring of the circuit to the electrolyte. The positive electrode is the anode and is the electrode through which the electric current enters the electrolyte. The negative electrode is the cathode and is the electrode through which the current leaves the electrolyte.

Electrolytic Cell: This is an assembly of two electrodes in an electrolyte, and is used for the electrolysis of a substance. In earlier days, the set up was known as voltameter.

Simple Electrolytic Cell

Mechanism of Electrolysis

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or when melted. For example, if electricity is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine. The electrodes connect to a battery. When they are electrically charged, one electrode becomes the negative terminal or cathode, and the other becomes the positive terminal or anode. The positive terminal attracts negatively charged ions and the negative terminal attracts positively charged ions.

Here is what happens during electrolysis:

  • Positively charged ions move to the negative electrode during electrolysis. They receive electrons and are reduced.
  • Negatively charged ions move to the positive electrode during electrolysis. They lose electrons and are oxidised.

The substance that is broken down is called the electrolyte.

Arrhenius Theory of Electrolytic Dissociation

In order to explain the properties of electrolytic solutions, Arrhenius put forth, in 1884, a comprehensive theory which is known as theory of electrolytic dissociation or ionic theory. The main points of the theory are:

(i) An electrolyte, when dissolved in water, breaks up into two types of charged particles, one carrying a positive charge and the other a negative charge. These charged particles are called ions. Positively charged ions are termed cations and negatively charged as anions.

AB –> A+  + B

NaCl –>  Na+ + CL

K2SO–> 2K++ SO42-

In its modern form, the theory assumes that solid electrolytes are composed of ions which are held together by electrostatic forces of attraction. When an electrolyte is dissolved in a solvent, these forces are weakened and the electrolyte undergoes dissociation into ions. The ions are solvated.

 A+B(aq) –>  A+(aq)+B (aq)

 (ii)    The process of splitting of the molecules into ions of an electrolyte is called ionization. The fraction of the total number of molecules present in solution as ions is known as degree of ionization or degree of dissociation. It has been observed that all electrolytes do not ionize to the same extent. Some are almost completely ionized while others are feebly ionized. The degree of ionization depends on a number of factors.

 (iii)    Ions present in solution constantly re-unite to form neutral molecules and, thus, there is a state of dynamic equilibrium between the ionized and non-ionized molecules, i.e.,

    AB <–>  A+ + B

Applying the law of mass action to above equilibrium

[A+ ][B ] /[AB] =K

K is known as ionization constant. The electrolytes having high  value of K are termed strong electrolytes and those having low value of K as weak electrolytes.

 (iv)   When an electric current is passed through the electrolytic solution, the positive ions (cations) move towards cathode and the negative ions (anions) move towards anode and get discharged, i.e., electrolysis occurs. The ions are discharged always in equivalent amounts, no matter what their relative speeds are.

(v)    The electrolytic solutions is always neutral in nature as the total charge on one set of ions is always equal to the total charge on the other set of ions. However, it is not necessary that the number of two sets of ions must be equal always.

NaCl <–>  Na+ + Cl (Both ions are equal)

BaCl2 <–>  Ba2+ + 2Cl   (Anions are double that of cations)

Na2SO4 <–> 2Na+ +   (Anions are double that of cations)

(vi)  The properties of electrolytes in solution are the properties of ions present in solution. For example, acidic solution always contains H+ ions while basic solution contains OH ions and characteristic properties of solutions are those of H+ ions and OH ions respectively.

(vii)  The conductivity of the electrolytic solution depends on the nature and number of ions as the current is carried through solution by the movement of ions.