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Chapter 6 – Electrolytes, Non-Electrolytes and Electrolysis

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction

Electrolyte is the chemical compound that conducts electricity in its molten or aqueous state and undergoes chemical decomposition due to the flow of electricity. It decomposes into ions and neutral atoms at the two electrodes. Electrolytes are ionic or covalent compounds.

Strong ElectrolytesWeak Electrolytes
Large amounts of electricity flow through them.A small amount of electricity flows through them.
They are good conductors of electricity.They are poor conductors of electricity.
These are completely dissociated into ions. These are partially dissociated into ions. 
The solution contains only ions.These solutions contain both ions and molecules.
Examples

1. Acids: Hydrogen Chloride, Sulphuric and Nitric acid

2. Bases: NaOH, KOH

3. Salts: NaCl, CuCl₂

Examples

1. Acids: carbonic acid, acetic acid

2. Bases: calcium hydroxide and ammonium hydroxide

3. Salts: sodium carbonate and  bicarbonate

They do not conduct electric current at all, whether in a solid or aqueous state. So, they do not undergo decomposition into ions in solution. They contain only molecules. 

Examples

Distilled water, alcohol, kerosene, cane sugar, carbon disulphide, benzene, glucose, and urea.

Electrolysis is the decomposition of chemical compounds in aqueous solutions or molten states accompanied by chemical change using a direct electric current.

When NaCl is electrolysed, it decomposes into sodium and chlorine gas.

Nacl ⇄ Na⁺ + Cl⁻

At cathode: Na⁺ + e⁻→ Na (Reduction)

At Anode: Cl⁻ – e⁻ →Cl

Cl+ Cl →Cl₂

Reaction: 2NaCl → Na + Cl₂

  • The passage of electricity through an electrolyte causes the positively charged ions to move towards the cathode and negatively charged ions to the anode.
  • The electrons lost at the anode are equal to the number of electrons gained by the cathode.
  • Electrolysis products are formed at the anode and cathode since the exchange of electrons occurs at the electrodes’ surface.
  • Hydrogen gas and metals will liberate at the cathode, and hence they are called electropositive elements. In contrast, non-metals are released at the anode and are called electronegative elements.
  • The process of electrolysis is a redox reaction.
  • Electrolysis is caused by direct current.

Metal plates or graphite rods are connected to the terminals of a battery and are immersed into the electrolyte, through which electricity enters and leaves the electrolytic cell.

The electrode connected to the positive pole of the battery is the anode. Anions migrate to the anode, and they are oxidised.

The electrode connected to the negative pole of the battery is the cathode. Cations migrate to the cathode and get reduced.

The atoms or group of atoms that carry a positive or a negative charge are known as ions. Atoms which have positive charge are cations, and negative charge is anions.

Examples

Cations: Na⁺, Ca²⁺, Al³⁺

Anions: Po₄³⁻, Cl⁻, OH⁻

The arrangement of ions depends on their ease of being neutralised or discharged during an electrolytic process. The ions are arranged in the order such that the ion at the top is removed with difficulty, and the one at the lower of the series is released quickly.

Selective discharge of ions at electrodes

When two or more ions of the same charge present in an electrolyte solution under similar conditions and are competing for discharge at the same electrode, one gets preferentially discharged. It is known as the selective discharge of ions, and it depends on three factors:

  1. Relative position of ions in the electrochemical series
    The ions placed lower in the electrochemical series get discharged more quickly.
    In an electrolyte solution containing Cu2+ and H+, Cu2+ is reduced at the cathode.
  1. The concentration of the ions in the electrolyte
    The higher concentration ions will be discharged quickly. During the electrolysis of conc. NaCl solution, both Na⁺ H₃O ⁺ migrate to the cathode. But sodium ions have increased concentration and get released at the cathode first. Both Cl⁻ and O.H.⁻ migrate to the anode, and chloride ions discharge at the anode.
    NaCl → Na⁺ + Cl⁻
    H₂O +H₂O →H₃O + OH⁻
  2. Nature of the electrode
    Electrodes used in the process of electrolysis are either inert or active. Active electrodes take part in the electrolytic reaction. Copper is an active electrode, and platinum is an inert electrode.

Electrolysis of molten lead bromide

ElectrolyteMolten lead bromide(PbBr₂)
Electrolytic cellCrucible made up of silica.
ElectrodesCathode: Iron or Graphite

Anode: Graphite plates. 

TemperatureAbove 380°C.
Electrode reactionsAt cathode: Pb²⁺+ 2e⁻→  Pb

At Anode: Br⁻ – e⁻ →Br

Br+ Br→ Br₂

Overall Reaction: PbBr₂→  Pb+ Br₂

Observation Greyish lead deposit at Cathode.

Dark reddish-brown vapours at Anode.

  1. Electroplating of metals
    Electroplating is a process in which a layer of metal like gold, silver, platinum gets coated on other metals with the help of electricity. The electroplating is used for decoration purposes to give the metal a radiant appearance and protect it from rust and corrosion.
Condition Reason
The article to be electroplated is placed at the cathode.The metal deposits at the cathode by a gain of electrons.
The metal to be plated on the article is always made anode and should be replaced frequently.The metal anode dissolves continuously as ions in solution and diminishes in size.
Low current should be used for an extended period.Because high current causes uneven deposition of the metal, so low current initiates thicker uniform deposition.
A direct current is used instead of A.C current.A.C current causes discharge and ionisation; thereby, no effective coating is provided.
The electrolyte must contain the ions of the metal to be plated on the article.Electrolyte dissociates to give metal ions which migrate towards cathode and deposits as neutral metallic atoms.

Electroplating with Nickel

You know that iron gets easily rusted when exposed to the air. To prevent rusting and corrosion, we use electroplating techniques. So, here iron particles are coated with nickel, tin, or zinc to prevent rusting. 

 

Electrolyte: Aqueous solution of Nickel sulphate (NiSO₄)

Cathode: Iron Article 

Reaction at cathode: Ni²⁺ + 2e⁻ →Ni (deposited)

Anode: Block of Nickel-metal

The reaction at the anode: Ni – 2e⁻ →Ni²⁺

  1. Electrolytic refining of metals
    It is a process in which metals containing impurities are purified electrolytically to give a pure metal.

 

Refining of a copper

The copper wire used for electric transmission has to be pure because even small impurities reduce its conductivity. When current passes through the electrolyte, copper ions attract the cathode and get deposited on sterile copper strips. The impure copper loses electrons and gives into a solution, and gets finished. The pure copper deposited at the cathode becomes thicker, and it is 99.9% pure.

Electrolyte: Solution of copper sulphate along with dil. Sulphuric acid.

Cathode: Thin strip of pure copper

Reaction at cathode: Cu²⁺ + 2e⁻ →Cu 

Anode: Impure copper

The reaction at the anode: Cu – 2e⁻ →Cu²⁺

Electrolytes are substances that dissociate into ions and conduct electricity. Non-electrolytes do not produce ions and do not allow current flow through them. Electrolysis is the process to separate a substance when an electric current passes through an electrolyte. Electrolysis is performed to prevent rusting, extract and purify metals.

  1. What are the conductors and non-conductors?
    Elements like copper, aluminium and other metals allow an electric current to pass through them. So, they are called conductors, and non-metals like sulphur and phosphorus do not conduct electricity and are called non-conductors.
  2. How to determine conductivity in compounds?
    When compounds dissolve in water, the forces of attraction between the ions are broken and can carry current effectively. The higher the concentration, the greater is the conductivity. In contrast, the compounds in solid-state, despite having ions, cannot conduct electricity.
  1. What is the degree of dissociation?
    It is defined as the fraction of its molecules dissociated in a given time.
    Degree of dissociation = (No. of molecules dissociated/total no. of molecules)100.
  2. What is electrometallurgy?
    It is the process of extraction of metals by electrolysis.
  3. What is the difference between electrolytic and electrochemical cells?i. Electrolytic cell
    The vessel in which the electrolysis takes place is called an electrolytic cell or voltammeter. It converts electrical energy into chemical energy.

    ii. Electrochemical cell
    The devices like simple voltaic cells and daniel cells are used to convert chemical energy into electric energy.

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