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38671 results in Cambridge Textbooks

Page 1023 of 1934

  • 16 - Electrochemistry

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 890-967

  • Summary

    Introduction

    Electrochemistry is the branch of science which deals with the relationship between electrical energy and chemical energy and the inter-conversion of one form into other. If electrical energy brings about chemical reactions to occur, then the process is called electrolysis; for example, water can be made to split into hydrogen and oxygen by the passage of electric current. On the other hand, electrical energy can be produced as a result of chemical change. Both the above changes are called electrochemical changes and the latter finds use in the manufacture of cells and batteries, which generate electricity due to chemical changes. Both types of changes have widespread applications in chemistry. This chapter, however, deals only with the conversion of chemical energy into electrical energy and its applications in chemistry.

    Types of Conductors

    Conductors are substances which allow electricity to pass through them, for example, copper, silver and gold. Those substances which do not allow the passage of electric current through them are called non–conductors or insulators; for example, wood, rubber, etc.

    Conductors can be of two types:

  • (i) Metallic conductors Metallic conductors allow electricity to pass through them without undergoing any chemical change. The flow of electric current through metallic conductors is due to the flow of electrons in the metal. Common examples of metallic conductors are metals like copper, silver, etc.

  • (ii) Electrolytic conductors These are substances which allow electricity to pass through them in their molten states or in their aqueous solutions. Electric current brings about decomposition in them. The charge is carried by the ions and conduction occurs due to the movement of ions. Thus, these substances do not conduct electricity in solid state but in molten state or in aqueous solutions, as the ions can move about freely in molten state or in aqueous state. Substances like these are termed as electrolytes. Example: Molten solution of NaCl. The differences between metallic and electrolytic conductors are summarised in Table 16.1.

  • (iii) Non-electrolytes These are substances which do not conduct electricity at all, neither in their molten state nor in their aqueous solution. Example: sugar, glucose, urea, etc.

  • 4 - Phase Rule

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 255-306

  • Summary

    Introduction

    The phase rule was given by Gibbs, which explains the equilibrium existing in heterogeneous systems. It states that the equilibrium between different phases is influenced by temperature, pressure and concentration only and not gravity, electrical or magnetic forces. The number of degrees of freedom (F), which will be explained later, is related to the number of components (C) and phases (P) by the following phase rule equation.

    Explanation of Terms

    Phase A phase is a homogenous, physically distinct and mechanically separable portion of a system which is separated from other parts of the system by a definite boundary.

    Examples

  • A gaseous mixture, being thoroughly miscible in all proportions constitutes one phase only.

  • If two liquids are immiscible (e.g., oil and water), they will constitute two separate phases.

  • Two miscible liquids (e.g., water and alcohol) constitute one phase only.

  • A solute completely dissolved in a solvent constitutes a single phase, for example, a solution of glucose in water, salt in water, etc.

  • At freezing point, water consists of three phases.

  • Each solid constitutes a separate phase. A mixture of rhombic and monoclinic sulphur is a two phase system.

  • A solid solution irrespective of the number of solids present is a single phase system.

  • Calcium carbonate decomposes as follows:

  • The above equilibrium has three phases (two solid and one gaseous)

  • Similarly,

  • Here there are two solid phases, Fe and FeO and one gaseous phase consisting of H2O(g) and H2(g). Hence, there are three phases in equilibrium.

  • A homogenous solution of Mohr's salt [FeSO4.(NH4)2SO4.6H2O] constitutes a single phase.

  • Components The number of components of a system at equilibrium is the smallest number of independently variable constituents by means of which the composition of each phase present can be expressed either directly or in the form of a chemical equation. While expressing the composition of a phase in terms of its components, zero and negative quantities are permissible. The components of a system do not represent the number of constituents or chemical individuals present in the system.

    • For example

  • (i) The sulphur system consists of four phases – rhombic, monoclinic, liquid and vapour.

Page 1023 of 1934