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106116 results in Materials Science

Page 135 of 5306

  • 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.

  • 5 - Engineering Materials

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 307-351

  • Summary

    Cement

    Introduction

    Cement is a binder, a substance that sets and hardens independently, and can bind other materials together. It is used in construction for binding materials like bricks, stones and tiles, and is also used for flooring and plastering purposes.

    Cements used in construction can be either hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement) set and harden because of hydration and hydrolysis reactions between the anhydrous cement powder and water.

    Non-hydraulic cement such as slaked limes (calcium hydroxide mixed with water) harden due to the carbonation reaction by carbon dioxide present naturally in the air.

    Cement alone cannot be used for construction work. It is used as a mixture with sand and crushed stones. It may be used as mortar or concrete. Mortar It is a mixture of cement, sand and water which is used for binding bricks together and for plastering.

    Concrete It is a mixture of cement, gravel (crushed stones), sand and water. It is used in flooring works, columns, beams, foundations, etc. If instead of cement, lime is used, then the concrete is known as lime concrete.

    Reinforced concrete construction (RCC) It is concrete reinforced with steel rods or heavy wire mesh. Reinforced concrete is used in floors, beams, girders, columns, slabs, bridges, etc. It imparts high tensile strength to structures.

    Portland cement was discovered by Joseph Aspdin in 1824. The name Portland cement comes from the fact that the colour and quality of the resulting concrete are similar to Portland stone, a kind of limestone found in England.

    Types of cements

    The different types of cements based on Indian Standards are as follows:

  • Ordinary Portland cement It is the most important type of cement and is commonly called OPC. It is made by grinding and mixing clay and lime bearing minerals in proper proportions and then heating the mixture to nearly 1400 °C. It is not in much use these days, having been replaced by Portland pozzolana cement (PPC).

  • Portland pozzolana cement It can be fly ash-based or calcined clay-based. PPC is obtained by mixing OPC with suitable pozzolans at a certain temperature. Pozzolans are deposits of volcanic ash produced by rapid cooling. When mixed with slaked lime, pozzolans show properties similar to cement when it comes in contact with water. PPC is commonly used for various construction activities.

  • 14 - Surface Chemistry

  • Shikha Agarwal
  • Book:
    Engineering Chemistry
    Published online:
    08 May 2019
    Print publication:
    16 May 2019, pp 757-792

  • Summary

    Introduction

    Various physical and chemical reactions occur at the interfaces or boundaries of two different phases like solid–liquid interface, solid–solid interface, solid–gas interface, liquid–gas interface, liquid–solid interface and liquid–liquid interface. These reactions have been of great interest to the chemists worldwide as they have important applications in industrial and domestic field.

    All these changes occurring at the boundaries of different phases are studied under a branch of chemistry called surface chemistry. Surface chemistry not only deals with the reactions occurring at the interfaces but also studies the nature of surfaces and the reactions taking place at the surfaces. Adsorption and colloidal properties are some of the important effects that occur at the solid – liquid interfaces.

    Adsorption

    The surface of solid or liquid has a tendency to attract and retain molecules of other species (gas or liquids). This phenomenon of attracting and retaining molecules on the surface is termed as adsorption (latin word surbere; to suck up). The material on the surface of which adsorption occurs is termed as adsorbent and the substance adsorbed on the surface is called adsorbate. Now, the question of why adsorption occurs arises. Adsorption occurs because of the presence of unbalanced forces at the surface of liquids and solids. In a solid or liquid, the molecules present in the bulk are attracted equally from all sides and the net force experienced by the molecule in the bulk is zero. However, the molecule that lies near the surface (known as surface molecule) is being attracted only by the molecules below it because there are no molecules above it (Fig. 14.1). Hence, a net downward force is experienced by the surface molecule. Because of this downward force, the surface molecule tends to attract and retain other species (a gas or a dissolved substance). These species do not penetrate inside the molecule but remain on the surface. Hence, their concentration is higher on the surface than in the bulk.

    Common examples of adsorption are

  • (i) Finely divided activated charcoal adsorbs a number of gases like ammonia, sulphurdioxide, chlorine, etc.

  • (ii) Animal charcoal adsorbs dyes and is used for decolorising a number of organic substances.

Page 135 of 5306