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Chemistry for Chemical Engineers 73 Kinetic theory of gases Kinetic theory of gases In order to understand the kinetic theory of gases, it is essential to appreciate the terminology of the area and the underlying dynamics of the different phases of matter. Solids Within a solid material, the molecules are in very close proximity, contacting each other; the only permissible motion is that of vibration. The arrangement of molecules can be regular or random. In a regular system, the solid can be termed as crystalline depending on the length scale under study; for random packings, the solid is usually termed amorphous. These arrangements are held in place by interactions between the molecules, which can be a bond (ionic, covalent, metallic) or an electrostatic interaction (hydrogen bonds, van der Waals forces). Liquids In liquid systems, the molecules again contact but there will be occasional spaces in the structure; these spaces allow molecule movement through the system and a random arrangement results. Similar forces exist as in solid systems but the energy of the molecules themselves have more energy and are, hence, able to overcome these forces preventing a fixed arrangement. Generally, as a result of these spaces in the structure, the density of the liquid is a little less than that of the solid (water is a notable exception to this as solid ice floats on liquid water). Gases For gas systems, molecules have large, 10s of molecule scale, distances between them and are free to move. At usual temperatures and pressures, the intermolecular forces are practically negligible. Phase changes Solid to Liquid If a heat is supplied to a solid, the vibrational energy will increase, allowing the molecules sufficient energy to adopt a less rigid arrangement and become a liquid, through a process known as melting. A specific amount of heat is required to convert one mole of a solid into a liquid and this is equal to the enthalpy of fusion. An inverse enthalpy of the same magnitude is evolved when a liquid becomes a solid, in the process of freezing, as the energy of the molecules reduces enough to allow the attractive interactions to arrange and bind the molecules into a solid form. The application of heat breaks the bonds within a system, and the formation of bonds, increasing order within the system, generates heat. Download free eBooks at bookboon.com
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Chemistry for Chemical Engineers 74 Kinetic theory of gases Liquid to gas If sufficiently more heat is supplied to a liquid, this causes the molecules to increase their motion and relative speed, eventually breaking all the attractive forces binding them into a liquid form and producing a gas as the liquid boils. The inverse process is called condensation as the gas molecules reduce their energy sufficiently to condense into a liquid. The heat involved in the liquid-gas phase transition is called the enthalpy of vaporisation. Occasionally, some solid species form a gas on heating (or the reverse gas
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