240_l18 - Phase Transformations Sections 8.1-8.3 7th Edition Sections 6.1-6.3 8th Edition Phases Components and Degrees of Freedom Definitions The

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Phase Transformations Phases, Components and Degrees of Freedom Sections 8.1-8.3, 7th Edition; Sections 6.1-6.3, 8th Edition Definitions The Phase Rule Two-Component Systems Vapour Pressure Diagrams Composition of the Vapour Interpretation of the Diagrams The Lever Rule Last updated: Nov. 29, 2006: updated slide 1.
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Phases, Components and Degrees of Freedom Phase diagrams are extremely useful for systems with multiple components, and serve to describe physical and chemical equilibria over a range of different compositions, as well as points where substances are mutually miscible, or even when a system has to be brought to a specific set of conditions for equilbrium to exist (e.g., pressure, temperature and composition) Phase diagrams are very important in the development of: # alloys # ceramic materials # steels # semiconductors # plastics # superconductors # cosmetics # petroleum product separations # food products # glasses All phase diagrams are developed around one relationship, the phase rule , which was derived by J.W. Gibbs - it can be applied to a wide variety of systems Phase diagrams: pictorial way of understanding the properties of a system
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Some Definitions Phase : Signifies a form of matter that is uniform throughout, not only in chemical composition but also in physical state Number of phases is denoted by P : P = 1 for gas, gaseous mixture, crystal, two miscible liquids, ice P = 2 for slurry of ice and water, immiscible metal alloys Sometimes it is not easy to decide how many phases there are - for example, a solution of solid A in solid B - homogeneous on molecular scale - A atoms are surrounded by B atoms, representative of composition on the whole (example a) a b A dispersion (example b) is uniform macroscopically, but not on the microscopic molecular level - for examples, regions of A are embedded withing a matrix of B, still uniform composition - very important in materials synthesis , especially production of steels, tailoring materials mechanical and electrical properties
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Some Definitions, 2 Constituent : A chemical species that is present in a system. For example, a mixture of water and ethanol has 2 consituents Component : A chemically independent component of the system. The number of components in a system, C , is the minimum number of independent species needed to define the composition of all of the phases present in the system # When no reaction takes place, Constituents = Components # When a reaction can occur, the number of components is the minimum number of species which specifies the composition of all of the phases (or: smallest number of independently variable chemical species to describe the composition of each phase ) CaCO 3 (s) W CaO(s) + CO 2 (g) Phase 1 Phase 2 Gas Phase • Number of Phases, P = 3 • Number of Constituents = 3 • Number of Components, C = 2* *CaCO 3 can be expressed in terms of 2 components in two different phases from the stoichiometry of the reaction
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Components Example Consider the thermal decomposition of ammonium chloride: NH 4 Cl (s) W NH 3 (g) + HCl (g) • Number of Phases, P = 2 • Number of Constituents = 3
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This note was uploaded on 10/24/2009 for the course CHEM 260 taught by Professor Staff during the Spring '08 term at University of Michigan.

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240_l18 - Phase Transformations Sections 8.1-8.3 7th Edition Sections 6.1-6.3 8th Edition Phases Components and Degrees of Freedom Definitions The

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