武汉大学：《物理化学 Physical Chemistry》课程教学资源（PPT课件讲稿）07 Phase rules

相律 Phase rules 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

相 律 Phase Rules

相律 Phase rules 1. fE Phase Signifies a form of matter that is uniform throughout, not only in chemical composition but also in physical state 2.相数 Phase number 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 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 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 b tailoring materials mechanical and electrical properties

相 律 Phase Rules 1. 相 Phase 2. 相 数 Phase Number

相律 Phase rules 3.物种数 Constituent: A chemical species that is present in a system. For example, a mixture of water and ethanol has 2 consituents 4.组分数C 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, Consituents= 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

相 律 Phase Rules 3. 物种数 4. 组分数 C

相律 Phase rules 例1 CaCO3(s)= Cao(s) Co2(g Phase 1 Phase 2 Gas phase Number of phases, P=3 Number of constituents 3 Number of Components, C=2 CacO, can be expressed in terms of 2 components in two different phases from the stoichiometry of the reaction 1jJ2 Consider the thermal decomposition of ammonium chloride NH,CI(s NH3(g) HcI (g) Number of phases P=2 Number of constituents 3 Number of Components, C=1 NH3 and HCl are fixed in stoichoimetric proportions by the reaction, and compositions of both phases can be specified by nhcl 组分数C=物种数一独立化学平衡数一独立浓度关系数 注意独立浓度关系只限于同一相中

相 律 Phase Rules 例1 例2 组分数 C = 物种数 – 独立化学平衡数 – 独立浓度关系数 注意:独立浓度关系只限于同一相中

相律 Phase rules 组分数C=物种数一独立化学平衡数一独立浓度关系数 注意独立浓度关系只限于同一相中 Fi]3 Consider the thermal decomposition of ammonium chloride NH4CI(s) NH3(g) HCl(g) N B, if additional HCl or NH3 were added to the system, decomposition of the Nhcl would not give the correct gas phase compositions, and either HCl or NH would be invoked as a 2nd component 例4 H2O() 1202(g) H2(g)(at room temp) Number of phases. P=2 Number of constituents 3 Number of Components, C= 3* At room temperature, O2(g)and H ,(g) do not react to form water they are not in equilibrium regarded as independent constituent,so

相 律 Phase Rules 例3 例4 组分数 C = 物种数 – 独立化学平衡数 – 独立浓度关系数 注意:独立浓度关系只限于同一相中

相律 Phase rules 5自由度 F Degrees of Freedom The variance, F, is the number of intensive variables in a system that can be changed independently without disturbing the number of phases in equilibrium 6.相律 Phase rules J.W. Gibbs, regarded as America's first theoretical scientist and the father of chemical thermodynamics wrote that the number of components, C, and the number of phases at equilibrium, P, for a system of any composition F=C-P+ 2

相 律 Phase Rules 5. 自由度 F Degrees of Freedom 6. 相律 Phase Rules

相图 Phase Diagram 相转换 Phase Transition Spontaneous conversion of one phase into another occurring at a characteristic temperature for a given pressure Consider water at p=1 atm Solid Ice is stable phase when T0 Gas Pice >Liquid water when T>0 Solid Transition Temperature, Ttrs stable stable stable Temperature when chemical potentials are equa; for example,μlce=μ uid water at T=0 Temperature, T

相 图 Phase Diagram 相 转 换

相图 Phase Diagram Phase diagram Shows regions of pressure and temperature where phases are thermodynamically stable Vapour pressure Critical Pressure of a gas Solid Sublimation Vapour point in equilibrium with Liquid Pressure the liquid phase Pressure of gas in equilibrium with solid phase Triple point Vapour Phase Boundaries pressure, p Vapour separate regions and show p and T where T two phases exist in equilibrium Temperature, T Triple point Critical Temperature Vapour pressures increase with higher T, as the or solid Boltzmann distribution populates the higher energy states with increasing temperature

相 图 Phase Diagram

单组分系统的相图 One Component systems 单组分F=3-P Phase Y Phase e Four phases in equilibrium Phase a forbidden) Phase F=2, one\p/Phase 8 phase F=0. three phases in equilibrium F= 1. tw phases in equilibrium Temperature, T

单组分系统的相图 One Component Systems 单组分 F = 3 - P

单组分系统的相图 One Component systems Consider pure water In gas phase, cooled at constant p, F=2 b Liquid appears at the phase transition( the boiling temperature), and F=1. We have specified constant p (single degree of Solid (ce freedom, so equilibrium T is not under our contro c Lowering t takes liquid to single phase Vapour liquid region, F=2, T can be varied at will Liquid-solid phase transition at T, with Tf T3 1, and at constant p, T is not under our Temperature, T contr Lowering T further results in single solid phase with F=2, where p and T can be varⅰed

单组分系统的相图 One Component Systems