2015/6/15
Physical Chemistry II
4
Thermodynamics of phase equilibria
The phase rule and its deduction
Phase, number of components, degrees is a form of matter that is homogeneous in
2015/6/15
Physical Chemistry II
5
Thermodynamics of phase equilibria
The phase rule and its deduction
Some basic conceptual issues need to take care when discussing ―phase‖ :
Gas, no matter how many kinds of them are mixed together, there
exists only one gas phase Liquid, depending on its miscibility, one and coexistence of two or three phases are possible Solid, one solid generally constitutes one phase by itself. No matter how well-mixed are two different solid powders, there still exist two phases (Except for solid solution, which is a single phase)
T = T = … =T
(2) Conditions for baric equilibrium: when reaching equilibrium, each phase is at the same pressure
p = p = … =p
2015/6/15
Physical Chemistry II
9
Thermodynamics of phase equilibria
The phase rule and its deduction
Phase, number of components, degrees of freedom
degree of freedom（自由度）
2015/6/15
Physical Chemistry II
6
Thermodynamics of phase equilibria
The phase rule and its deduction
III) In general, mixtures of gases only form a single phase. Since
8
Thermodynamics of phase equilibria
The phase rule and its deduction
(3) Conditions for phase equilibrium: Each substance B in all phases has the same chemical potential, equilibrium phase transition is reached
The phase rule and its deduction
Phase equilibrium is one of the most important parts of
thermodynamics in chemistry and chemical engineering. Many processes, for example, dissolution, distillation, extraction, recrystallization, purification and polymorph analysis, needs the knowledge of phase equilibria in heterogeneous systems.
chemical composition and physical state. Two immiscible liquids (or liquid/solid mixtures with different compositions) separated by a distinct boundary or interfaces. The change of macroscopic properties on the interface is very huge. The number of phases in thermodynamic equilibrium with each other is denoted as
I) Phase is homogeneous, but not necessarily continuous. A single phase could be separated into many particles or liquid drops, or dispersed in other phases. Therefore, a clear interface must exist between two different phases, while the existence of an interface does not necessarily mean that there are different phases. II) Being homogeneous but with different properties means that they are not the same phase. For example: iron and sulfur powers can be grinded into very fine and homogeneous state seen with the naked eye, but microscopic observation still reveal two phases. Since no matter how finely grinded, molecularly mixed state will never be reached, i.e., molecular level dispersion not possible, there still exist two phases.
all different types of gases can be molecularly mixed in any ratio, no interface exists between them.
Liquid, depending on its miscibility, there can be one or two phases. Two totally miscible liquids form a single phase, while partly miscible ones form two phases. For example: the benzene-water system consists of two phases, whereas the ethanol-water system consists of only one phase.
Normally, unless they form solid solution, there exist many distinct solid phases. For example: alloy consists of one phase, as alloy is a solid solution formed by molecularly mixed two types of metals.
2015/6/15
Physical Chemistry II
7
Thermodynamics of phase equilibria
The phase rule and its deduction
General requirements for equilibrium in heterogeneous systems In a closed heterogeneous system, heat/ material exchange and work delivery could occur between different phases. For the thermodynamic equilibrium between multiple phases, it actually contains the following four equilibrium conditions: (1) Conditions for thermal equilibrium: consider the system consisting of ,,, phases, when reaching equilibrium, each phase has the same temperature
Phase rule and its deduction Phase diagrams for one-component systems Second-order phase transitions Two-component phase diagrams Three-component phase diagrams
Extraction
2015/6/15
Recrystallization
Physical Chemistry II
2
Thermodynamics of phase equilibria