Explain the components approach to the phase rule, Chemistry

Explain the Components approach to the phase rule?

The derivation of the phase rule in this section uses the concept of components. The number of components, C, is the minimum number of substances or mixtures of fixed composition from which we could in principle prepare each individual phase of an equilibrium state of the system, using methods that may be hypothetical. These methods include the addition or removal of one or more of the substances or fixed-composition mixtures, and the conversion of some of the substances into others by means of a reaction that is at equilibrium in the actual system.

It is not always easy to decide on the number of components of an equilibrium system. The number of components may be less than the number of substances present, on account of the existence of reaction equilibria that produce some substances from others. When we use a reaction to prepare a phase, nothing must remain unused. For instance, consider a system consisting of solid phases of CaCO3 and CaO and a gas phase of CO2. Assume the reaction CaCO3(s) → CaO(s) + CO2 (g) is at equilibrium. We could prepare the CaCO3 phase from CaO and CO2 by the reverse of this reaction, but we can only prepare the CaO and CO2 phases from the individual substances. We could not use CaCO3 to prepare either the CaO phase or the CO2 phase, because CO2 or CaO would be left over. Thus this system has three substances but only two components, namely CaO and CO2.

 In deriving the phase rule by the components approach, it is convenient to consider only intensive variables. Suppose we have a system of P phases in which each substance present is a component (i.e., there are no reactions) and each of the C components is present in each phase. If we make changes to the system while it remains in thermal and mechanical equilibrium, but not necessarily in transfer equilibrium, we can independently vary the tem- perature and pressure of the whole system, and for each phase we can independently vary the mole fraction of all but one of the substances (the value of the omitted mole fraction comes from the relation Σixi = 1). This is a total of 2+P (C-1) independent intensive variables.

When there also exist transfer and reaction equilibria, not all of these variables are in- dependent. Each substance in the system is either a component, or else can be formed from components by a reaction that is in reaction equilibrium in the system. Transfer equilibria establish P 1 independent relations for each component (μβi = μαi, μγi = μαi etc.) and a total of C(P-1) relations for all components. Since these are relations among chemical potentials, which are intensive properties, each relation reduces the number of independent intensive variables by one. The resulting number of independent intensive variables is

F = [2 + P(C-1)] - C(P-1) = 2 + C- P

If the equilibrium system lacks a particular component in one phase, there is one fewer mole fraction variable and one fewer relation for transfer equilibrium. These changes cancel in the calculation of F , which is still equal to 2 + C - P. If a phase contains a substance that is formed from components by a reaction, there is an additional mole fraction variable and also the additional relation Σiviμi = 0 for the reaction; again the changes cancel.

We conclude that, regardless of the kind of system, the expression for F based on components is given by F = 2 + C - P. By comparing this expression and F = 2 + s - r- P, we see that the number of components is related to the number of species by

C = s - r

Posted Date: 6/26/2013 5:54:52 AM | Location : United States







Related Discussions:- Explain the components approach to the phase rule, Assignment Help, Ask Question on Explain the components approach to the phase rule, Get Answer, Expert's Help, Explain the components approach to the phase rule Discussions

Write discussion on Explain the components approach to the phase rule
Your posts are moderated
Related Questions
Price ceiling problems: The persistent shortage created by price ceiling will in turn pose the following problems:  Rationing Government may have to apportion the quantity s


how much glucose C6H12O6 you need to start with to prepare 4.6 times 10 to power 4 Kg of C2H5OH if the yield in the reaction is %64.5 C6H12O6+3O2= 4CO2+C2H5oh+3H2O


Local Anesthetics: These influence just only a part of the body insensitive to pain or feeling. Common local Anesthetics are: xylocaine (employed in jelly form), ethyl chlorid

what are the different types of method for the preparation of DDT?

Explain the Phase Diagrams - Ternary Systems? A ternary system is one with three components. We can independently vary the temperature, the pressure, and two independent compos

Chloride of metal is MCl 2  . The formula of its phosphate will be: (1) M 2 PO 4                                                (2)  M 3 (Po 4 ) 2 (3)  M 2 (

Q. Name the three ways in which consumer testing can be done? The three ways by which consumer testing can be done are: inhouse laboratory testing, home testing and institution

what are the signs associated with electron affinity?