Basic Thermodynamics 3_Part_2

Basic Thermodynamics 3_Part_2 - Variation of number of...

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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Variation of number of particles and composition If the number of particles and/or composition of the system changes during a process, then two independent variables are not sufficient to describe the state of the system. Chemical reactions or exchange with surroundings can lead to the change in composition (number of moles of different species, n i , n j , n k , …). The Gibbs free energy (as well as other thermodynamic potentials) is an extensive property – depends on size of the system and on number of moles of different species, G=G(T,P,n i ,n j ,n k ,…). dG=( G/ P) T,ni,nj,… dP+( G/ T) P,ni,nj,… dT+( G/ n i ) P,T,nj,nk,… dn i + ( G/ n j ) P,T,ni,nk,… dn j + etc. If composition is not changing, dG = VdP – SdT and therefore S = – ( G/ T) P,ni,nj,… and V = ( G/ P) T,ni,nj,… Therefore: i ,... n , T , P k i 1 i i dn n G VdP SdT dG i = = + + = Where i ,... n
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Basic Thermodynamics 3_Part_2 - Variation of number of...

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