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Thermodynamics Lecture 5 (What is equilibrium (Two opposing processes…

- - - - At T=0, all the motion of the atoms is eliminated and there is no thermal disorder. Moreover if the substance is perfectly crystalline, with every atom in a well defined location, there is no spatial disorder either,
        We can therefore say that when T = 0 the ENTROPY = 0 also
      - The third law of thermodynamics, the entropies of all perfectly crystalline substances are the same at T=0
- - - - The change in the Gibb's free energy, DELTA G is then the criterion for the feasibility of a chemical change- remember if it tells us nothing about the rate of a reaction or the mechanism that it may follow - these lie in the experimental science of kinetics
        
        If DELTA G < 0 = spontaneous or irreversible process
        
        if DELTA G > 0 = process is not feasible
        
        If DELTA G = 0 equilibrium
    - - Equilibrium at the boiling point between a liquid and a vapor
        DELTA H>0 = latent heat of vaporization absorbed by the liquid
        DELTA S> 0 = because the gaseous state is much more disordered
        DELTA G = DELTA H - T DELTA S
        DELTA G = 0 at equilibrium
        at sufficiently high T (ie when water reaches boiling point) T DELTA S becomes equal to DELTA H
- - - - The final and most USEFUL equation is found by taking the complete differential of G and substituting for dH from the second matter equation, G=H-TS dG=dH-TdS-SdT therefore dG=Vdp-SdT
- - - - Vapor pressure increases with temperature because as the temperature is raised more molecules ave sufficient energy to leave their neighbors in the liquid and escape
- - - - If we were considering the equilibrium between a solid and a liquid the focus would be different in this case we would typically be interested in the change in melting point as the pressure is increased, the relation between dT and dp that ensures that in either case the two phases remain in equilibrium is given by the CLAPEYRON EQUATION for the slope of the phase boundary at any temperature dp/dT = d trans H/T d trans V
        or dp/dT= dS/dV
        This is because DELTA S = DELTA q rev/T = DELTA H/T
        
        When converting from solid to gas or liquid to gas we can safely assume that the molar volume of gas is much greater than that of either liquid or solid, next we suppose that the vapor behaves as a perfect gas and write its molar volume as Vm(g) = RT/p
        then dp/dT=DELTA vap H/T(RT/p) =
        
        We have seen that as the temperature of a liquid is raised, its vapor pressure increases, what we observe, however, depends on whether the heating takes place in a closed or an open container
        OPEN VESSEL
        at a certain temperature the vapor pressure becomes the same as the external pressure, at this temperature the vapor can expand indefinitely, moreover because there s no constraint on expansion bubbles of vapor can form throughout the body of the liquid the condition known as boiling
        The pressure at which the vapor pressure of a liquid is equal to the external pressure is called the boiling temperature
        
        CLOSED VESSEL, when a liquid is heated in a sealed container the density of the vapor phase increases, the density of the liquid phase decreases, there comes a stage at which the two densities are equal and the interface between the two fluids disappears, this disappearance occurs at the critical temperature