Pure Substances Flashcards
What equation gives the change of Gibbs energy with pressure and temperature?
dG = Vdp - SdT
What equation gives the change of molar Gibbs energy at constant temperature?
dG = Vdp - SdT
SdT = 0 at constant T where Vm is constant in the pressure range of interest, so;
dGm = VmdP, which gives:
∆Gm = Vm∆P
(This assumes Vm is constant in the pressure range of interest)
What equation gives the variation of Gibbs energy at constant pressure? (Can assume molar quantities are constant)
At constant P:
dG = Vdp - SdT = -SdT
Hence;
∆Gm = -Sm∆T
Why does molar Gibbs energy increase as pressure increases?
Because in ∆Gm = Vm∆P, all molar volumes are positive, hence ∆G increases with pressure.
What must you assume to use ∆Gm = Vm∆P?
This assumes that Vm is constant (at constant temperature) in the pressure range of interest
Why is the line for gases not straight, like solids & liquids, when ∆Gm = Vm∆P is plotted graphically?
Because you cannot assume Vm is constant with changes in pressure. This can be done for solids and liquids
What is pascals in SI units?
Pa = Nm-2 = kgm-1s-2
Why does ∆Gm = Vm∆P only apply to solids and liquids?
Because the molar volume is effectively independent of pressure, and so the relationship is linear
What is the change in molar Gibbs energy for an ideal gas at constant temperature?
dGm = VmdP
Vm = RT/P
So, ∆Gm = RT ln pf/pi
How does constant pressure affect dGm = Vmdp - SdT? What does this mean?
The Vmdp term is zero
This means an increase in temperature decreases ∆Gm, as the entropy is positive
What is dynamic equilibrium?
It is any point on a phase boundary, in a phase diagram, at which the two adjacent phases are in “dynamic equilibrium”
What is the Clapeyron equation?
When is the Clapeyron equation applied?
The equation is applied to small variation in pressure and temperature where ∆trsH and ∆trsV can be treated as constant
What equation describes the liquid-vapour phase boundary?
The Clausius-Clapeyron equation
When does the liquid-vapour phase boundary terminate?
At the critical point.
At the triple point solid, liquid and vapour are in dynamic equilibrium
