Week 2 Flashcards
(18 cards)
What is entropy?
A state function,
For reversible carnot cycle:
dS = dQrev / T
What is the first law?
What is the first law in terms of state functions?
dU = dQ + dW
heat, E and work, W are not state functions
dU =TdS - pdV
what is the entropy change for any isolated system?
dS >= 0
The entropy always increases, or stays the same for reversible reactions.
Note, the universe is an isolated system, this gives direction of time.
What is a Joule expansion?
A type of free expansion.
For a thermally isolated system, the gas is free to expand, so does not do work on the environment as the total volume is fixed (e.g opening valve to another container).
This process is irreversible, as it is quick, so energy is lost to sound etc and the gas cannot be moved back on it’s own (without putting in work).
What happens if state functions are combined?
An infinite number of new state functions can be made, but only few are useful.
What is internal energy and it’s natural variables?
a state function defining the energy os a substance is the absence of fields (on it’s own).
S, entropy, and V, volume are the natural variables of internal energy.
What is Helmholtz free energy, it’s equation and its natural variables?
Helmholtz free energy is the maximum work you can get out of a system at constant temperature and volume. It is minimised at equilibrium.
F = U - TS = -p.dV -S.dT
Volume and temperature are the natural variables.
What is Gibbs Free Energy and it’s natural variables?
Gibbs Free energy is the maximum work you can get out of a system at postance pressure and temperature.
G = U - TS - pV = -S.dT +v.dp
Gibbs Free energy natural variables are pressure and temperature.
What is Enthalpy and it’s natural variables?
Enthalpy is H = U +pV
the natural variables of enthalpy are entropy, S and pressure, p.
note, constane entropy is the same as constant heat: adiabatic.
What is the reciprocal theorem?
dx/dy = 1 / (dy/dx)
What is the Reciprocity theorem (triple product rule?
(dx/dy)_z * (dy/dz)_x * (dz/dx)_y = -1
How would you turn an exact differential into a partial differential os S(T,V)?
dS = (dS/dT)_V .dT + (dS/dV)_T .dV
= (dS/dT)_V + (dS/dV)_T * (dV/dT)_p
How do you derive the maxwell equations?
Using F, H atc and the laws
How can you cool gases?
- Adiabatic expansion - so work is done on the environment to expand
What is a Joule-Thomson (-Kelvin) expansion?
work is done on a piston to push the gas through a plug into another container. Enthalpy is conserved.
What is the inversion temperature?
the temperature below which you can get cooling by the Joule-Thompson expansion.
The gas can be cooled to this temperature by adiabatic expansion first.
What is the element most like an ideal gas and why?
Helium is most like an ideal gas since it is monoatomic and has a full outershell.
When do real gases act like ideal gases?
At high temperatures (room temp+) and low pressures. At low temp/high pressures the intermolecular forces become more important so the ideal gas law has to be replaced by the Van der Wall equation.
