The Properties Of Gases Flashcards
What is the ideal gas equation?
pV = nRT
When does a real gas behave more like an ideal gas?
As its pressure is reduced towards 0, and at high temperature.
What is the relationship between pressure and volume? (Boyle’s Law)
They are inversely proportional to each other at constant temperature.
What is Charle’s Law?
At constant pressure, volume is proportional to temperature
What is Dalton’s Law?
The pressure exerted by a mixture of ideal gases is the sum of the pressures that each gas would exert if it were alone in the container at the same temperature.
Why is the potential energy of an ideal gas 0?
As the only contribution to the total energy for an ideal gas comes from the kinetics of the molecules - as molecular interactions aren’t considered.
Potential energy = attractions + repulsions
Total energy = kinetics + potential
What is the critical point of a compound?
Critical point is where the substance is in 1 phase, no gas or liquid.
This is called supercritical.
How do attractions affect potential energy?
Attraction forces reduce the potential energy of a gas.
What does Pc, Vc, Tc and SCF stand for?
The pressure, molar volume, and temperature at the critical point.
SCF = Supercritical fluid - The dense fluid obtained by compressing a gas when its temp is higher than its critical temperature, not a true liquid as has no surface that separates from vapour phase. It is also too dense to be a gas.
What is the difference between vapour and gas?
A vapour is the gaseous phase of a substance below its critical temperature which can be liquefied by compression.
A gas is the gaseous phase of a substance above its critical temperature that cannot be liquefied by compression alone.
What is the compression factor?
Z = (Vm / Vm˚)
Where Vm is the molar volume of a gas, and Vm˚ is the molar volume of an IDEAL gas under the same conditions.
What does it mean when Z (compression factor) = 1?
When Z = 1, the gas is ideal.
Hence, deviations from 1 are a measure of how far a real gas is from behaving ideal.
What does it mean if Z < 1?
The molar volume is smaller than that of an ideal gas, and so attractive interactions are dominant.
What does it mean if Z > 1?
The molar volume is greater than that expected of an ideal gas, and so repulsive forces are dominant.
What is the viral equation of state?
Z = (pVm / RT) = 1 + (B/Vm) + (C/Vm2) + …
As Vm = V / n
Under most conditions, C/Vm2 «< B/Vm and so is neglected.
When is C/Vm2 considerable in the viral equation of state?
C/Vm2 becomes large when under highly compressed gases.
What does the van der Waals equation of state consider?
Repulsive interactions: this implies molecules cannot come closer than a certain distance and therefore the actual volume is reduced to V - nb. b is an empirical constant.
Attractive interactions: These reduce the pressure exerted by the gas as attractions slow molecules down, they strike the walls less frequently and with less force.
What are the attractive forces of a gas proportional to?
These attractive forces are proportional to concentration, n/v.
Reduction in pressure = ∂ (n/v)^2
Where ∂ (alpha) is the constant of proportionality.
When can attractive interactions be ignored in the van der Waals equation of state?
They are neglected at high temperatures, as amount of energy input reduces effect of attractive forces
When can repulsive interactions be ignored in the van der Waals equation of state?
At low pressures; the molar volume is so large that V-nb is effectively V.
What are the 3 critical constants that are related to the van der Waals coefficients?
Vc = 3b
Tc = 8a / 27Rb
Pc = a / 27b^2
When does the van der Waals equation of state become the ideal gas equation?
When at high temperature and low pressure. This is when both attractive and repulsive interactions can be ignored.
What is joules in terms of Si units?
Well F=ma, hence N = Kg m s^-2
And Joules = Nm
So, J = Kg m^2 s^-2
What is the conversion from atm to Pa?
1atm = 101325Pa
What can be said about the total pressure of a mix of gases (real and ideal)?
That Pj = Xj P
Where Pj is the partial pressure of a gas
Xj is the mole fraction of gas j. (nj / ntotal)
And P is the total pressure