REAL VS IDEAL GASES

Ideal Gases

Ideal gas may be defined as a gas which obeys the gas equation (PV=nRT) under all conditions of temperature and pressure; and hence the gas equation is also known as Ideal Gas Equation.

Real Gases

However,no gas is perfect and the concept of perfect gas is only theoretical.Gases tend to show ideal behaviour more and more as the temperature rises above the boiling points of their liquified forms and the pressure is lowered.Such gases are known as real gases.Thus a real gas may be defined as a gas which obeys the gas laws under low pressure or high temperature.

Ideal Gas Real GasIt obeys gas laws (PV=nRT) under all conditions of temperature and pressure.

It obeys gas laws (PV=nRT) only at high temperature and low pressures.

It is hypothetical i.e.does not exist. Nitrogen,helium and hydrogen which do not liquefy easily come nearest to behaving as ideal gases.

All gases are real.

Volume occupied by molecules is negligible as compared to volume of container

The volume occupied by the molecules is not negligible as compared to the total volume of gas

There is no intermolecular force of attraction between the molecules.

The force of attraction are not negligible

Real gases obey the van der Waals equation

wherep is the pressure of the fluid V is the total volume of the container containing the fluid a is a measure of the attraction between the particles         b is the volume excluded by a mole of particles         n is the number of moles R is the universal gas constant,         T is the absolute temperature

Correction for molecular attration

Correction for volume of molecules

The van der Waals constants a and b are different for different gasses

They generally increase with an increase in mass of the molecule and with an increase in the complexity of the gas molecule (i.e. volume and number of atoms)

Substance a (L2 atm/mol2) b(L/mol)

He 0.0341 0.0237

H2 0.244 0.0266

O2 1.36 0.0318

H2O 5.46 0.035

CH4 20.4 0.1383

Deviations From Ideal Gas BehaviourPlotting PV/RT for various gasses as a function

of pressure, P:

The deviation from ideal behavior is large at high pressure. The deviation varies from gas to gas. At lower pressures(<10 atm) the deviation from ideal behavior is typically small, and the ideal gas law can be used to predict behavior with little error.

Deviation from ideal behavior is also a function of

temperature:

As temperature increases the deviation from ideal behavior decreases. As temperature decreases the deviation increases, with a maximum deviation near the temperature at which the gas becomes a liquid.

Liquefaction of Gases

For some gases boiling points and type of intermolecular forces are given.

Gases Boiling Point (0C)

Type of Intermolecular Forces

H2O 100 Hydrogen Bonding, Dipole-Dipole, London Forces

CH3OH 64.96 Hydrogen Bonding, Dipole-Dipole, London Forces

SO2 -10 Dipole-Dipole, London Forces

Cl2 -34.6 London Forces

CO2 -78.5 London Forces

O2 -182.9 London Forces

F2 -188.1 London Forces

He -268.6 London Forces

As the intermolecular forces are weaker, the gas is closest to ideal behaviour. As the strength of intermolecular forces increases, the gas liquefies and deviates from ideal behaviour.

JOULE THOMSON EFFECT

Joule-Thomson effect, the change in temperature that accompanies expansion of a gas without production of work or transfer of heat. At ordinary temperatures and pressures, all real gases except hydrogen and helium cool upon such expansion; this phenomenon often is utilized in liquefying gases.

The phenomenon was investigated in 1852 by the British physicists James Prescott Joule and William Thomson (Lord Kelvin). The cooling occurs because work must be done to overcome the long-range attraction between the gas molecules as they move farther apart. Hydrogen and helium will cool upon expansion only if their initial temperatures are very low because the long-range forces in these gases are unusually weak.

PHASE DIAGRAMSThe simplest phase

diagrams are pressure-temperature diagrams of a single simple substance, such as water. The axes correspond to the pressure and temperature. The phase diagram shows, in pressure-temperature space, the lines of equilibrium or phase boundaries between the three phases of solid, liquid, and gas.

Critical Temperature Gases can be converted to liquids by

compressing the gas at a suitable temperature. Gases become more difficult to liquefy as the

temperature increases because the kinetic energies of the particles that make up the gas also increase.

The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied.

Vapor vs Gas

A vapor condenses very readily to the liquid state under small changes of temperature or pressure or both, and constantly does so under ordinary conditions in nature. It may be said to be very close to the liquid state.

A gas, on the other hand, exists under ordinary conditions in the gaseous state. To change it to the liquid state extreme changes in gaseous and liquid state is required. A gas may be said to be far removed from the liquid state, and can not change to it under ordinary natural conditions.

Vapors are gases which can be liquefied, so a gas above its critical temperature can not be referred to as a vapor.

Helium: Critical temperature: -268 0C. All our nature and life conditions exist above its critical temperature. We will consider it as a gas not a vapor.

Propane : Critical temperature: 97 0C. This substance when in gaseous state will be considered as vapor.At room temperature it will remain gaseous. By placing it in a container and pressurizing it, we can turn it into liquid at room temperature.

Water: Critical temperature: 374 0C. This substance when in gaseous state will be considered as vapor.At room temperature certain amount of water will be liquid. We can turn water into gas by placing it in a container and lowering the pressure.

Properties of Refrigerants

It has a low boiling point, so that at room conditions it stays gaseous.

It has high critical point so that it can be liquefied and vaporized under applicable pressure.

It should be unreactive, cheap, consume less energy

It should not be toxic, flammable , not cause environmental damage and corrosion

Helium: not a refrigerant. It has a low boliling and critical point

Water: not a refrigerant. Liquid at room conditions.

NH3:not a good refrigerant. It has a low boiling point and high critical point but it is toxic.

CCl2F2:not a good refrigerant. It has a low boiling point and high critical point but it damages ozon layer.

Puron (50% difluorometan, 50%penta fluoro etan): a good refrigerant. It has a low boiling point and high critical point.