REAL VS IDEAL GASES

1. REAL VS IDEAL GASES

2. 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. • The ideal gas obeys the kinetic molecular theory of gases.

3. 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 and high temperature.

4. Deviation from the ideal occurs at high pressure and low temperature. At these conditions, the gas molecules are "squished" together. When the gas molecules are so close together, they experience intermolecular interactions. Also, the molecular volume becomes significant when the total volume is squished down so much. The intermolecular attractions will cause collisions to be sticky and inelastic. At the extremely high pressures and low temperatures, gases cease to be gases at all - they condense into liquids.

6. b for bounce. The term with the constant b is the repulsion term. The greater b is, the more repulsion, which leads to greater pressure.Its value is the volume of one mole of the atoms or molecules. • a for attraction. The term with the constant a is the attraction term. The greater a is, the more attraction, which leads to less pressure. It provides a correction for the intermolecular forces A modification of the ideal gas law was proposed by Johannes D. van der Waals in 1873 to take into account molecular size and molecular interaction forces.

7. Real gases obey the van der Waals equation Correction for molecular attration Correction for volume of molecules • where • p 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 occupied by a mole of particles         • n is the number of moles • R is the universal gas constant,         • T is the absolute temperature

8. 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)

9. Deviations From Ideal Gas Behaviour • Plotting PV/RT for various gases 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.

10. 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.

11. Liquefaction of Gases • For some gases boiling points and type of intermolecular forces are given.

12. 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.

13. 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.

14. 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.

16. PHASE DIAGRAMS • The 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.

17. 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.

18. 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.

19. 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.

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

21. 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.

22. 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.

23. 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.

24. 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

25. Helium: not a refrigerant. It has a low boiling 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.