Ideal and Real Gases Properties
Compare ideal and real gases by one gas equation assumed that the gaseous molecule which obeys ideal gas law under all conditions of temperature and pressure are called ideal gases but which does not obey are the example real one. Ideal gas law formula can use for comparisons of the physical property like density, molecular weight, diffusion, thermal expansion, compression, etc for ideal and real gases in physics or physical chemistry. An ideal gas, really hypothetical one, because under the maintained temperature and pressure they transformed into the liquid state but we assumed that perfect gases molecule have no intermolecular attraction.
Physical Properties of the Ideal Gases
- An ideal gas can not be liquefied because the gaseous molecule has no inter-molecular attraction.
- The coefficient of thermal expansion(ɑ) depends on the temperature of the gases and does not depends on nature.
- The coefficient of compressibility(β) similarly depends on the pressure and will be the same for all gases.
- When pressure plotted against volume at a constant temperature a rectangular hyperbola curve obtained.
- When PV is plotted against pressure at a constant temperature a straight line parallel plot obtained.
- If the gases molecule passes through a porous plug from higher pressure to lower pressure within the insulated enclosure, there will be no change in the specific heat or temperature. This confirms that the ideal gases have no inter-molecular attraction.
Thermal Expansion and Compressibility Formula
The thermodynamics formula of the coefficient of thermal expansion and compressibility of a gas
- For 1-mole ideal gases, PV = RT, hence α = R/PV = 1/T. Therefore, the thermal expansion will be independent of nature and will be a function of temperature only. For example, the coefficient of thermal expansion for hydrogen and carbon dioxide gases 2.78 × 10-7 and 3.49 × 10-7 respectively at 0°C and 500 atm pressure.
- Compressibility factor (β) = RT/P2V = 1/P. Therefore, β should be a function of pressure only and the same for all gases. But experimentally the coefficient of compressibility has been found to be individual property.
Physical Properties of Real Gases
- Real gas could be liquefied because the gaseous molecule has the property of intermolecular attraction which helps to coalesce the molecule.
- The coefficient of thermal expansion (α) depends on the nature of the gaseous molecule.
- The coefficient of compressibility (β) also found to depend on the nature of the gases.
- When pressure plotted against volume a rectangular hyperbola curve obtained only at a high temperature above the critical temperature.
- But a temperature below the critical temperature (TC), the molecule can be liquefied after certain pressure depends on temperature. Liquid and gas can be indistinguishable property in the critical point of the gases.
When PV is plotted against pressure for real or Van der Waals gases, the Amagat curve obtained.
- Real gases pass through porous plug from higher pressure to lower pressure within the insulated enclosure, there occurs a change of temperature.
- Real gases have inter-molecular attraction and when expands, the molecules have to spend more kinetic energy to overcome inter-molecular attraction compare to an ideal gas. Therefore, the temperature drops down.
Example of compression of the Gas
The value of Z decreases attains minimum and then increases with the increased pressure. Hydrogen and helium gas have baffled this compression trend and the curve rise with increased pressure from the very beginning. Carbon dioxide can be easily liquified and Z dips sharply below the ideal gas line in the low-pressure region.
TB called Boyle temperature, the initial slope at TB zero. At TB, the Z vs P line of the tangent to that of real gas when pressure approaches zero but latter rises above the ideal line only very slowly. Therefore, at TB real gas behaves ideally over a wide range of pressure. Because the effect of the size of the gaseous molecule and intermolecular forces roughly balanced each other. Hence, the Boyle temperature of hydrogen, helium molecules is -156°, -249°C respectively.
For hydrogen and helium, TB lowers then 0⁰C temperature so Z values greater than unity. For nitrogen, hydrocarbon like methane, and ammonia gas TB greater compare then 0⁰C, thus Z values less than unity in the low-pressure region.
Compare Compressibility of ideal and real Gases
An example of a single parameter called the compressibility used to compare the extent of deviation of the real gases from ideal behavior in learning chemistry or physics. For ideal gas molecules, Z = PV/RT.
- When Z=1, there is no deviation from the ideal behavior.
- When Z ≠ 1, the departure of the unit value of Z is a measure of the extent of non-ideality.
- Zく1, the gas is a more compressible then ideal gases, and when Z 〉1, less compressible then ideal gases.