# Ideal and Real Gases , Thermodynamic Relations | Interview Questions and Answers

Ideal and Real Gases , Thermodynamic Relations | Interview , viva , oral Questions and Answers

1. Define Ideal gas.

It is defined as a gas having no forces of intermolecular attraction. These gases will follow the gas laws at all ranges of pressures and temperatures.

2. What are the properties of ideal gas?

1. An ideal gas consists of a large number of identical molecules.
2. The volume occupied by the molecules themselves is negligible compared to the volume occupied by the gas.
3. The molecules obey Newton’s laws of motion, and they move in random motion.
. The molecules experience forces only during collisions; any collisions are completely elastic, and take a negligible amount of time.

3. Define Real gas.

It is defined, as a gas having the forces of attraction between molecules tends to be very small at reduced pressures and elevated temperatures.

4. What is equation of state?

The relation between the independent properties such as pressure, specific volume and temperature for a pure substance is known as the equation of state.

5. State the Vander Waal’s equation of state.

The van der Waals equation (or van der Waals equation of state) is an equation relating the density of gases and liquids (fluids) to the pressure (p), volume (V), and temperature (T) conditions (i.e., it is a thermodynamic equation of state).

6. State Boyle’s law.

It states that volume of a given mass of a perfect gas varies inversely as the absolute pressure when temperature is constant.

7. State Charle’s law.

It states that if any gas is heated at constant pressure, its volume changes directly as its absolute temperature.

8. Explain the construction and give the use of generalized compressibility chart.

The general compressibility chart is plotted with Z versus Pr for various values of Tr. This is constructed by plotting the known data of one of mole gases and can be used for any gas. This chart gives best results for the regions well removed from the critical state for all gases.

9. What do you mean by reduced properties?

The ratios of pressure, temperature and specific volume of a real gas to the corresponding critical values are called the reduced properties.

10. Explain law of corresponding states.

If any two gases have equal values of reduced pressure and reduced temperature, then they have same values of reduced volume.

11. Explain Dalton’s law of partial pressure.

The pressure of a mixture of gases is equal to the sum of the partial pressures of the constituents. The partial pressure of each constituent is that pressure which the gas would expect if it occupied alone that volume occupied by the mixtures at the same temperatures.

m = mA+mB+mC+……. = mi
mi = mass of the constituent.
P=PA+PB+PC+……. = Pi, Pi – the partial pressure of a constituent.

The number of moles of any gas is proportional to the volume of gas at a given pressure and temperature.

13. What is compressibility factor?

The gas equation for an ideal gas is given by (PV/RT) = 1, for real gas (PV/RT) is not equal to 1
(PV/RT) = Z for real gas is called the compressibility factor.

14. What is partial pressure?

The partial pressure of each constituent is that pressure which the gas would exert if it occupied alone that volume occupied by the mixtures at the same temperature.

15. Define Dalton’s law of partial pressure.

The total pressure exerted in a closed vessel containing a number of gases is equal to the sum of the pressures of each gas and the volume of each gas equal to the volume of the vessel.

16. How does the Vander Waal’s equation differ from the ideal gas equation of state?

The ideal gas equation pV=mRT has two important assumptions,
1. There is little or no attraction between the molecules of the gas.
2. That the volume occupied by the molecules themselves is negligibly small compared to the volume of the gas.
This equation holds good for low pressure and high temperature ranges as the Inter molecular attraction and the volume of the molecules are not of much significance.
As the pressure increases, the inter molecular forces of attraction and repulsion increases and the volume of the molecules are not negligible. The real gas deviates considerably from the ideal gas equation [p+(a/V2)](V-b) = RT

17. Explain Joule-Kelvin effect. What is inversion temperature?

When a gas (not ideal gas) is throttled, the temperature increases up to a point and then decreases. This is known as Joule Kelvin effect. The temperature at which the slope of a throttling curve in T-p diagram is zero is inversion temperature.

18. What is the law of corresponding states?

According to Vander Waals, the theorem of corresponding states (or principle of corresponding states) indicates that all fluids, when compared at the same reduced temperature and reduced pressure, have approximately the same compressibility factor and all deviate from ideal gas behaviour to about the same degree.

19. In what way the Clausius Clapeyron equations is useful?

• Apply the Clausius-Clapeyron equation to estimate the vapor pressure at any temperature.
• Estimate the heat of phase transition from the vapor pressures measured at two temperatures.

20. What are the assumptions made to derive ideal gas equation analytically using the kinetic theory of gases?

The assumptions are:

• Gases are made up of molecules which are in constant motion in straight lines.
• The molecules behave as rigid spheres.
• Pressure is due to collisions between the molecules and the walls of the container.
• All collisions, both between the molecules themselves, and between the molecules and the walls of the container, are perfectly elastic.
• The temperature of the gas is proportional to the average kinetic energy of the molecules.

21. . Write down the two Tds equations.

22. What is Clausius Clapeyron Equation?

Clapeyron equation which involves in the relationship between the saturation pressure, saturation temperature, the enthalpy of evaporation and the specific volume of the two phases involved.

23. State Helmholtz function.

Helmholtz function is property of system and it is given by subtracting the product of absolute temperature (T) and entropy (s) from the internal energy u.
i.e. Helmholtz function = u-Ts

24. State Gibbs Function.

Gibbs function is property of system and is given by
G = u – Ts + pv = h – Ts {since h=u+pv}

Where h = enthalpy

T = Temperature
S = Entropy

25. Have you ever encountered any ideal gas? If so, where?

No. In actual practice, there is no ideal gas which strictly follows the gas laws over the entire range of temperature and pressure. However, hydrogen, oxygen, nitrogen and air behave as an ideal gas under certain temperature and pressure limits.

27. What is meant by equation of state? Write the same for an ideal gas.

The relationship which exists for the state variables of the system in equilibrium is called equation of state.
The equation of state for ideal is given by pV=mRT
Where p – Pressure of gas, V – Volume of gas, m- Mass of gas, R – Gas constant, T – Temperature.

28. Determine the molecular volume of any perfect gas at 600 N/m2 and 300C. Universal gas constant may be taken as 8314 kJ/kg mole-k.

Given:
P = 600 N/m2
T = 300C = 303 K
R = 8314 kJ/kg mole-k
Solution:
Ideal Gas equation, pV=mRT
V=mRT/p=1 x 8314 x 303/600= 4198 m3
/kg-mole.

29. State Charle’s law.

Charle’s law states that “the volume of a given mass of a gas varies directly as its absolute temperature, when the pressure remains constant”.

30. State Regnault’s law.

Regnault’s law states that specific heats of a gas always remain constant.