Behavior Of Real Gases

Behavior Of Real Gases

Deviation From Ideal Gas Behavior: 

                                                   When we do different experiments, we find that real gases don’t follow PV=nRT Equation of ideal gases. So real gases don’t follow Boyle’s law means, if we plot graph between PV and P then we don’t get parallel straight line at all pressures with X-axis.

Real gases in above graph show some significant deviation from ideal gas behavior. As we see-

1) Dihydrogen and helium shows positive deviation means PV value increases with increase in pressure.

2) Methane and Carbon monoxide shows negative deviation and positive deviation means first with increase in pressure, PV value decreases and reaches the minimum then starts increasing with increasing pressure.

• ·       Real gases don’t follow Boyle’s law, Charles law and Avogadro’s law perfectly under the all conditions, So real gases liquefy when they cooled and compressed.

• ·       Under very high pressure attraction forces start operating between molecules of gases so pressure exerted by real gases is lower than that of ideal gas, because in ideal gas there is no attraction force exists at high pressure.

P_ideal=P_real+(an²/V²) …………..1

P_real = observed pressure

an² /V² = correction term

Where ‘a’ is constant

• ·       Under very high pressure gas molecule don’t move freely but restricted to (V-nb) Volume, Where nb is actual Volume occupied by the gas molecules themselves. So we can write gas equation for real gases as :

{P+(an²/V²)}(V-nb)=nRT …………2

Equation 2 is known as van der walls equation.

Where n is no. of moles of the gas.

a and b is vander walls constant and value depends on gas characteristics.

• ·       Deviation from ideal behavior from real gases is measured with compressibility factor z.

Compressibility factor,

 z = PV/nRT  …………3

If z =1 gas is ideal gas because PV = nRT

If z > 1 or z<1, gas is real gas and If z > 1 then it is more difficult to compress gas.

• ·       Boyle’s temperature or Boyle’s point is a temperature at which real gas behaves like ideal gas under appreciable range of pressure.

• ·       Compressibility factor is also defined as ratio between actual molar volume and calculated molar volume

                             i.e. z = V_real / V_ideal

As per above discussion we say that gases behave ideally -

• 1.   At low temperature and high pressure.

• 2.   or, If volume occupied by the gas is very large therefore volume occupied by gas molecule can be neglected in comparison to it.

Liquification Of Gases

·       The process of converting gas into liquid is known as liquification of gas

·       The highest temperature at which gas start liquefying is known as critical temperature (T_c)

·       Volume of one mole of the gas at this critical temperature is known as critical volume (V_c)

·       Pressure at this critical temperature is known as critical pressure (P_c)

·       Gases are cooled below their critical temperature for the liquefication of gases

·       When we apply cooling as well as compression, gases liquefy easily