Title: Lesson 11 Ideal and Real Gas Behaviour

Learning Objectives:

– Understand how Ideal Gas assumptions may not be valid for a real gas

An ideal gas is defined as one that obeys the ideal gas law PV=nRT under all conditions. E.g. For one mole of gas, the relationship PV/RT should be equal to 1.

However, this just theory, there is no such thing as an ideal gas. All gases that exist are know as REAL GASES, and these will deviate from ideal behaviour.

Graph showing variation from 1 of real 1 molar gases at different conditions

Q: What conclusions can we draw from the graph and the deviation in relation to pressure and temperature?

• The gas behaves most like an ideal gas at low pressure and shows greatest deviation at high pressure.

•The gas behaves most like an ideal gas at high temperature and shows greatest deviation at low temperature.

BUT WHY??? WE NEED TO QUESTION THE VALIDITY OF THE TWO ASSUMPTIONS WE MAKE ABOUT IDEAL GASES…

1. The volume of the gas particles in negligible2. There are no attractive forces between particles. (Hence an ideal gas can never

liquefy!)

Assumption - VolumeAt low pressure, e.g. 1x105 Pa (STP), the volume

occupied by the particles of a typical gas is only 0.05% of the total volume = negligible = Valid assumption…

But if you increase the pressure to 5x105 Pa, the volume of the particles is about 20% of the total volume = not so negligible…

Therefore, the volume of a real gas at high pressure is larger than that predicted by the ideal gas law and PV/nRT > 1

= nAssumptions The volume of the gas particles in negligible

Assumption – Interactive ForcesAt low pressure, e.g. 1x105 Pa (STP), the particles are so

widely spaced apart, interactive forces are highly unlikely = Valid assumption!

But if you increase the pressure to 3x107 Pa, the particles are much closer, so interactive forces strengthen. This will cause a reduction of pressure of the gas.

Therefore, the pressure of a real gas when at high pressure is smaller than that predicted by the ideal gas law and PV/nRT < 1

= nAssumption There are no attractive forces between

particles.

Temperature At low temperature, the lower kinetic energy of the

particles increase the inter-particle forces. Therefore, the pressure of a real gas when at low

temperature is smaller than that predicted by the ideal gas law and PV/nRT < 1

= n

CONCLUSION: Real gases deviate from ideal behaviour when either or both assumptions are not valid.

Assumptions1. The volume of the gas particles in negligible2. There are no attractive forces between particles. (Hence an

ideal gas can never liquefy!)

Assumption There are no attractive forces between

particles.

Time to practice more Ideal Gas Equations…

• Complete the test yourself questions page 39.

• Questions 33-39.

• Check your answers on page 559.