Everything Science Grade 11

CHAPTER 5. THERMAL PROPERTIES AND IDEAL GASES 5.8

3. Substitute these valuesinto the original equation.

V =

nRT

p

V =

1 mol× 8. 3 J· K−^1 mol−^1 × 273 K

101 300 Pa

4. Calculate the volume of 1 mole of gas under these conditions
   The volume of 1 mole of gas at STP is 22.4× 10 −^3 m^3 = 22.4 dm^3.

5.8 Ideal gases and non-ideal gas behaviour

ESBBF

In looking at the behaviour of gases to arrive atthe ideal gas law, we have limited our examination to

a small range of temperature and pressure. Almost all gases will obey these laws most of the time, and

are called ideal gases. However, there are deviations at high pressures and low temperatures. So what

is happening at these two extremes?

Earlier when we discussed the kinetic theory ofgases, we made a number of assumptions about the

behaviour of gases. Wenow need to look at twoof these again because they affect how gases behave

either when pressures are high or when temperatures are low.

1. Molecules do occupy volume
   When pressures are very high and the molecules are compressed, the volume of the molecules
   becomes significant. This means that the total volume available for the gas molecules to move is
   reduced and collisions become more frequent. This causes the pressureof the gas to be higher
   than what would normally have been predictedby Boyle’s law (Figure 5.6).

ideal gas

real gas

Volume

Pressure

Figure 5.6: Gases deviate from ideal gas behaviour at high pressure.

2. Forces of attraction doexist between molecules
   At low temperatures, when the speed of the molecules decreases and theymove closer together,
   the intermolecular forces become more apparent. As the attraction between molecules increases,
   their movement decreases and there are fewer collisions between them.The pressure of the gas
   at low temperatures istherefore lower than what would have been expected for an ideal gas
   (Figure 5.7). If the temperature is low enoughor the pressure high enough, a real gas will
   liquefy.