KEY2CHEM

Real Gases

Ideal gases are those where interparticle interaction is assumed to be negligible and whose particle volume is assumed to be insignificant relative to the overall volume of the container. At low temperatures and high pressures, interparticle interaction and particle volume becomes significant. Interparticle interactions cause an overestimation of the gas pressure based on the Ideal Gas Law, and particle volume causes an underestimation of gas pressure based on the Ideal Gas Law.

Example 1.

Which gas is expected to have the largest deviation from ideal behavior?  Assume all the gases are at the same temperature and external pressure.

A. $$\require{mhchem}\ce{He}$$

B. $$\require{mhchem}\ce{NH3}$$

C. $$\require{mhchem}\ce{O2}$$

Solution

B. $$\require{mhchem}\ce{NH3}$$

$$\require{mhchem}\ce{NH3}$$ displays strong hydrogen bonding interactions between its molecules (the others have dipole-dipole). Because the electrostatic forces holding $$\require{mhchem}\ce{NH3}$$ molecules together are stronger, the interparticle forces are more significant, causing deviation from ideal behavior.

Example 2.

The _____________ between particles in ideal gases are assumed to be negligible. In real gases, it is significant.

A. interparticle attraction

B. interparticle spacing

C. interparticle collision

Solution

A. interparticle attraction

Ideal gases are those whose particles do not have interparticle attraction and the volume of the particles is negligible. The particles are considered to be far apart, in constant random motion, and undergoing elastic collisions with other particles and the container.

Example 3.

When interparticle interactions are significant, the measured gas pressure for a real gas will be _______ the pressure calculated for an ideal gas.

A. greater than

B. less than

C. the same as

Solution

B. less than

The Ideal Gas Law overestimates the gas pressure compared to a real gas when interparticle forces are significant. Interparticle forces essentially slow down moving particles as they collide with container walls. This decreases the force per unit area, decreasing the actual pressure compared to its ideal calculated value.