Ideal Behavior

Under what conditions do real gases behave most like an ideal gas?

• Low temperature and low pressure

• Low temperature and high pressure

• High temperature and low pressure

• High temperature and high pressure

Answer explanation

At low temperatures and high temperatures, gas particles move slower and spend more time around each other. When the gas particles are close together, this can cause more attractive forces forces between particles. When particles are attracted to each other, IMFs become significant and the particles aren't hitting the walls of the container as often. This would cause deviations from the ideal gas law.

To avoid deviations, a gas must be at a high temperature (far from the gas condensing into a liquid) and low pressure.

What gases deviate most from ideal behavior?

• polar molecules with small molar masses

• polar molecules with large molar masses

• nonpolar molecules with small molar masses

• nonpolar molecules with large molar masses

Answer explanation

Polar molecules and larger molecules behave less ideally than smaller non-polar molecules. The IMFs between polar molecules and larger polarizable molecules can cause these gas molecules to exert attractive forces on one another, deviating from ideal behavior.

Which of the following behaves most like an ideal gas at the conditions indicated?

• H₂(g) molecules at 10^-3 atm and 200°C

• O₂(g) molecules at 20 atm and 200°C

• SO₂(g) molecules at 20 atm and 200°C

• NH₃(g) molecules at 20 atm and 200°C

• NH₃(g) molecules at 20 atm and 300°C

Answer explanation

Polar molecules like SO₂ and NH₃ behave less ideally than non-polar molecules like H₂ and O₂. The IMFs between polar molecules and larger polarizable molecules can cause these gas molecules to exert attractive forces on one another, deviating from ideal behavior.

The table below contains information about samples of four different gases at 273 K. The samples are in four identical rigid containers numbered 1 through 4.

The best explanation for the lower pressure in container 4 is that SO₂ molecules

• have a larger average speed than the other three gases

• occupy a larger portion of the container volume than the other three gases

• have stronger intermolecular attractions than the other three gases

• contain 𝜋 bonds, while the other gases contain σ only bonds

Answer explanation

When particles are attracted to each other, IMFs become significant and the particles aren't hitting the walls of the container as often. This would cause the pressure to be lower than predicted by the ideal gas law.

Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference?

• The particle volume of Ar is greater than that of Ne.

• Ar atoms have more valence electrons than Ne atoms have, so atoms have greater interparticle forces.

• The intermolecular forces between Ne atoms are greater than those between Ar atoms.

• Ar atoms are more attracted to the walls of the container than Ne atoms are.

Answer explanation

The ideal gas law assumes that the volume of gas particles is negligible compared to the volume of their container. At extremely high pressures, the particles are relatively close together and the space taken up by the gas particles is not negligible. This factor is more significant for gases composed of larger particles such as Ar.

Ar and Ne are in the same group of the periodic table and have the same number of valence electrons, so valence electrons cannot explain the difference in behavior.

Interparticle forces do cause deviation from ideal behavior, but the interparticle forces between Ne atoms are smaller than those between Ar atoms. Greater interparticle forces result in greater deviation from ideal behavior.

Gas particles collide with the walls of their container, but it is attraction to other gas particles that leads to nonideal behavior.

Under which of the following conditions of temperature and pressure will H₂ gas be expected to behave most like an ideal gas?

• 50 K and 0.10 atm

• 50 K and 5.0 atm

• 500 K and 0.10 atm

• 500 K and 50 atm

Answer explanation

At low temperatures and high pressures, gas particles move slower and spend more time around each other. When the gas particles are close together, this can cause more attractive forces forces between particles. When particles are attracted to each other, IMFs become significant and the particles aren't hitting the walls of the container as often. This would cause deviations from the ideal gas law.

To avoid deviations, a gas must be at a high temperature (far from the gas condensing into a liquid) and low pressure.

Which of the following best helps explain why the pressure of a sample of CH₄ (molar mass 16 g/mol) is closer to the pressure predicted by the ideal gas law than a sample of NH₃ (molar mass 17 g/mol)?

• NH₃ molecules are polar while CH₄ molecules are not, and the greater attractions between NH₃ molecules cause the molecules to collide with the walls of the container with less force.

• NH₃ molecules have a greater molar mass than CH₄ molecules, so the NH₃ molecules collide with the walls of the container with more force.

• CH₄ molecules have more hydrogen atoms than NH₃ molecules, so CH₄ molecules have more hydrogen bonding and greater intermolecular forces.

• CH₄ molecules are larger than NH₃ molecules, so the actual CH₄ molecules take up a significant portion of the volume of the gas.

Answer explanation

The ideal gas law assumes that attractions between gas particles are negligible. The intermolecular forces between NH3 molecules are greater than those between CH4 molecules due to hydrogen bonding. Hydrogen atoms connected to carbon atoms does not result in hydrogen bonding. Gas pressure is due to collisions of gas particles with the walls of the container. If gas particles about to hit the walls of a container are attracted to nearby gas particles, they will hit the walls of the container with less force than expected.

The relative masses of gas particles do not account for differences in the ideality of gases.

Gases made of larger gas particles tend to deviate more from ideal behavior than gases made of smaller particles.

The diagrams above represent two samples of Xe gas in containers of equal volume at 280 K. Which of the following correctly compares the two samples in terms of their deviation from ideal gas behavior and explains why?

• The gas in sample 1 would deviate more from ideal behavior because the average distance an Xe atom travels before colliding with another Xe atom is greater.

• The gas in sample 2 would deviate more from ideal behavior because the Xe atoms are closer together, leading to an increase in intermolecular attractions.

• The gas in sample 2 would deviate more from ideal behavior because the average speed of the Xe atoms is less, leading to an increase in intermolecular attractions.

• The gases in both sample 1 and sample 2 would show the same deviation from ideal behavior.

Answer explanation

There will be more deviation from ideal gas behavior in sample 2 because the molecules are, on average, closer together, leading to a larger impact from the intermolecular attractive forces, which increase as distance between the molecules decreases.

A vessel contains Ar(g) at a high pressure. Which of the following statements best helps to explain why the measured pressure is significantly greater than the pressure calculated using the ideal gas law?

• The molar mass of Ar is relatively large.

• A significant number of Ar₂ molecules form.

• The attractive forces among Ar atoms cause them to collide with the walls of the container with less force.

• The combined volume of the Ar atoms is too large to be negligible compared with the total volume of the container.

Answer explanation

The ideal gas law assumes that the volume of gas particles is negligible compared to the volume of their container. At extremely high pressures, the particles are relatively close together and the space taken up by the gas particles is not negligible.

The relative masses of gas particles do not account for differences in the ideality of gases.

If gas particles about to hit the walls of a container are attracted to nearby gas particles, they will hit the walls of the container with less force than expected, resulting in lower pressure.

Ar₂ will not form as Ar is an inert gas.

Which gas deviates most from ideal behavior?

• SO₂

• Ne

• CH₄

• N₂

• H₂

Answer explanation

Polar molecules like SO₂ behave less ideally than non-polar molecules like Ne, CH₄, N₂, and H₂. The IMFs between polar molecules and larger polarizable molecules can cause these gas molecules to exert attractive forces on one another, deviating from ideal behavior.