http://www.ck12.org Chapter 14. The Behavior of Gases
Shown below (Figure14.12) is a graph of PV/RT plotted against pressure for 1 mol of a gas at three different
temperatures: 200 K, 500 K, and 1000 K. An ideal gas would have a value of 1 for that ratio at all temperatures and
pressures, and the graph would simply be a horizontal line, which is clearly not the case. As the pressure begins to
rise, the attractive forces become more significant, causing the volume of the gas to be less than expected and the
value of PV/RT drops under 1. At very high pressures, the volume occupied by the particles themselves starts to
become significant, and the value of PV/RT rises above 1. Notice that the magnitude of the deviation from ideality
is greatest for the gas at 200 K and smallest at 1000 K.
FIGURE 14.12
This graph shows how real gases deviate
from ideal gases at high pressures and at
low temperatures.The ideality of a gas also depends on the strength and type of intermolecular attractive forces that exist between the
particles. Gases whose attractive forces are weak, are more ideal than those with strong attractive forces. At the
same temperature and pressure, neon is more ideal than water vapor because neon’s atoms are only attracted to one
another by weak dispersion forces, while water vapor’s molecules are attracted to each other by relatively stronger
hydrogen bonds. Helium is a more ideal gas than neon because it has even weaker dispersion forces, due to its
smaller number of electrons per particle.
Lesson Summary
- The ideal gas law is derived from the combined gas law and allows the number of moles of a gas to be evaluated
in a single equation along with pressure, volume, and temperature. The value of the ideal gas constant depends
upon the unit of pressure that is used in the problem. - The ideal gas law can be used to calculate pressure, volume, temperature, or number of moles of a gas when
three of the variables are known and the gas is not undergoing a change in conditions. - The ideal gas law can also be used to calculate the density of a gas and to perform stoichiometry problems
with chemical reactions that involve one or more gases. - Real gases deviate most from ideal behavior at low temperature and high pressure. Strong intermolecular
attractive forces between the gas particles results in larger deviations from ideality at a given temperature and
pressure.