MCAT General Chemistry Review by Princeton Review

Author:Princeton Review [Review, The Princeton]
Language: eng
Format: epub
ISBN: 978-1-101-88241-2
Publisher: Random House Children's Books
Published: 2015-03-16T16:00:00+00:00

= 16.6 atm – 0.9 atm = 15.7 atm

This represents a 4% decrease in pressure from that predicted by the ideal gas law.

To underscore the concept that gases behave more ideally at higher temperatures, if we increase the temperature of the system for any gas, the first term in the van der Waals equation approaches the pressure of the ideal gas while the second term remains unchanged. For example, if the temperature of our systems above is increased by 100 K (to 600 K), two moles of an ideal gas would exert 19.7 atm of pressure, while the van der Waals equation predicts pressures of 19.7 atm and 19.1 atm for oxygen and water, respectively. Therefore, we can see that at increased temperature the real gas (H2O) behaves more ideally since it now deviates by only 3% from the pressure predicted by the ideal gas law.

So conceptually, why do higher pressures and lower temperatures cause larger deviations from ideal behavior? As pressure increases, gas particles become closer to one another. This accentuates the effects of attractive intermolecular forces, causing a decrease in observed pressure (Preal < Pideal). Similarly, at low temperatures intermolecular forces become more important, and when taken to an extreme, cause condensation to occur. Liquids aren’t very ideal gases. In addition, when gas particles are packed closer to one another at high pressures, particle volume of the gas itself begins to limit the free space in which the gas particles can move (Vreal < Videal). However under extremely high pressure, these particles can begin to repel one another leading to an increase in volume.

To summarize, those gases that behave most ideally have the weakest intermolecular forces and the smallest molecular weights (and volumes). Furthermore, by maintaining conditions of high temperature and low pressure, the potential interactions between particles are minimized and particle volume remains insignificant compared to the container size, helping to favor more ideal behavior for all gases.

Example 8-8: Of the following, which gas would likely deviate the most from ideal behavior at high pressure and low temperature?

A) He(g)

B) H2(g)

C) O2(g)

D) H2O(g)

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