Structure 1.5.2—Real gases deviate from the ideal gas model, particularly at low temperature andhigh pressure.

Structure 1.5.2—Real gases deviate from the ideal gas model, particularly at low temperature and
high pressure.

What You’ll Learn:

  • Explain the limitations of the ideal gas model.


ideal gas, moving particles, negligible volume, no intermolecular forces, elastic collisions, real gases, deviation, low temperature, high pressure, molar volume, constant, specific temperature, specific pressure, pressure, volume, temperature, amount, ideal gas equation, PV=nRT, combined gas law

Syllabus Links

Structure 2.2—Under comparable conditions, why do some gases deviate more from ideal behaviour than others?

Real gases deviate from the ideal gas model due to the assumptions made while developing the ideal gas concept. The ideal gas model assumes that the gas particles have negligible volume and that there are no intermolecular forces between them. These assumptions simplify the mathematical representation of gas behavior, but they do not always hold true, especially at low temperatures and high pressures. Here’s why:

  1. Low temperatures: At low temperatures, the kinetic energy of the gas particles decreases. As a result, the particles move more slowly, and the effects of intermolecular forces become more pronounced. In the case of real gases, attractive forces between particles cause them to stick together or move closer, deviating from the ideal gas behavior. This deviation is more significant in polar molecules due to their stronger dipole-dipole interactions or hydrogen bonding. At very low temperatures, real gases can condense into liquids or solids, which is not predicted by the ideal gas model.
  2. High pressures: When the pressure is increased, gas particles are compressed and forced closer together. This means that the volume occupied by the particles themselves can no longer be considered negligible compared to the total volume of the gas, as assumed in the ideal gas model. Additionally, as the particles come closer together, the intermolecular forces between them start playing a more significant role, further affecting the gas behavior. In these conditions, the ideal gas model fails to accurately predict the properties and behavior of real gases.

One application of the understanding of real gas behavior and deviations from the ideal gas model is in the design and operation of liquefied natural gas (LNG) plants. LNG is natural gas that has been cooled and compressed to its liquid state, primarily consisting of methane, with small amounts of other hydrocarbons and impurities. The liquefaction process involves cooling the natural gas to around -162°C (-260°F) at near-atmospheric pressure. This process is used for the efficient storage and transportation of natural gas.

Understanding the deviations of real gases from the ideal gas model is crucial in the design and operation of an LNG plant for several reasons:

  1. Accurate modeling of gas behavior: Engineers need to accurately predict the behavior of natural gas under various conditions of temperature and pressure throughout the liquefaction process. Since real gases deviate from the ideal gas model at low temperatures and high pressures, alternative models like the van der Waals equation or other equations of state are used to account for these deviations and design the process more accurately.
  2. Optimizing the liquefaction process: The efficiency of the LNG plant depends on the proper management of heat exchange and compression during the liquefaction process. Understanding the real gas behavior helps engineers optimize these processes by accounting for the specific properties of natural gas, such as the effect of intermolecular forces and the volume occupied by the gas particles.
  3. Separation of impurities: Natural gas often contains impurities such as water, carbon dioxide, and heavier hydrocarbons that need to be removed before the liquefaction process. These impurities can freeze and cause blockages or damage equipment when the gas is cooled to cryogenic temperatures. Understanding the real gas behavior of these impurities and their interactions with methane under various conditions allows engineers to design efficient separation processes.
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