Grade 11

Grade 11States of matter


Real gases and deviations from ideal behaviour


The study of gases is an essential part of chemistry, and understanding the behavior of gases provides a foundation for understanding many other concepts in this subject. In an ideal world, gases behave in a way described by the ideal gas law, which is a simple model that works under certain standard conditions. However, in reality, gases do not always strictly follow this model. Real gases deviate from ideal behavior due to a number of factors. In this lesson, we will explore the reasons behind these deviations, how they are taken into account, and the importance of understanding real gas behavior in chemistry.

Ideal gas law

Before we learn about real gases, let's revisit the basics of the ideal gas law. This law is often expressed in this equation:

PV = nRT

Where:

  • P is the pressure of the gas
  • V is the volume of the gas
  • n is the number of moles
  • R is the ideal gas constant
  • T is the temperature in Kelvin

This equation assumes that gas particles are point particles that do not interact with each other. In many situations, this assumption simplifies the behavior of the gas and proves useful in calculations. However, it is important to remember that the ideal gas law describes an "ideal" situation that does not always match reality.

Characteristics of real gases

Real gases differ from ideal gases mainly due to the following reasons:

  1. Molecular size: Gases have molecules that occupy space, and this space becomes important at high pressures or low temperatures.
  2. Intermolecular forces: Real gases experience attractive and repulsive forces between their molecules.

Van der Waals equation

The van der Waals equation for real gases adjusts the ideal gas law to take into account the volume occupied by the gas molecules and the intermolecular forces. This equation is given as:

(P + a(n/V)^2)(V - nb) = nRT

Here:

  • a is a constant that corrects the intermolecular forces
  • b is a constant that corrects the volume occupied by the gas molecules

The terms a(n/V)^2 and nb in the ideal gas law adjust for pressure and volume, giving a more accurate description of the behavior of a gas under different conditions.

Visual example: pressure versus volume at constant temperature

Below is a graphical representation of how pressure and volume are related for ideal and real gases. The graph highlights the deviations from ideal behavior as pressure is increased.

Volume Pressure ideal gas Real gas

As shown in this graph, the ideal gas follows a smooth curve. The real gas, on the other hand, experiences a sharp drop in volume as intermolecular forces become significant.

Conditions for real gas divergence

There are certain specific conditions under which real gases deviate the most from ideal behaviour:

  • High Pressure: At high pressure the molecules are forced closer to each other such that the volume occupied by the molecules becomes negligible.
  • Low Temperatures: At low temperatures, the intermolecular forces have a greater effect because the kinetic energy is low, making attractive forces more significant.

If we consider two gas containers at different pressure and temperature, the container with higher pressure or lower temperature will show more deviation from the ideal gas behaviour.

Lesson example: comparison of ideal and real gases

Consider two containers:

  • Container A contains an ideal gas at 1 atm and 273 K.
  • Container B contains real gas at 1 atm and 273 K.

Upon analysis, container A strictly follows the ideal gas law equation. However, container B will require adjustments according to the van der Waals equation, as the volume and interactions of the molecules will affect the calculation of pressure or volume.

Mathematical adjustments

The need to mathematically correct real gases comes from the need to understand and predict how gases will behave under different conditions. This understanding is important for applications ranging from chemical reactions to industrial processes where accurate pressure and volume measurements are required.

Practical implications

Understanding the behavior of real gases is important in a variety of fields. For example, in engineering and technology, systems involving gases require careful calculations to ensure safety and effectiveness. In medical applications, it is important to control gas mixtures for patients. Thus an accurate understanding of real gas behavior becomes indispensable, and science constantly absorbs new findings to refine existing models.

Visual example: the effects of intermolecular forces

The concept of intermolecular forces in gases can be represented graphically.

ideal gas Real gas

In this illustration, ideal gas particles are small and well-spaced, exhibiting uniform motion. Real gas particles exhibit more compact shapes, and their proximity leads to strong interactions, significantly affecting their behavior.

Conclusion

The study of real gases and their deviations from ideal behavior is important in advancing chemical knowledge. While the ideal gas law provides a foundation, real gases and van der Waals adjustments provide a more complete and applicable understanding, especially under non-ideal conditions of high pressure and low temperature. From scientific research to applied sciences, these concepts form a vital mindset for discovering advanced theories and implementing solutions that require a deeper chemical understanding.


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