Ideal and Real Gases and Mixtures Reference Page

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Intro to Ideal and Real Gases and Mixtures

Gases are often modeled using the ideal gas law, which provides a simple relationship between pressure, volume, temperature, and the amount of gas present. For many engineering calculations the ideal gas approximation works remarkably well.

However, real gases deviate from ideal behavior, particularly at high pressures and low temperatures. In these cases, engineers must use more advanced models and equations of state to describe gas behavior accurately.

The videos on this page introduce the ideal gas law, explain how real gases deviate from ideal behavior, and demonstrate how real-gas and real-gas mixture calculations are performed.

Ideal Gases EXPOSED: Are They Even Real?!!

This video introduces the ideal gas law and explains the assumptions behind the ideal gas model.

Visuals

When Getting Gassed Isn’t Ideal, Part 1

This video introduces real-gas behavior and explains why gases deviate from the ideal gas law under certain conditions.

Visuals

When Getting Gassed Isn’t Ideal, Part 2

This video continues the discussion of real gases and demonstrates how compressibility charts can be used to estimate real-gas behavior.

Visuals

REAL Gas Mixtures EXPLAINED – Don’t Let This Cost You Points on Your Exam! Part 1

This video introduces the behavior of real gas mixtures and explains how mixture properties can be estimated.

Visuals

REAL Gas Mixtures EXPLAINED – Don’t Let This Cost You Points on Your Exam! Part 2

This video continues the discussion of real gas mixtures and demonstrates example calculations.

Visuals

Examples and Definitions

Definitions

Ideal Gas
A theoretical gas that follows the ideal gas law

\[ PV = nRT,\ \ P\hat{V} = R T \]

In the ideal gas model, gas molecules have no volume and no intermolecular forces. These assumptions allow simple mathematical descriptions of gas behavior.

Real Gas
A gas whose behavior deviates from the ideal gas law because real molecules occupy volume and interact with each other through intermolecular forces.
Compressibility Factor, \(z\)
A measure of how much a gas deviates from ideal behavior.

\[ z \equiv \frac{PV}{nRT} = \frac{P\hat{V}}{RT} \]

For an ideal gas \(z = 1\).
Values different from 1 indicate non-ideal behavior.

Critical Point
The temperature and pressure at which the liquid and gas phases of a substance become indistinguishable. At this point the substance becomes a supercritical fluid.
Critical Pressure, \(P_\mathrm{c}\)
The pressure at the critical point.
Critical Temperature, \(T_\mathrm{c}\)
The temperature at the critical point.
Reduced Pressure, \(P_\mathrm{r}\)
The ratio of the system pressure to the critical pressure.

\[ P_\mathrm{r} = \frac{P}{P_\mathrm{c}} \]

Reduced Temperature, \(T_\mathrm{r}\)
The ratio of the system temperature to the critical temperature.

\[ T_\mathrm{r} = \frac{T}{T_\mathrm{c}} \]

Acentric Factor, \(\omega\)
A parameter used in many equations of state to characterize how a real fluid deviates from simple spherical molecular behavior.
Equation of State
A mathematical relationship between pressure, volume, and temperature that describes the behavior of a substance. Common real-gas equations of state include the Soave–Redlich–Kwong and Peng–Robinson equations.
Law of Corresponding States
A principle stating that many fluids exhibit similar behavior when their properties are expressed in terms of reduced pressure and reduced temperature. This concept allows generalized charts and correlations to be used to estimate real-gas properties.