8: Gases

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Of the three basic phases of matter—solids, liquids, and gases—only one of them has predictable physical properties: gases. In fact, the study of the properties of gases was the beginning of the development of modern chemistry from its alchemical roots. The interesting thing about some of these properties is that they are independent of the identity of the gas. That is, it doesn’t matter if the gas is helium gas, oxygen gas, or sulfur vapors; some of their behavior is predictable and very similar. In this chapter, we will review some of the common behaviors of gases. Gases have no definite shape or volume; they tend to fill whatever container they are in. They can compress and expand, sometimes to a great extent. Gases have extremely low densities, a one-thousandth or less of the density of a liquid or solid. Combinations of gases tend to mix together spontaneously—that is, they form solutions. Air, for example, is a solution of mostly nitrogen and oxygen. Any understanding of the properties of gases must be able to explain these characteristics.

8.1: Solids, Liquids, and Gases- A Molecular Comparison
The state of a substance depends on the balance between the kinetic energy of the individual particles (molecules or atoms) and the intermolecular forces. The kinetic energy keeps the molecules apart and moving around, and is a function of the temperature of the substance and the intermolecular forces try to draw the particles together.
8.2: Pressure - The Result of Constant Molecular Collisions
Pressure is a force exerted over an area. Pressure has several common units that can be converted.
8.3: Kinetic Molecular Theory- A Model for Gases
The physical behavior of gases is explained by the kinetic theory of gases. An ideal gas adheres exactly to the kinetic theory of gases.
8.4: Simple Gas Laws
A gas law is a simple mathematical formula that allows you to model, or predict, the behavior of a gas.
8.5: The Ideal Gas Law
The Ideal Gas Law relates the four independent physical properties of a gas at any time. The Ideal Gas Law can be used in stoichiometry problems with chemical reactions involving gases. Standard temperature and pressure (STP) are a useful set of benchmark conditions to compare other properties of gases. At STP, gases have a volume of 22.4 L per mole. The Ideal Gas Law can be used to determine densities of gases.
8.6: Stoichiometry and the Ideal Gas Law
The relationship between the amounts of products and reactants in a chemical reaction can be expressed in units of moles or masses of pure substances, of volumes of solutions, or of volumes of gaseous substances. The ideal gas law can be used to calculate the volume of gaseous products or reactants as needed.
8.7: Mixtures of Gases
The pressure of a gas in a gas mixture is termed the partial pressure. Dalton’s Law of Partial Pressures states that the total pressure in a gas mixture is the sum of the individual partial pressures. Collecting gases over water requires that we take the vapor pressure of water into account. Mole fraction is another way to express the amounts of components in a mixture.
8.8: Gases (Exercises)
These are exercises and select solutions to accompany Chapter 6 of the "Beginning Chemistry" Textmap formulated around the Ball et al. textbook.