Properties of Gases and Pressure: Study Notes

Properties of Gases

Introduction to Gases

Gases are one of the fundamental states of matter, characterized by their ability to expand and fill any container. Unlike solids and liquids, gases have unique properties due to the large distances between their particles and their constant, random motion.

• Compressibility: Gases can be compressed easily because the particles are far apart.

• Expansion: Gases expand to fill the volume of their container.

• Low Density: The density of gases is much lower than that of solids and liquids.

• Diffusion: Gases mix evenly and completely when combined due to the random motion of particles.

• Fluidity: Gases can flow and take the shape of their container.

Example: Air in a balloon expands to fill the entire balloon, demonstrating the property of expansion.

Pressure

Definition of Pressure

Pressure is defined as the force exerted per unit area by gas particles as they collide with the surfaces of their container.

• Formula: $P = \frac{F}{A}$ where $P$ is pressure, $F$ is force, and $A$ is area.

• Physical Meaning: The more frequent and forceful the collisions, the higher the pressure.

Units of Pressure

Pressure can be measured in several units, each commonly used in chemistry and physics.

• Atmosphere (atm): Standard atmospheric pressure at sea level is 1 atm.

• Millimeters of Mercury (mm Hg): Also called torr; 1 atm = 760 mm Hg = 760 torr.

• Pascals (Pa): SI unit of pressure; 1 atm = 101,325 Pa.

• Bar: 1 bar = 100,000 Pa.

Conversion of Pressure Units

It is often necessary to convert between different units of pressure in chemical calculations.

• Key Conversion Factors:

  • 1 atm = 760 mm Hg

  • 1 atm = 760 torr

  • 1 atm = 101,325 Pa

  • 1 bar = 100,000 Pa

• Example Conversion: To convert 2 atm to mm Hg: $2 \text{ atm} \times \frac{760 \text{ mm Hg}}{1 \text{ atm}} = 1520 \text{ mm Hg}$

Variables Affecting Pressure

Several variables influence the pressure exerted by a gas in a container.

• Volume (V): Decreasing the volume increases pressure (Boyle's Law).

• Temperature (T): Increasing temperature increases pressure (Gay-Lussac's Law).

• Number of Moles (n): More gas particles result in higher pressure.

Combined Gas Law: Relates pressure, volume, and temperature: $\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$

Example: If the volume of a gas is halved at constant temperature, the pressure doubles.

Unit	Symbol	Equivalent to 1 atm
Atmosphere	atm	1
Millimeter of Mercury	mm Hg	760
Torr	torr	760
Pascals	Pa	101,325
Bar	bar	1.013

Additional info: Boyle's Law ($P_1 V_1 = P_2 V_2$), Charles's Law ($\frac{V_1}{T_1} = \frac{V_2}{T_2}$), and Gay-Lussac's Law ($\frac{P_1}{T_1} = \frac{P_2}{T_2}$) are important relationships describing how gases respond to changes in pressure, volume, and temperature.