Chapter 10: Gases – Properties, Laws, and Applications

Physical Characteristics of Gases

General Properties

Gases are a fundamental state of matter with unique physical characteristics that distinguish them from solids and liquids. Understanding these properties is essential for studying their behavior and applications.

• Composition: Most gases are composed mainly of nonmetallic elements with simple formulas and low molar masses.

• Expansion: Gases expand to fill their containers, taking both the shape and volume of the container.

• Compressibility: Gases are highly compressible compared to liquids and solids.

• Density: Gases have extremely low densities under standard conditions.

• Mixing: Two or more gases can form a homogeneous mixture, regardless of their identities or proportions.

Some Common Gases

Examples of Gaseous Compounds at Room Temperature

Many compounds exist as gases at room temperature. These gases have diverse properties and uses, and some are hazardous.

Formula	Name	Characteristics
HCN	Hydrogen cyanide	Very toxic, slight odor of bitter almonds
H2S	Hydrogen sulfide	Very toxic, odor of rotten eggs
CO	Carbon monoxide	Toxic, colorless, odorless
CO2	Carbon dioxide	Colorless, odorless
CH4	Methane	Colorless, odorless, flammable
C2H4	Ethene (Ethylene)	Colorless, ripens fruit
C3H8	Propane	Colorless, odorless, bottled gas
N2O	Nitrous oxide	Colorless, sweet odor, laughing gas
NO2	Nitrogen dioxide	Toxic, red-brown, irritating odor
NH3	Ammonia	Colorless, pungent odor
SO2	Sulfur dioxide	Colorless, irritating odor

Pressure

Definition and Measurement

Pressure is defined as the amount of force applied per unit area. It is a key variable in describing the state of a gas.

• Formula:

$P = \frac{F}{A}$

• Atmospheric Pressure: The pressure exerted by the weight of air in the atmosphere on Earth's surface.

• All gases exert pressure on any surface they contact.

Units of Pressure

Common Units and Conversions

Pressure can be measured in several units, each useful in different contexts.

• Pascals (Pa): The SI unit of pressure. $1\ \text{Pa} = 1\ \text{N/m}^2$

• Bar: $1\ \text{bar} = 10^5\ \text{Pa} = 100\ \text{kPa}$

• Millimeters of mercury (mm Hg) or Torr: Based on the height of a mercury column in a barometer. $1\ \text{atm} = 760\ \text{torr} = 760\ \text{mm Hg}$

• Atmosphere (atm): Standard atmospheric pressure. $1\ \text{atm} = 101.325\ \text{kPa} = 1.01325\ \text{bar}$

Measuring Pressure: The Manometer

Principle and Use

A manometer is a device used to measure the pressure of a gas in a vessel relative to atmospheric pressure. It typically consists of a U-shaped tube containing mercury or another liquid.

• If the gas pressure ($P_{\text{gas}}$) is higher than atmospheric pressure ($P_{\text{atm}}$):

$P_{\text{gas}} = P_{\text{atm}} + P_h$

• $P_h$ is the pressure due to the height difference in the mercury column.

• If the gas pressure is lower, the equation is $P_{\text{gas}} = P_{\text{atm}} - P_h$.

Example: If the height difference in the manometer is 136 mm and atmospheric pressure is 760 mm Hg, then $P_{\text{gas}} = 760 + 136 = 896$ mm Hg.

Standard Pressure

Definition and Importance

Standard atmospheric pressure is the pressure at sea level and is used as a reference point in many calculations.

• $1\ \text{atm} = 760\ \text{torr} = 760\ \text{mm Hg} = 101.325\ \text{kPa} = 1.01325\ \text{bar}$

• Standard Temperature and Pressure (STP): 0°C (273.15 K) and 1 atm.

Additional info:

• Further topics in the chapter (not shown in the images) typically include the gas laws (Boyle's, Charles's, Avogadro's), the ideal gas law, kinetic molecular theory, real gases, and applications such as gas stoichiometry and partial pressures.