General Chemistry: Chapter 10 – Gases (Study Notes)

Chapter 10: Gases

10.1 Physical Characteristics of Gases

Gases are a fundamental state of matter with unique physical properties that distinguish them from solids and liquids. Understanding these characteristics 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: Unlike liquids and solids, gases expand to fill their containers completely.

• Compressibility: Gases are highly compressible, meaning their volume can decrease significantly under pressure.

• Density: Gases have extremely low densities compared to solids and liquids.

• Mixtures: Two or more gases can form a homogeneous mixture, meaning their components are evenly distributed.

Example: Air is a homogeneous mixture of nitrogen, oxygen, argon, and other gases.

10.1 Some Common Gases

Many compounds exist as gases at room temperature. Their properties and hazards vary widely.

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

10.2 Pressure

Pressure is a key concept in the study of gases, describing the force exerted by gas molecules on the walls of their container.

• Definition: Pressure is the amount of force applied to an area.

• Formula:

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

• All gases exert pressure on any surface they contact.

• Atmospheric pressure is the weight of air per unit of area at Earth's surface.

Example: Atmospheric pressure at sea level is approximately 101,325 Pa.

10.2 Units of Pressure

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

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

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

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

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

Example: Blood pressure is often measured in mm Hg.

10.2 The Manometer

A manometer is a device used to measure the pressure of a gas in a vessel relative to atmospheric pressure.

• Open-end manometer: Measures the difference in pressure between atmospheric pressure and the gas.

• Formula for gas at higher pressure than atmosphere:

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

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

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

10.2 Standard Pressure

Standard atmospheric pressure is a reference value used in many calculations and is defined as:

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

Application: Standard pressure is used to define standard temperature and pressure (STP) conditions for gases.

*Additional info: More advanced gas laws, kinetic-molecular theory, and real gas behavior are covered in subsequent sections of the chapter, which are not included in these slides but are essential for a complete understanding of the topic.*