Section 5 – Gases

Introduction to Gases

Gases are one of the fundamental states of matter, characterized by their ability to expand and fill any container. Although relatively few substances exist as gases under typical conditions, they play a crucial role in both natural and industrial processes.

• Atmosphere: The layer of gases surrounding Earth, essential for supporting life, absorbing harmful radiation, and acting as a reservoir for exhaust gases.

• Pressure: All gases exert pressure on their surroundings due to the force exerted by gas molecules as they strike surfaces.

• Common atmospheric gases: N2, O2, Ar, CO2, Ne, HCl, etc.

Atmospheric Pressure and Its Measurement

Atmospheric pressure is the force per unit area exerted by the weight of the atmosphere. It is commonly measured using a barometer.

• Barometer: An instrument invented by Evangelista Torricelli to measure atmospheric pressure. It typically consists of a column of mercury whose height changes with atmospheric pressure.

• Standard atmospheric pressure: 760 mm Hg (also called 1 atm).

Factors Affecting Atmospheric Pressure

• Altitude: Atmospheric pressure decreases with increasing altitude because there is less air above the measurement point.

• Weather (Humidity): Higher humidity means more water vapor in the air, which can slightly decrease atmospheric pressure since water vapor is less dense than dry air.

Units of Pressure

Pressure can be measured in several units, and conversion between these units is often necessary in calculations.

• 1 atm = 760 mm Hg = 760 torr = 101,325 Pa = 14.7 psi

• Common units: atm, mm Hg (torr), Pa, psi

Example: To convert 49 torr to atm, mm Hg, and pascals:

Gas Laws

The behavior of gases can be described by several empirical laws, which relate pressure, volume, temperature, and amount of gas.

Boyle's Law

At constant temperature, the volume of a fixed amount of gas is inversely proportional to its pressure.

• Equation:

• Example: If the pressure on a gas increases, its volume decreases, provided temperature is constant.

Charles's Law

At constant pressure, the volume of a fixed amount of gas is directly proportional to its temperature (in Kelvin).

• Equation:

• Example: Heating a balloon causes it to expand as the gas inside increases in volume with temperature.

Avogadro's Law

At constant temperature and pressure, the volume of a gas is directly proportional to the number of moles of gas present.

• Equation:

• Example: Doubling the amount of gas (in moles) at constant temperature and pressure doubles the volume.

The Combined Gas Law

The combined gas law merges Boyle's, Charles's, and Avogadro's laws to relate pressure, volume, and temperature for a fixed amount of gas.

• Equation:

• Used when the amount of gas is constant but pressure, volume, and temperature change.

The Ideal Gas Law

The ideal gas law combines all the simple gas laws into a single equation that relates pressure, volume, temperature, and number of moles for an ideal gas.

• Equation:

• P: Pressure (atm)

• V: Volume (L)

• n: Number of moles

• R: Universal gas constant ( L·atm·mol−1·K−1)

• T: Temperature (K)

Example Calculation Using the Ideal Gas Law

• Calculate the volume of 0.85 mol of nitrogen gas at a pressure of 694.2 torr and a temperature of 315 K.

First, convert pressure to atm: Apply the ideal gas law:

Sample Problems and Applications

• Use the combined gas law to solve for changes in volume, pressure, or temperature when the amount of gas is constant.

• Use the ideal gas law to solve for any variable when the other three are known.

• Always convert temperatures to Kelvin and pressures to the appropriate units for the gas constant used.

Summary Table: Gas Laws

Additional info: These notes cover the foundational concepts of gases in general chemistry, including the measurement of pressure, the relationships between gas variables, and the use of the ideal gas law for calculations. For more advanced topics (e.g., Dalton's Law, Kinetic Molecular Theory, Real Gases), refer to the full version of the notes or a comprehensive textbook.