Gas Laws and Properties

Kinetic Molecular Theory of Gases

The Kinetic Molecular Theory provides a model for understanding the behavior of gases. It explains how gas particles move and interact, and forms the basis for the gas laws.

• Random Motion: Gas consists of small particles that move randomly with high velocity, resulting in no definite shape.

• Weak Attractive Forces: The attractive forces between gas particles are minimal, so particles are far apart.

• Volume: The actual volume occupied by gas molecules is extremely small compared to the volume the gas occupies; gases fill any container and are easily compressed.

• Constant Motion: Gas particles move rapidly in straight paths, exerting pressure on the container walls.

• Kinetic Energy and Temperature: The average kinetic energy of gas molecules is proportional to the Kelvin temperature; higher temperature means faster movement and greater pressure.

Basic Properties of Gases

Four fundamental properties describe gases: pressure, volume, temperature, and amount of gas. Each property is measurable and affects gas behavior.

• Pressure (P): The force exerted by gas particles against the walls of the container. Measured in atmosphere (atm), millimeters of mercury (mmHg), torr (Torr), kilopascals (kPa), and pounds per square inch (psi).

• Volume (V): The space occupied by a gas, equal to the size of the container. Measured in liters (L) and milliliters (mL).

• Temperature (T): Determines the kinetic energy of gas particles. Measured in degrees Celsius (°C) and Kelvin (K); Kelvin is required for calculations.

• Amount (n): The quantity of gas present, usually measured in grams (g) or moles (n); moles are required for gas law calculations.

Pressure and Atmospheric Pressure

Pressure is created when gas particles collide with the walls of their container. Atmospheric pressure is the force exerted by air molecules on Earth's surface.

• Atmospheric Pressure: A column of air from the atmosphere to Earth's surface produces a pressure of about 1 atm.

• Altitude Effect: Atmospheric pressure decreases at higher altitudes due to fewer air particles.

• Pressure Units: 1 atm = 760 mmHg = 760 Torr = 101.325 kPa = 14.7 psi.

• Pressure Formula:

Gas Laws

The behavior of gases is governed by several fundamental laws that relate their properties. These laws allow prediction of how changes in one property affect others.

Boyle’s Law: Pressure and Volume

Boyle’s Law describes the inverse relationship between the pressure and volume of a gas, when temperature and amount of gas are constant.

• Inverse Relationship: As pressure increases, volume decreases, and vice versa.

• Formula:

• Example: If a sample of oxygen gas has a volume of 12.0 L at a pressure of 600 mmHg, what is the final pressure when the volume changes to 36.0 L (at constant T and n)?

Charles’ Law: Temperature and Volume

Charles’s Law describes the direct relationship between the temperature and volume of a gas, when pressure and amount of gas are constant.

• Direct Relationship: As temperature increases, volume increases, and vice versa.

• Formula:

• Example: A sample of oxygen gas has a volume of 420 mL at a temperature of 18°C. At what temperature (in °C) will the volume be 640 mL (P and n are constant)?

Gay-Lussac’s Law: Temperature and Pressure

Gay-Lussac’s Law describes the direct relationship between the pressure and temperature of a gas, when volume and amount of gas are constant.

• Direct Relationship: As temperature increases, pressure increases, and vice versa.

• Formula:

• Example: A gas has a pressure of 645 Torr at 128°C. What is the temperature (in °C) if the pressure increases to 824 Torr (V and n remain constant)?

Application: Boiling Point of Water at Different Altitudes

Water boils at a lower temperature in the mountains than at sea level because atmospheric pressure is lower at higher altitudes. Lower pressure means water molecules require less energy to escape into the gas phase, thus boiling occurs at a lower temperature.

References

Timberlake, K. (2018). Chemistry: Introduction to general, organic and biological chemistry (13th ed.). Pearson Education.