Gas Pressure

Gas pressure is the force exerted per unit area by gas molecules as they collide with surfaces. This pressure arises from the constant motion and collisions of gas particles.

• Definition: Pressure is the force per unit area exerted by gas molecules.

• Factors affecting pressure: Number of gas particles, volume of the container, and average speed of gas particles.

• Example: Atmospheric pressure is the result of air molecules colliding with surfaces on Earth.

Atmospheric Pressure Effects

Atmospheric pressure varies with altitude and weather conditions. High-pressure regions are associated with clear weather, while low-pressure regions are linked to unstable weather. Pressure decreases with altitude because there's fewer gas particles per unit volume at higher altitudes.

Pressure: The Result of Molecular Collision

• Higher concentration: More collisions, higher pressure.

• Lower concentration: Fewer collisions, lower pressure.

Gas Pressure and Particle Density

The density of gas particles in a given volume directly affects the pressure exerted by the gas.

• Low density: Low pressure. High density: High pressure.

The Barometer

A barometer measures atmospheric pressure using a column of mercury. Atmospheric pressure supports a column of mercury about 760 mm tall at sea level. Mercury's density allows for a shorter column compared to water.

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

Common Pressure Units

• Atmosphere (atm)

• Millimeter of mercury (mm Hg)

• Torr

• Pascals (Pa)

The Manometer

A manometer measures the pressure of a gas in a container by comparing it to atmospheric pressure using a U-shaped tube filled with liquid.

• Gas pressure greater than atmospheric: Liquid level higher on the side open to air.

• Gas pressure less than atmospheric: Liquid level higher on the side connected to the gas.

The Simple Gas Laws

Four basic properties of gases—pressure (P), volume (V), temperature (T), and amount in moles (n)—are interrelated. The simple gas laws describe relationships between pairs of these properties.

Boyle’s Law

Boyle’s law states that the volume of a gas is inversely proportional to its pressure at constant temperature and amount.

• Equation:

Charles’s Law

Charles’s law states that the volume of a fixed amount of gas at constant pressure increases linearly with increasing temperature (in kelvins).

• Equation: (at constant P and n)

Avogadro’s Law

Avogadro’s law states that the volume of a gas is directly proportional to the number of moles at constant pressure and temperature.

• Equation: (at constant P and T)

Ideal Gas Law

The ideal gas law combines the simple gas laws into a single equation relating all four properties.

• Equation:

• R: Gas constant (0.0821 L·atm·mol-1·K-1)

STP

Standard temperature and pressure (STP) are defined as 0°C (273 K) and 1 atm. The molar volume of a gas at STP is 22.4 L.

Molar Mass of a Gas

• Equation:

Mixtures of Gases and Partial Pressure

Gas mixtures, such as air, can be treated as a single gas for many calculations. The pressure due to each component is its partial pressure.

• Dalton’s Law:

• Mole fraction:

• Partial pressure:

Collecting Gases and Vapor Pressure

Gases collected over water contain water vapor. The partial pressure of water vapor depends on temperature and must be subtracted from the total pressure to find the pressure of the dry gas.

Kinetic Molecular Theory

The kinetic molecular theory models gases as particles in constant motion, with negligible size and no intermolecular attractions.

• Postulates: Gas molecules are negligibly small; average kinetic energy is proportional to temperature; collisions are elastic.

• Pressure: Result of collisions with container walls.

Temperature and Molecular Velocities

Average kinetic energy depends on mass and velocity. At the same temperature, lighter molecules move faster than heavier ones.

• Root Mean Square Velocity:

Diffusion and Effusion

Diffusion is the spread of molecules from high to low concentration. Effusion is the escape of molecules through a small hole into a vacuum.

• Graham’s Law of Effusion:

Real Gases

Real gases deviate from ideal behavior at high pressure and low temperature due to intermolecular attractions and finite molecular volume.

• Van der Waals equation: Accounts for molecular volume and attractions.

• Equation: where a and b are Van der Waals constants specific to each gas.

Table of Gas Laws and Relationships