Gas Laws Wasilla High School

Kinetic Molecular Theory and Gas Behavior  The word kinetic refers to motion.  The energy an object has because of its motion is called kinetic energy.  According to the kinetic theory, all matter consists of tiny particles that are in constant motion.  The particles in a gas are considered to be small, hard spheres with an insignificant volume.  The motion of the particles in a gas is rapid, constant, and random.  All collisions between particles in a gas are perfectly elastic.

Gases share some general characteristics. a) The rapid, constant motion of particles in a gas causes them to collide with one another and with the walls of their container. b) The particles travel in straight-line paths between collisions. c) A gas fills all the available space in its container.

KMT and Gas Behavior  Gas pressure results from the force exerted by a gas per unit surface area of an object.  An empty space with no particles and no pressure is called a vacuum.  Atmospheric pressure results from the collisions of atoms and molecules in air with objects.  Gas pressure is the result of simultaneous collisions of billions of rapidly moving particles in a gas with an object.

Features of Gases  Compressibility is a measure of how much the volume of matter decreases under pressure.  When a person collides with an inflated airbag, the compression of the gas absorbs the energy of the impact.  Gases are easily compressed because of the space between the particles in a gas.  The distance between particles in a gas is much greater than the distance between particles in a liquid or solid.  Under pressure, the particles in a gas are forced closer together.

 A barometer is a device that is used to measure atmospheric pressure.

Kinetic Energy and Temperature  The particles in any collection of atoms or molecules at a given temperature have a wide range of kinetic energies. Most of the particles have kinetic energies somewhere in the middle of this range.

Kinetic Energy and Temperature  The Kelvin temperature of a substance is directly proportional to the average kinetic energy of the particles of the substance.  Absolute zero (0 K, or –273.15°C) is the temperature at which the motion of particles theoretically ceases.  Particles would have no kinetic energy at absolute zero.  Absolute zero has never been produced in the laboratory.

Gas Laws

Pressure Conversions  The SI unit of pressure is the pascal (Pa).  One standard atmosphere (atm) is the pressure required to support 760 mm of mercury in a mercury barometer at 25°C.

Factors Affecting Gas Pressure  Four variables are generally used to describe a gas. The variables and their common units are  pressure (P) in kilopascals  volume (V) in liters  temperature (T) in kelvins  the number of moles (n).

Boyle’s Law  Boyle’s law states that for a given mass of gas at constant temperature, the volume of the gas varies inversely with pressure.

Sample Problem #1

Charles’ Law: Temperature and Volume  As the temperature of an enclosed gas increases, the volume increases, if the pressure is constant.

Sample Problem #2

Gay-Lussac’s Law: Pressure and Temperature  As the temperature of an enclosed gas increases, the pressure increases, if the volume is constant.

Sample Problem #3

The Combined Gas Law  The combined gas law describes the relationship among the pressure, temperature, and volume of an enclosed gas.

Sample Problem #4

Ideal Gas Law  The gas law that includes all four variables—P, V, T, and n—is called the ideal gas law.  The ideal gas constant (R) has the value 8.31 (L·kPa)/(K·mol).

Sample Problem #5

Ideal Gases and Real Gases  There are attractions between the particles in an ideal gas. Because of these attractions, a gas can condense, or even solidify, when it is compressed or cooled.  Real gases differ most from an ideal gas at low temperatures and high pressures.  We assume that ideal gases are NOT affected by these attractions

Dalton’s Law  The contribution each gas in a mixture makes to the total pressure is called the partial pressure exerted by that gas.

Sample Problem #6

Graham’s Law  Diffusion is the tendency of molecules to move toward areas of lower concentration until the concentration is uniform throughout.  During effusion, a gas escapes through a tiny hole in its container.  Gases of lower molar mass diffuse and effuse faster than gases of higher molar mass.

Graham’s Law  Comparing Effusion Rates  A helium filled balloon will deflate sooner than an air-filled balloon.  Helium atoms are less massive than oxygen or nitrogen molecules. So the molecules in air move more slowly than helium atoms with the same kinetic energy.