Physical Characteristics of Gases

TEKS Objectives (9) The student understands the principles of ideal gas behavior, kinetic molecular theory, and the conditions that influence the behavior of gases. The student is expected to: (C) describe the postulates of kinetic molecular theory (A) describe and calculate the relations between volume, pressure, number of moles, and temperature for an ideal gas as described by Boyle's law, Charles' law, Avogadro's law, Dalton's law of partial pressure, and the ideal gas law (B) perform stoichiometric calculations, including determination of mass and volume relationships between reactants and products for reactions involving gases; and

Kinetic-Molecular (K-M) Theory All matter is made up of particles The particles are constantly in motion Explains the properties of solids, liquids and gases

Five Assumptions for Gases Gases consist of large numbers of tiny particles that are very far apart Collisions in gases are elastic (no loss of kinetic energy) Particles are in continuous, rapid, random motion No forces of attraction or repulsion between particles Average kinetic energy depends on the temperature of the gas

Keep in mind… At the same temperature, a gas’s kinetic energy is proportionate to its size Small size <---------------> Large High temperature <------> Low More energy <------------> Less Ex: At the same temperature, hydrogen has more energy than oxygen

Physical Properties of Gases 1. Expansion: no definite shape or volume 2. Fluidity: particles glide past each other and behave as a fluid 3. Very low density 4. Compressibility: can physically change the volume

Physical Properties of Gases (continued) 5. Diffusion: spreading and mixing of gases depends on the speeds, diameters, and the attractive force between them 6. Effusion: the ability of a gas under pressure to pass through a small opening and is directly proportional to the velocity and mass of the gas

Celsius & Kelvin Conversions When using gas laws, temperature must be in Kelvin Absolute Zero is the theoretical temperature at which all kinetic energy stops Kelvin (K) = Celsius (C°) + 273 Celsius (C°) = Kelvin (K) - 273

Four Properties Needed to Describe a Gas 1. Pressure 2. Volume: how much space it occupies 3. Temperature: a measure of the average kinetic energy 4. n = # of moles of gas ( 1 mole = 6.022 x 1023 molecules)

Pressure Definition: Force per unit area on a surface (how much a specifically sized area is being pushed on) SI unit for force is the Newton (N)

Gas molecules exert pressure on any surface they collide with This pressure is determined by the temp, volume, and # of molecules present

Measuring Pressure/ Units of pressure The atmosphere exerts pressure on the surface of the earth Atmospheric pressure is measured by a barometer A manometer is used to measure pressure in a container At sea level, & zero Celsius, atmospheric pressure will support a column of mercury 760 mm tall 1 mm Hg also known as 1 torr

One atmosphere of pressure (atm) is defined as 760 mm of Hg One pascal (Pa) is the pressure of one N acting on 1 square meter Pa are usually expressed in kiloPascals (kPa) 101.325 kPa = 1atm = 760 mm Hg = 760 torr

STP When comparing volumes of gases, you must know the temp and pressure (they affect volume) For comparison, standard conditions for gases are needed “Standard temperature and pressure” (STP) STP is 1 atm of pressure and 0o C