Chapter 13 Kinetic - Molecular Theory

The Nature of Gases The word “Kinetic” means motion The energy an object has due to its motion is called kinetic energy The kinetic theory states that tiny particles in all forms of matter are in constant motion!

The Nature of Gases Three basic rules of the kinetic theory for gases: 1.Gas is made of particles- usually molecules or atoms Small, hard spheres Insignificant volume; relatively far apart from each other No attraction or repulsion between particles

The Nature of Gases 2. Particles in a gas move rapidly in constant random motion Move in straight paths, changing direction only when colliding with something else Average speed of O 2 in air at 20 o C is an amazing 1660 km/h! Random walk is a very short distance

The Nature of Gases 3. Collisions are perfectly elastic meaning kinetic energy is transferred without loss from one particle to another the total kinetic energy remains constant no energy is lost between particles

The Nature of Gases Gas Pressure – defined as the force exerted by a gas per unit surface area of an object Due to: a) force of collisions, b) number of collisions No particles present? Then there cannot be any collisions, and thus no pressure – called a vacuum

The Nature of Gases Atmospheric pressure results from the collisions of air molecules with objects Decreases as you climb a mountain because the air layer thins out as elevation increases Decreases as you climb a mountain because the air layer thins out as elevation increases Barometer is the measuring instrument for atmospheric pressure; dependent upon weather

The Nature of Gases The SI unit of pressure is the “pascal” (Pa) At sea level, atmospheric pressure is about kilopascals (kPa) Older units of pressure include 760 Torr760 mmHg 14.7 psi76 cmHg 1 atmosphere

The Nature of Gases Mercury Barometer –a straight glass tube filled with Hg, and closed at one end; placed in a dish of Hg, with the open end below the surface At sea level, the mercury would rise to 760 mm high at 25 o C- called one standard atmosphere (atm)

The Nature of Gases As the atmospheric pressure outside pushes the Hg in the dish, the level raises in the tube and can be measured.

The Nature of Gases For gases, it is important to relate measured values to standards: Standard conditions for gases are a temperature of 0 o C and a pressure of kPa, or 1 atm This is called Standard Temperature and Pressure, or STP