Chapter 6 The States of Matter 6.1 Observed Properties of Matter 6.2 Kinetic Molecular Theory

Kinetic Molecular Theory of Matter Postulates of the kinetic molecular theory are: Matter is composed of tiny particles called molecules. The particles are in constant motion and therefore possess kinetic energy The particles possess potential energy as a result of at attracting or repelling each other. The average particle speed increases as the temperature increases. The particles transfer energy from one to another during collisions in which no net energy is lost from the system.

Definitions Shape – depends on the physical state of matter. Compressibility – The change in volume of a sample resulting from a pressure change acting on the sample. Thermal Expansion – The change in volume of a sample resulting from a change in temperature of the sample.

Density Density is the number given when the mass of a sample of a substance is divided by the volume of the same sample. d = m(g)/V(ml or cm3)

Learning Check Samples of plumber’s solder, rubbing alcohol, and air are collected. The volume and mass of each sample are determined at 20oC as follows: solder – volume = 28.6 ml, mass = 268.8 g; alcohol – volume = 100.0 ml, mass = 78.5 g; air – volume = 500.0 ml, mass = 0.602 g. Calculate the density of each substance in grams per milliliter.

Solution Solder: d = 268.8 g = 9.40 g/ml 28.6 ml Alcohol: d = 78.5 g = 0.785 g/ml 100.0 ml Air: d = 0.602 g = 0.00120 g/ml 500.0 ml

Kinetic and Potential Energy These postulates contain two new terms, kinetic and potential energy. Potential energy is stored energy. Kinetic energy is energy in motion. Mathematically, kinetic energy is calculated as: KE = ½ mv2 where m (g) is the particle mass and v (cm2/sec2) is the velocity.

Learning Check Calculate the kinetic energy of two particles with masses of 2.00 g and 3.00 g if they are both moving with a velocity of 15.0 cm/s.

Solution The kinetic energy of the 2.00 g particle is KE = ½ mv2 = ½ (2.00 g)(15.0 cm/s)2 = 225 g cm2 sec2 The kinetic energy of the 3.00 g particle is KE = ½ mv2 = ½ (3.00 g)(15.0 cm/s)2 = 338 g cm2 The more massive particle has more kinetic energy.