Kinetic Theory of Matter

The Kinetic Theory explains the differences between the three states of matter. It states that all matter is made up of moving particles which are molecules or atoms. In solids, the particles are so tightly bound to each another that they can only vibrate but not move to another location.

In liquids, the particles have enough free space to move about, but they still attract one another. In gases, the particles are far apart and can move about freely since there is much free space. Solids change into liquids, and liquids into gases, when the particles gain more kinetic energy, like when being heated and are able to move apart from one another. When the molecules vibrate more quickly upon heating, some of it escapes from the matter. This is what the Kinetic Theory is about.

ü       All matter is composed of small particles.

ü       The particles of matter are in constant motion.

ü     All collisions between the particles of matter are perfectly elastic

Particle motion: atoms move in a straight line between collisions.

·                     An O2 molecule in the air travels about 443 m/s (about 1700 km/hour) at 25 oC.

·                     The diameter of an O2 is about 0.339 nanometers. Each molecule travels about 314 times its own diameter between collisions. At this rate, each oxygen molecule has over four and a half billion collisions per second.

Pressure: the force of particles striking a surface.

·                     The SI unit for pressure is the Pascal.

·                     The Pascal is a derived SI unit equal to Newtons / m2.

·                     1 N = (1 kg)(1 m) / s2

·                     The average air pressure at sea level is 101.325 kilopascals (kPa).

·                     Scientists agree to call this average "standard atmospheric pressure".

Temperature: a measure of the average kinetic energy of the particles of matter.

·                     Kinetic energy is the energy an object possesses because of its motion.

·                     The SI units for energy are the joule.

·                     The joule is a derived SI unit equal to (kilograms)(meters) / s2.

·                     KE = mv2 / 2

·                     The calorie is sometimes used to describe to describe heat. This unit is defined as the amount of heat needed to raise the temperature of one gram of water one degree Celsius. A "food calorie" is 1000 calories, or one Kilocalorie.

Physical states of matter: solid, liquid, gas, and plasma.

The physical states of matter can commonly be defined by their physical characteristics. A solid has a definite volume and shape, a liquid has a definite volume but not a definite shape, and a gas has neither definite volume nor shape. While these are true characteristics, they are not the best description of the physical states.

The kinetic theory can be used to describe the physical states of matter:

·                     Solid - a substance whose particles have a low kinetic energy. The particles of a solid are held close together by intermolecular forces of attraction. Because the particles are so close together, they appear to vibrate around a fixed point.

When the temperature of a solid is raised, the velocity of the particles increases. The collisions between the particles occur with greater force, causing the particles to more farther apart. The ordered arrangement of the solid breaks down and a change in physical state occurs.

·                     Liquid - a substance whose particles have enough kinetic energy to stretch the intermolecular forces of attraction. Collisions between the particles a strong enough to force the particles apart. The particles appear to have a moving vibration because they are still under the influence of the intermolecular forces of attraction.

As the temperature of a liquid is raised, the velocity of the particles increases. The collisions eventually become so great that the particles break all intermolecular forces, begin moving independently between collisions, and a change in physical state occurs.

·                     Gas - a substance whose particles have enough kinetic energy to break all intermolecular forces of attraction. The particles of a gas move independently of each other. The particles move at random because they have overcome the intermolecular forces of attraction.

When a gas is raised to extreme temperatures, over 5000 oC, they have so much kinetic energy that their collisions will break electrons out of the atoms, and a change in physical state occurs.

·                     Plasma (a special form of gas) - plasma can be defined as a charged gas. The particle collisions are violent enough to break electrons out of the atoms, producing particles with charges (electrons and positive ions).

·                     Physical state at room temperature (25 oC) and standard atmospheric pressure: Under these conditions, the physical state of a substance depends mostly on the chemical bond characteristics of the substance. Ionic compounds have strong electric charges holding the ions together and exist as solids. Nonpolar molecular compounds of low molecular mass tend to be gases. Greater molecular mass and greater polarity both tend to make substances denser, producing either liquid or solid.

You may find out more about Kinetic Theory of Matter through this flash movie. You can access it by clicking [Here].

·                     Intermolecular forces of attraction:

Within an atom, these forces are called weak forces, because they are much weaker than chemical bonds between atoms. Weak forces involve the attraction of the electrons of one atom for the protons of another atom. When these forces interact between molecules, they are known as van der Waals forces. Here are some forms of van der Waals forces:

·                     Dipole-dipole forces - two molecules that are both permanent dipoles are attracted to each other.

 A water molecule is a dipole (polar molecule). The hydrogen electron is attracted toward the eight protons of oxygen. The shape of the water molecule causes one side of the molecule to have a slightly positive charge and the other side of the molecule to have a slightly negative charge. Water molecules are attracted to each other because of their polar characteristics. The negative side of one molecule is attracted to the positive side of another molecule. Many of the special properties of water are caused by these dipole to dipole forces.

·                     Dipole-induced dipole forces - a dipole transforms a non-polar molecule into a dipole and an attraction occurs.

·                     Dispersion forces - the attraction of two non-polar molecules. Dispersion forces account for over 85% of the van der Waals forces.