The Kinetic Molecular Theory Monday, April 25 th, 2016

Review Matter can exist in several states:  Solids have a definite shape and volume, and are not compressible.  Liquids have a definite volume but not a definite shape. They are not compressible.  Gases do not have a definite shape or volume. Gases are compressible.

The Kinetic Molecular Theory Kinetic molecular theory (KMT):  Explains the observed properties of matter in its different states. Particles are in constant, rapid, random motion. The speed of this motion depends on temperature. Collisions between particles are elastic. (No kinetic energy lost).

Particles in Solids, Liquids, and Gases The motion of gas molecules is rapid, constant, and random.

The Kinetic Molecular Theory Collisions between particles are elastic. In a perfectly elastic collision, the kinetic energy of each molecule might change, but the total kinetic energy stays the same.

States of Matter The state (solid, liquid, or gas) of a substance depends on three things:  Chemical identity, Temperature and Pressure Chemical identity matters because:  Different substances have different degrees of intermolecular forces.  The molecules of O 2 are small and non-polar. Attracted to each other only by London forces. O 2 has a very low boiling point and is a gas at room temperature.  NaCl is a salt with very strong ionic bonds between ions. NaCl has a very high melting point and is solid at room temperature.

States of Matter In solids, attractive forces outweigh kinetic energy of particles.  Particles stick together in rigid position. In liquids, attractive forces are balanced by kinetic energy.  Particles stick together but can move past each other. In gases, kinetic energy outweighs attractive forces.  Particles fly rapidly past each other as if there are no attractive forces between them.

Pressure Pressure = force / area

Pressure One gas molecule exerts a tiny force against the side of a balloon.

Pressure When you have a huge number of gas molecules colliding against the sides of a balloon, the force adds up. Force spread out over the inner surface of the balloon is pressure.

Units of Pressure SI Unit: Pascals (Pa)  1 Pa = 1 Newton / square meter Other units:  atmospheres (atm)  millimeters of mercury (mmHg)  torr  pounds per square inch (psi)

Standard Pressure Standard Pressure = 1 atm Standard pressure is the normal air pressure at sea level.

Pressure Conversion Factors 1 atm = 760 mmHg 1 atm = kPa 1 atm = 14.7 psi 1 atm = 760 torr

Atmospheric Pressure As you move upward through the atmosphere, the density decreases. This is because most air molecules are held close to Earth’s surface by gravity. As the density decreases, there are fewer molecules colliding with surfaces; hence, less pressure.

Temperature Temperature – measure of the average kinetic energy of a substance’s molecules.  The molecules in a hot object are moving faster than the molecules in a cold object on average!

Temperature Absolute temp. is measured in Kelvins (K)  No degree symbol! To convert:  from °C to K. K = °C  from °C to °F °F = 9/5 (°C) + 32  from °F to °C °C = 5/9 (°F – 32) EXAMPLE:  Water boils at 100°C, or 373 K.  Water freezes at 0°C, or 32°F.  Avg. human body temp. is 97.8°F, or 36.5°C.

Standard Temperature Standard Temperature = 0°C.  Also, 273 K. Absolute Zero = -273°C = 0 K.  Lowest temperature.  All molecular motion stops.

Kinetic Molecular Theory Postulates of the Kinetic Molecular Theory of Gases 1.Gases consist of tiny particles (atoms or molecules) 2.These particles are so small, compared with the distances between them, that the volume (size) of the individual particles can be assumed to be negligible (zero). 3. The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas. 4. The particles are assumed not to attract or to repel each other. 5. The average kinetic energy of the gas particles is directly proportional to the Kelvin temperature of the gas

Kinetic Molecular Theory Postulates Evidence 1. Gases are tiny molecules in mostly empty space. The compressibility of gases. 2. There are no attractive forces between molecules. Gases do not clump. 3. The molecules move in constant, rapid, random, straight-line motion. Gases mix rapidly. 4. The molecules collide classically with container walls and one another. Gases exert pressure that does not diminish over time. 5. The average kinetic energy of the molecules is proportional to the Kelvin temperature of the sample. Charles’ Law

Kinetic Molecular Theory (KMT) 1.…are so small that they are assumed to have zero volume 2.…are in constant, straight-line motion 3.…experience elastic collisions in which no energy is lost 4.…have no attractive or repulsive forces toward each other 5.…have an average kinetic energy (KE) that is proportional to the absolute temp. of gas (i.e., Kelvin temp.) AS TEMP., KE  explains why gases behave as they do  deals w /“ideal” gas particles…

Characteristics of Gases Gases expand to fill any container.  random motion, no attraction Gases are fluids (like liquids).  no attraction Gases have very low densities.  no volume = lots of empty space Courtesy Christy Johannesson

Characteristics of Gases Gases can be compressed. nno volume = lots of empty space Gases undergo diffusion & effusion. rrandom motion Courtesy Christy Johannesson