Kinetic Molecular Theory (KMT)

1. Kinetic Molecular Theory (KMT)

2. KMT • What does the word Kinetic mean? • Now apply the word kinetic to matter and you will have the 3 basic assumptions of KMT as it applies to gases. (page 329 in your text book)

3. 1st Basic Assumption • A gas is composed of particles, usually molecules or atoms • These particles are considered to be small, hard spheres that have insignificant volume and are relatively far apart from one another. • Between the particles there is empty space. • No attractive or repulsive forces exist between the particles.

4. 2nd Basic Assumption • The particles in a gas move rapidly in constant random motion. • They travel is straight paths and move independently of each other. • Gases fill their containers regardless of the shape and volume of the containers. • The gas particles only change direction when they rebound from collisions with each other or the container • They move around 1660 km/h at room temperature.

5. 3rd assumption • All collisions are elastic. • This means that during collisions kinetic energy is not lost only transferred from one particle to another. • The total energy of the system remains constant.

6. The result of all those collisions • Moving bodies exert a force when the collide (force = mass x acceleration) • Each gas particle has a very small mass and therefore exerts a small force. • Therefore the force is going to be the result of how many particles are colliding. • This force is termed GAS PRESSURE.

7. Gas Pressure • Gas Pressure is defined as the force exerted by a gas per unit surface area of an object. • If there are no particles, there is not pressure and is this called a vacuum. • The most common form of gas pressure you experience is called atmospheric pressure.

8. Atmospheric pressure • Defined as the air pressure on Earth that results from gravity holding the air molecules in the atmosphere. • Atm. Pressure decreases as elevation increases because gravity decreases.

9. Measurements of Atm. Pressure • Atm. Pressure is measured in: • Pascal (Pa) • Atmospheres (atm) • mm Hg • Torr • At sea level • 101.3 Pa = 1 atm = 760 mm Hg = 760 Torr

10. Temperature • Temperature is defined as the average kinetic energy of a substance. • Therefore the greater the average speed of the particles the greater the temperature. • The point at which all particles stop moving is called absolute zero.

11. Temperature Scales • There are two important temp scales • Kelvin (K) • Celsius (ºC) 0 ºC = 273 K

12. How are liquids the same and different • Particles in liquids are also in motion. • The particles are free to slide past each other which is why liquids are a fluid. • Attractive forces exist between the particles in a liquid. This attractive forces are called intermolecular forces. • The move less rapidly. Their average kinetic energy is lower, therefore their temperature is lower.

13. Liquids continued • The molecules in liquids do not move independently of each other. • They form to the shape of their container but they have a fix volume due to the attractive forces between the particles. • They have less room between the particles and therefore they are more dense then gases.

14. Two important physical properties of liquids • The interplay between the motion of the particles and the attractive forces between them determine the physical properties of liquids. • The two that we will explore are: • Vapor pressure • Boiling point

15. Vapor pressure • Vapor pressure is defined as the force that results from the gas above the liquid. • It is the vapor pressure that determine at what temperature a gas will evaporate and boil at. • The larger the vapor pressure of a gas the more energy is needed to make the pressure in the liquid equal to the vapor pressure. Therefore the greater the temperature of the liquid needs to be for the liquid to evaporate.

16. Evaporation and Boiling • Evaporation and boiling are both types of vaporization. • Vaporization occurs when the particles in the liquid have enough energy to break free of the attractive forces keeping them in the liquid state and therefore they can become a gas.

17. Evaporation • Evaporation occurs when the surface of a liquid is heated. • Evaporation is a cooling process because when the particles escape the liquid they take their energy with them therefore the average kinetic energy of the system is reduced.

18. Boiling • Boiling is defined as vaporization that occurs throughout the liquid. • The boiling point is defined as the temperature at which the vapor pressure of the liquid is just equal to the external pressure. • The lower the external pressure the lower the boiling point. • Boiling is also a cooling process.

19. How are solids different • There is almost no empty space between the particles. They are tightly packed in an organized manner. (Crystal structure) • They do not move independently. • They have a fix shape and volume do to very strong attractive forces between the particles. • They only vibrate in place.

20. Melting point • The melting point is the point were the particles in the solid gains enough energy to overcome the interactions that hold them together. • At this temperature the particles will have enough energy to become a liquid.