1. Kinetic Theory Solids, Liquids and Gases

2. The Nature of Gases Objectives: Objectives: Describe the motion of gas particles according the kinetic theory Describe the motion of gas particles according the kinetic theory Interpret gas pressure in terms of kinetic theory Interpret gas pressure in terms of kinetic theory

3. Kinetic Theory Most compounds exist in only one state at a time – Most compounds exist in only one state at a time – Water can be Water can be Ice (solid) Ice (solid) Water (liquid) Water (liquid) Steam (gas) Steam (gas)

4. Kinetic Theory However, the state of existence is easily changed through However, the state of existence is easily changed through Changes in temperature Changes in temperature Cool water down and it freezes into ice Cool water down and it freezes into ice Heat water up and it boils into steam Heat water up and it boils into steam

5. Kinetic Theory The word kinetic means The word kinetic means Motion Motion Kinetic energy is Kinetic energy is The energy held by an object because of its motion The energy held by an object because of its motion

6. Kinetic Theory Kinetic theory is Kinetic theory is The concept that the particles in all matter are in constant motion. The concept that the particles in all matter are in constant motion.

7. Kinetic Theory For gases, the following assumptions have been applied: For gases, the following assumptions have been applied: 1. The particles of a gas are small, hard spheres with insignificant volume and large distances between one another. These particles are not attracted to one another; nor are they repulsed from one another.

8. Kinetic Theory For gases, the following assumptions have been applied: For gases, the following assumptions have been applied: 2. Each particle in a sample of a gas is in constant, straight-line motion. They will only change direction when they run into something, like another particle of gas or the container holding them. This straight-line motion is called a “random walk.”

9. Kinetic Theory For gases, the following assumptions have been applied: For gases, the following assumptions have been applied: 3. Energy is not lost when the particles hit each other. This means all of the collisions between the particles are called perfectly elastic. Overall, this mean the gas does not lose the energy it has because it is always running into itself… it doesn’t wear itself out.

10. Gas Pressure Everything that is moving will Everything that is moving will Exert a force when it collides with another object Exert a force when it collides with another object Imagine getting hit by a car, a bicycle and a rubber ball that are all moving at the same speed Imagine getting hit by a car, a bicycle and a rubber ball that are all moving at the same speed Which one hits you the hardest? Which one hits you the hardest? Which one hits you the softest? Which one hits you the softest? Why? Why? car rubber ball More mass = harder hit

11. Gas Pressure Well, gas particles exert force too. Well, gas particles exert force too. The force applied by one gas particle is extremely tiny The force applied by one gas particle is extremely tiny A container of gas will have lots of particles, so billions of gas particles will run into the container simultaneously. A container of gas will have lots of particles, so billions of gas particles will run into the container simultaneously. All of these forces hitting at the same time add up to a measurable amount of force. All of these forces hitting at the same time add up to a measurable amount of force.

12. Gas Pressure Gas pressure is Gas pressure is The sum of the forces applied by all the gas particles divided by the area over which the forces are applied. The sum of the forces applied by all the gas particles divided by the area over which the forces are applied. This is a measurement of how “hard” the gas is hitting the container. This is a measurement of how “hard” the gas is hitting the container. How could we increase the gas pressure? How could we increase the gas pressure? More particles, make them move faster, smaller area, etc. More particles, make them move faster, smaller area, etc.

13. Gas Pressure What if there were no gas particles present to run into anything, like out in space? What would the pressure of that sort of system be? What if there were no gas particles present to run into anything, like out in space? What would the pressure of that sort of system be? The pressure would be zero. The pressure would be zero. This is called a vacuum. This is called a vacuum.

14. Gas Pressure In this class, we will express pressure using the units of the SI system, Pascals (Pa); and an older unit called the atmosphere. In this class, we will express pressure using the units of the SI system, Pascals (Pa); and an older unit called the atmosphere. We will sometimes see other units also; just convert them to one that is easy to use. We will sometimes see other units also; just convert them to one that is easy to use. 101325 Pa = 1 atm = 14.7 psi = 760 mmHg 101325 Pa = 1 atm = 14.7 psi = 760 mmHg

15. Gas Pressure As a review, what does STP stand for? As a review, what does STP stand for? Standard temperature and pressure Standard temperature and pressure What is STP What is STP 1 atm and 0 o C 1 atm and 0 o C What is that temperature in Kelvin? What is that temperature in Kelvin? 273.15 K 273.15 K

16. Kinetic Energy and Kelvin Temperature When you heat something up, a little of the energy is stored as potential energy while the rest speeds the particles up. When you heat something up, a little of the energy is stored as potential energy while the rest speeds the particles up. That is, it increases the kinetic energy of the particles in the matter. That is, it increases the kinetic energy of the particles in the matter. The particles will NOT all have the same kinetic energy The particles will NOT all have the same kinetic energy Some (about half) are moving faster than average Some (about half) are moving faster than average Some (the other half) are moving slower than average Some (the other half) are moving slower than average

17. Kinetic Energy and Kelvin Temperature Theoretically, if you cooled something down enough all molecular motion would stop. This would happen at Theoretically, if you cooled something down enough all molecular motion would stop. This would happen at Absolute zero (0 Kelvin) Absolute zero (0 Kelvin) We’ve never been able to cool anything down to absolute zero in a lab We’ve never been able to cool anything down to absolute zero in a lab

18. Kinetic Energy and Kelvin Temperature In the Kelvin scale, temperature and kinetic energy are directly related to each other. What does this mean? In the Kelvin scale, temperature and kinetic energy are directly related to each other. What does this mean?

19. The Nature of Liquids Objectives: Objectives: Describe the nature of a liquid in terms of the attractive forces between the particles Describe the nature of a liquid in terms of the attractive forces between the particles Differentiate between evaporation and boiling of a liquid, using kinetic theory Differentiate between evaporation and boiling of a liquid, using kinetic theory

20. The Nature of Liquids What makes a liquid different from a solid? What makes a liquid different from a solid?

21. A Model for Liquids Does the kinetic theory apply to liquids? Does the kinetic theory apply to liquids? Yes – the particles in a liquid are in motion. Yes – the particles in a liquid are in motion. The particles move around more slowly than gas particles. The particles move around more slowly than gas particles. The particles are attracted to each other. The particles are attracted to each other. These attractions are called intermolecular forces. These attractions are called intermolecular forces. These forces are what cause liquids to have a defined volume. These forces are what cause liquids to have a defined volume. The forces are too weak to lock the particles into place, which is why liquids flow. The forces are too weak to lock the particles into place, which is why liquids flow.

22. Evaporation What happens to a saucer of water left sitting out for a long time? What happens to a saucer of water left sitting out for a long time? The water turns from a liquid into a gas – it evaporates The water turns from a liquid into a gas – it evaporates

23. Evaporation Vaporization: Vaporization: The conversion of a liquid to a gas or vapor. The conversion of a liquid to a gas or vapor. Evaporation: Evaporation: Vaporization that occurs at the surface of a liquid that is not boiling Vaporization that occurs at the surface of a liquid that is not boiling Faster than average liquid particles at the surface of the liquid can “escape” into the gas state Faster than average liquid particles at the surface of the liquid can “escape” into the gas state Cools a liquid down (the hot particles leave) Cools a liquid down (the hot particles leave)

24. Evaporation Vapor pressure: Vapor pressure: The pressure produced when vapor particles above a liquid in a closed container collide with the container walls The pressure produced when vapor particles above a liquid in a closed container collide with the container walls A dynamic equilibrium exists between the vapor and the liquid. A dynamic equilibrium exists between the vapor and the liquid.

25. Evaporation What does dynamic mean? What does dynamic mean? In motion In motion What is the opposite of dynamic? What is the opposite of dynamic? Static (still) Static (still) What does equilibrium mean? What does equilibrium mean? A state of balance or equality between opposing forces A state of balance or equality between opposing forces

26. Evaporation How can an equilibrium be dynamic? How can an equilibrium be dynamic? Everything going one direction has an opposite that goes the other way Everything going one direction has an opposite that goes the other way In a dynamic equilibrium, the two opposing sides are equal – essentially canceling each other out In a dynamic equilibrium, the two opposing sides are equal – essentially canceling each other out For evaporation, a dynamic equilibrium means the rate of particles going from liquid to gas is equal to the rate of particles going from gas to liquid For evaporation, a dynamic equilibrium means the rate of particles going from liquid to gas is equal to the rate of particles going from gas to liquid

27. Boiling Point How do you make a liquid evaporate faster? How do you make a liquid evaporate faster? Heat it up – hotter particles move faster and are more likely to escape into the gas state Heat it up – hotter particles move faster and are more likely to escape into the gas state

28. Boiling Point Boiling point: Boiling point: The temperature at which the vapor pressure of a liquid is just equal to the external pressure on the liquid The temperature at which the vapor pressure of a liquid is just equal to the external pressure on the liquid

29. Boiling Point Boiling: Boiling: Vaporization throughout a liquid Vaporization throughout a liquid Bubbles of vapor form all over the sample Bubbles of vapor form all over the sample Normal boiling point: Normal boiling point: Boiling point at a pressure of 1 atm (air pressure at sea level) Boiling point at a pressure of 1 atm (air pressure at sea level) While boiling the temperature will remain constant While boiling the temperature will remain constant The addition of more heat will vaporize more liquid rather than raise the temperature The addition of more heat will vaporize more liquid rather than raise the temperature

30. The Nature of Solids Objectives: Objectives: Describe how the degree of organization of particles distinguishes solids from liquids and gases Describe how the degree of organization of particles distinguishes solids from liquids and gases Distinguish between a crystal lattice and a unit cell Distinguish between a crystal lattice and a unit cell

31. Review Name three molecular compounds Name three molecular compounds Carbon dioxide, water, sulfur trioxide Carbon dioxide, water, sulfur trioxide Name three ionic compounds Name three ionic compounds Sodium chloride, magnesium hydroxide, calcium carbonate Sodium chloride, magnesium hydroxide, calcium carbonate How are molecular and ionic compounds different? How are molecular and ionic compounds different? Molecular compounds exist as molecules. Ionic compounds are huge collections of ions. Molecular compounds exist as molecules. Ionic compounds are huge collections of ions.

32. A Model for Solids Do the particles in solids move? Do the particles in solids move? Yes, but with much less freedom than the particles in liquids. Yes, but with much less freedom than the particles in liquids. They vibrate around a fixed point. They vibrate around a fixed point. These particles are locked into position, so they can’t flow – solids have both a definite volume and a definite shape. These particles are locked into position, so they can’t flow – solids have both a definite volume and a definite shape.

33. A Model for Solids Melting point: Melting point: The temperature at which a solid turns into a liquid. The temperature at which a solid turns into a liquid. Also the temperature at which a liquid turns into a solid Also the temperature at which a liquid turns into a solid Melting point = freezing point Melting point = freezing point

34. A Model for Solids Melting is a result of adding heat to an object. Melting is a result of adding heat to an object. More heat = higher temperature = increased kinetic energy of the particles = faster movement of the particles More heat = higher temperature = increased kinetic energy of the particles = faster movement of the particles At the melting point, the particles are moving so fast that they are able to break free of the intermolecular forces holding them together. At the melting point, the particles are moving so fast that they are able to break free of the intermolecular forces holding them together. They begin to slide past one another as in a liquid They begin to slide past one another as in a liquid

35. A Model for Solids Why are the melting points of ionic compounds higher than molecular compounds? Why are the melting points of ionic compounds higher than molecular compounds? The ionic compounds have stronger forces holding them together. The ionic compounds have stronger forces holding them together. Stronger forces = higher melting point Stronger forces = higher melting point Can all compounds be melted? Can all compounds be melted? No – some, like wood and various plastics, will decompose when heated. They never melt. No – some, like wood and various plastics, will decompose when heated. They never melt.

36. Crystal Structure and Unit Cells Many solids are crystalline in nature. Many solids are crystalline in nature. The atoms, ions or molecules of the solid are arranged in an orderly, repeating, 3D pattern called the crystal lattice. The atoms, ions or molecules of the solid are arranged in an orderly, repeating, 3D pattern called the crystal lattice. Crystals are classified into seven systems. Crystals are classified into seven systems. You don’t have to learn their names. You don’t have to learn their names.

37. Crystal Structure and Unit Cells The smallest group of particles within a crystal that retains the shape of the crystal is called The smallest group of particles within a crystal that retains the shape of the crystal is called The unit cell The unit cell

38. Crystal Structure and Unit Cells Three types of unit cells Three types of unit cells 1. Simple cubic 2. Body-centered cubic 3. Face-centered cubic

39. Crystal Structure and Unit Cells An allotrope is An allotrope is A different form of an element. A different form of an element. For example, carbon has 3 allotropes: For example, carbon has 3 allotropes: Graphite Graphite Coal Coal Diamond Diamond

40. Crystal Structure and Unit Cells Not all solids are crystalline – Not all solids are crystalline – Some lack the organized structure found in crystalline solids Some lack the organized structure found in crystalline solids These unorganized solids are called amorphous solids These unorganized solids are called amorphous solids Amorphous means “without shape” Amorphous means “without shape”

41. Crystal Structure and Unit Cells There are two key difference between crystalline and amorphous solids: There are two key difference between crystalline and amorphous solids: 1. Crystalline solids have a sharply defined melting point. Amorphous solids don’t – they will have temperature range over which they soften and then melt. 2. When struck, crystals will fragment into pieces with the same shape as the original. Amorphous solids will break into pieces with sharp edges (like glass) that are shaped randomly.