Matter changes phase when energy is added or removed

Transcription

1 Section 12.4 Phase Changes Explain how the addition and removal of energy can cause a phase change. Interpret a phase diagram. Matter changes phase when energy is added or removed

2 Energy Changes Accompanying Phase Changes Gas Vaporization Condensation Energy of system Melting Liquid Sublimation Freezing Deposition Solid Brown, LeMay, Bursten, Chemistry 2000, page 405

3 Section 12.4 Phase Changes That Require Energy Melting occurs when heat flows into a solid object. The process by which a solid changes into a liquid. The melting point of a crystalline solid is the temperature at which the forces holding the crystal lattice together are broken and it becomes a liquid. Add energy to overcome intermolecular forces.

4 Section 12.4 Phase Changes That Require Energy (cont.) Vaporization is the process by which a liquid changes to a gas or vapor. -Vaporization is an endothermic process - it requires heat. -Energy is required to overcome intermolecular forces -Responsible for: -cool earth -Why we sweat Two types of vaporization seen are, evaporation and boiling.

5 Section 12.4 Phase Changes That Require Energy Evaporation is vaporization only at the surface of a liquid. To evaporate, molecules must have sufficient energy to break IM forces. Molecules at the surface break away and become gas. Only those with enough KE escape. Breaking IM forces requires energy. The process of evaporation is endothermic. Evaporation is a cooling process. It requires heat.

6 Section 12.4 Phase Changes That Require Energy (cont.) In a closed container, the pressure exerted by a vapor over a liquid is called vapor pressure.

7 Section 12.4 Phase Changes That Require Energy (cont.) The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure.

8 Section 12.4 Phase Changes That Require Energy (cont.) Sublimation is the process by which a solid changes into a gas without becoming a liquid.

9 Energy Changes Accompanying Phase Changes Gas Vaporization Condensation Energy of system Melting Liquid Sublimation Freezing Deposition Solid Brown, LeMay, Bursten, Chemistry 2000, page 405

10 Section 12.4 Phase Changes That Release Energy Freezing is the process by which a liquid changes into a solid. As heat flows from water to the surroundings, the particles lose energy. The freezing point is the temperature at which a liquid is converted into a crystalline solid.

11 Section 12.4 Phase Changes That Release Energy (cont.) The process by which a gas or vapor becomes a liquid is called condensation. Achieves a dynamic equilibrium with vaporization in a closed system. What is a closed system? A closed system means matter can t go in or out. (put a cork in it) What the heck is a dynamic equilibrium?

12 Dynamic Equilibrium When first sealed, the molecules gradually escape the surface of the liquid. As the molecules build up above the liquid - some condense back to a liquid. The rate at which the molecules evaporate and condense are equal.

13 Dynamic Equilibrium As time goes by the rate of vaporization remains constant but the rate of condensation increases because there are more molecules to condense. Equilibrium is reached when: Rate of Vaporization = Rate of Condensation Molecules are constantly changing phase dynamic The total amount of liquid and vapor remains constant equilibrium

14 Section 12.4 Phase Changes That Release Energy (cont.) Deposition is the process by which a gas or vapor changes directly to a solid, and is the reverse of sublimation.

15 Heating Curves Temperature ( o C) Gas- KE Liquid- KE Vaporization- PE, KE remains the same Melting- PE, KE remains the same Solid- KE Time

16 Phase Diagrams Section 12.4 A phase diagram is a graph of pressure versus temperature that shows in which phase a substance will exist under different conditions of temperature and pressure. The triple point is the point on a phase diagram that represents the temperature and pressure at which all three phases of a substance can coexist.

17 Section 12.4 Phase Diagrams (cont.)

18 The phase diagram for different substances are different from water.