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Energy and Molecular Order Changes: Phase Changes We now take a closer look at how substances change phase. It is important to remember that phase change depends on the nature of the substance and on the temperature and pressure of its environment. It involves gain/absorption of heat by the substance from the environment (endothermic) or loss/release of heat by the substance to the environment exothermic). The different processes associated with the phase changes and the accompanying energy changes are shown in the following diagram: Heat Energy Absorption) Kinetic Energy Increase Section Objectives At the end of this section you should be able to describe the nature of the solid-ligald, oquid- vapor, and sold-vapor phase changes in terms of energy changes and increase or decrease in molecular order and 2 define the terms associated with phase changes Sublimation Fusion (melting) Vaporization (Bolling) Solidification (Freezing) Condensation vie) Deposition Heat Energy Release/Ginetie Energy Decrease Figure 41. Diagram of Energy Changes Legend: 1. Terms on the arrows are processes that refer to the changes. 2. An arrow indicates the direction of phase change (e.g. from solid to gas) Exercise: Study the diagram in Figure 4.1 and determine the relationships indicated by the words and arrows. 8. Answer the following questions. 1. What phase change happens in the process of: a) sublimation? to b) fusion? to c) vaporization? to 2. What happens to heat energy during the phase changes in number 1?
Section 2 Heating and Cooling Curves, Phase Diagrams Section Objectives At the end of the section you should be able to 1 explain the heating and cooling curves of substances, and 2 interpret and an Information from phase diagrams When the amount of heat energy added to a solid is plotted against the temperature changes as time progresses, a heating curve results and much information can be obtained from it. Let us learn from the heating curve shown below: 3502 300 200 Τεπιφαιρικό Ο 150 100 Liquid S Solid o free 20 10 80 80 100 120 140 100 100 200 Heutrurgy added Figure 4.2. Heating Curve of a Solid As seen from the Figure 4.2, (1) from the start the temperature of the solid rises as heat energy is added. This is an indication that the heat energy is being used to increase the average kinetic energy of the particles. (2) After some time, even while heat energy is still being added, the temperature remains constant. At this point, a phase change is occurring. (3) What is this phase change? (4) At what temperature does the phase change occur? When the phase change has been completed, (5) the temperature of the liquid begins to rise again as the average kinetic energy of the particles is increasing. Then, the point is reached when the added heat energy does not raise the temperature again as a phase change occurs. (6) What is the phase change that occurs at this point? (7) What is the temperature of the liquid at this point? At the completion of the phase change the temperature again rises with the addition of heat. (8) How much heat energy has been added before the second phase change began? (9) How much energy has been added at the completion of this phase change? (10) What temperature was reached when 2003 of heat was added? Exercise: 1. Read the paragraph that describes the heating curve (Figure 4.2). 2. Read again and as you read, trace the curve to identify the part that indicates what is described by or asked about the numbered statements. 3. Write the location of each identified part or answer the given question, as the case may be. (Sample answer: 1. line from OJ, 20°C to 40), 100°C) 38 General Chemistry 2
Heating Curve Data A. 1. line from OJ. 20°C point to 40), 100°C 6. 7. 2. 8. 3. 9. 10. 5. B. explain the observation that the temperature remains constant while a phase What inference regarding the kinetic energy of the molecules can be given to change occurs? narov