Heating Curves and Thermodynamics Chemistry Chapter 15

Phase Change Overview What are the names of the phase changes? Gas Solid Liquid Energy Absorbed Energy Released Endothermic Exothermic a b c d e f ENDOTHERMIC Phase changes EXOTHERMIC Phase Chages a. Sublimation d. Deposition b. e. Condensation Fusion c. Vaporization f. Solidification

Heating Curve What happens when we keep adding energy to a solid ? Solid becomes a liquid then the liquid becomes a gas As this energy is added, KE increases and so does temp. BUT, does temp uniformly increase over time? NO!

Heating Curve If we heat a solid, it’s temperature increases steadily until we finally reach the temperature at which the solid begins to melt It begins to go through a phase change: solid  liquid This phase change is called fusion

Solid/Liquid Phase Transition As the solid begins to melt, something unusual happens. As we continue to add heat to the solid the temperature stops rising as the solid continues to melt. But why does this happen? Solid Melting Solid

Solid/Liquid Phase Transition Solids exist in a rigid, closely packed, highly structured pattern Liquids however have no such rigid structure. As we reach the solid’s melting point there is just enough energy to begin overcoming the intermolecular forces between molecules holding them together in the solid state Molecules begin to separate The farther apart molecules become, the more PE they have

Heat of Fusion Added additional heat energy goes into separating more and more molecules As molecules move from solid to liquid the PE increases. But since the temperature doesn’t rise the KE remains constant. The energy necessary to melt 1 GRAM of a solid is called the … It is unique for every substance. For water, the heat of fusion = 334 Joules/gram HEAT OF FUSION (Hf)

Solid/Liquid Phase transition Remember: heat ALWAYS flows from hot to cold Until the last piece of solid melts the temperature the solid/liquid mixture remains constant Any excess heat in the liquid immediately flows back into the colder solid Once there’s no solid left all additional heat added begins to increase the temperature of the liquid. H C

Solid/Liquid Phase transition When heat is added what happens to Kinetic Energy, Temperature, & Potential Energy ? Only a single solid (s) phase is present KE (T) increases PE is Constant Dual L/S phase is present KE (T) is Constant PE increase Only a single liquid (l) phase is present PE is constant Solid solid & liquid Liquid Heating Curve

Liquid/Vapor Phase Transition B.P. Any additional heat added to the liquid increases temperature and vapor pressure. Once VP = Patm the liquid boils As in the transition from solid to liquid, two phases are now present and the temperature of the boiling water remains constant Solid solid & liquid Liquid

Heat of Vaporization Molecules in the liquid phase form a close but loosely organized structure Molecules in the gas phase have no structure and are widely separated (PE increases) Separating these molecules this much takes lots of energy. This energy is called the HEAT OF VAPORIZATION (Hv) Liquid S & L Solid Gas L & G

Heat of Vaporization For water, Hv = 2,260 Joules/g Note Hv > Hf As long as the liquid is boiling, temperature and kinetic energy will be constant Potential energy will increase as Molecules move further apart in the gas phase Liquid & gas Solid & liquid M.P B.P. The Heating Curve

Summary (b.p.) T const. / KE const. /PE inc ( 2 phases ) T inc. / KE inc / PE Const. ( 1 phase ) T const. / KE const. /PE inc ( 2 phases ) Hf = 334 J/g (m.p.) (b.p.) Hv = 2,260 J/g

Heating Curve of Water

Heating Curve A heating curve illustrates the changes of state as a solid is heated. uses sloped lines to show an increase in temperature. uses plateaus (flat lines) to indicate a change of state. Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings

Heating curve of Water… can you label the important temperatures, phases and phase changes?

Heating curve of Water… can you label the important temperatures, phases and phase changes? Ice Solid

Heating curve of Water… can you label the important temperatures, phases and phase changes? Ice Solid

Heating Curve of Water… can you label the important temperatures, phases and phase changes? Melting / Freezing Ice Solid

Heating curve of Water… can you label the important temperatures, phases and phase changes? Liquid Water Liquid Melting / Freezing Ice Solid

Heating curve of Water… can you label the important temperatures, phases and phase changes? 100 Liquid Water Liquid Melting / Freezing Ice Solid

Heating curve of Water… can you label the important temperatures, phases and phase changes? 100 Boiling / Condensation Liquid Water Liquid Melting / Freezing Ice Solid

Heating curve of Water… can you label the important temperatures, phases and phase changes? Vapor 100 Boiling / Condensation Gas Liquid Water Liquid Melting / Freezing Ice Solid

How energy moves from one place to another:   Heat (q): The movement of energy from one thing to another through the motion of molecules (thermal energy). Heat spontaneously moves from hot things to cold. This is why a hot pan can burn you and you can’t burn a hot pan – the energy goes only from the pan to you because it’s hotter. Heat and temperature are NOT the same thing: Heat is the transfer of energy, temperature is a measure of the kinetic energy of the object once the energy has finished transferring. Example: If I give you five dollars, the money is what I give you (heat) and your net value is what goes up after you get the money (temperature). Your wealth is not a five dollar bill, but the five dollar bill does affect it.

System: Whatever we’re studying. In chemistry, heat is a lot more useful than work because chemistry generally deals with the movement of atoms and molecules a lot more often than it deals with the movement of large machines.     Since heat (energy that’s being transferred through thermal motion) is more important than work (for our purposes), how do we measure it? More definitions of use: System: Whatever we’re studying. This can be practically anything. If we are studying what happens when we heat a pan on the stove, the pan will be the system we are studying. If a system gets energy added to it, the amount of energy it has after the change is positive. Because of this, an endothermic process is any process in which energy is absorbed by the system we’re talking about. If a system has energy taken away from it, the amount of energy it has after the change is negative. Because of this, an exothermic process is any process in which energy is given off by the system we’re talking about. 

Potential Energy Diagrams

Potential Energy Diagrams The amount of energy in the products is greater than the energy in the reactants…. so this process is ENDOTHERMIC. This means the reaction ABSORBS energy The

Potential Energy Diagrams The

Potential Energy Diagrams The amount of energy in the products is less than the energy in the reactants…. so this process is EXOTHERMIC. This means the reaction GIVES OFF energy The