2. 1 Mr. ShieldsRegents Chemistry U03 L03

3. 2 Phase changes This transition represents a change from one state of matter to another Consider the following: Steam (G )  Water (L)  Ice cube (S) >100 deg C 0-100 deg C <0 deg C A Change of state is also known as a PHASE CHANGE

4. 3 Phase Changes  Consider Ice (s) going through a phase change to water (l).  H 2 0 (s)  H 2 0 (l)  In both solid and liquid phases, the molecules are H 2 O. The identity of a substance during a phase change is not altered.

5. 4 Phase Changes What’s happening to the molecules as we make these transitions? Let’s consider the phase changes in our water example Steam (G )  Water (L)  Ice cube (S) >100 deg C 0-100 deg C <0 deg C

6. 5 Phase Changes As temperature decreases K.E. decreases - The force of attraction between particles begins to play a bigger and bigger role Molecules are like magnets - they feel a force of attraction between one another So what exactly is going on?

7. 6 Phase Changes 1.As T decreases Molecules begin to attract one another more strongly 2.The Molecules start arranging themselves in a more orderly fashion - they grab on to each other and stay in place Liquid phase  solid phase As they cool down further…

8. 7 Phase Changes 3.Motion of the molecule becomes more restricted - The molecule is trapped in place by it’s neighbor unorganized Little organization Highly organized High KE (T)Lower KE Even Lower KE

9. 8 Phase Change & Energy There are 2 specific energy terms associated With Phase changes Increasing T represents an Increase in Energy Decreasing T represents a Decrease in Energy Which energy term applies depends upon which way the temperature is changing 1. Temperature (T) is either increasing 2. Temperature (T) is either decreasing

10. 9 Phase Change Diagram We can show this relationship between Energy, T and Phase change in a Phase Change Energy diagram In an endothermic phase change the particles absorb energy In an exothermic phase change the particles release energy Does the T of the phase increase or decrease? Gas Solid Liquid Energy Absorbed Energy Released Endothermic Exothermic T decreases T increases

11. 10 Let’s look at the Liquid to Gas phase changes first Phase Change – Liquid / Gas When a liquid absorbs energy it’s temperature increase. Eventually the liquid becomes a gas (steam). Is this an endothermic or exothermic process? endothermic exothermic What about the reverse process (Gas to Liquid)?

12. 11 Phase Change – Liquid / Gas Gas Solid Liquid Vaporization “endo” Condensation “exo” What happens if the gas is cooled (i.e. it gives up energy)? The gas phase undergoes a phase change to the liquid phase. This is called CONDENSATION. If enough energy is absorbed a liquid becomes a gas. This is called VAPORIZATION.

13. 12 Let’s next look at Solid to Liquid phase changes Phase Change – Solid / Liquid When a solid absorbs energy it’s temperature increase. The solid becomes a liquid. Is this an endothermic or exothermic process? endothermic exothermic What about the reverse process (liquid to solid)?

14. 13 Phase Change – Solid / Liquid Gas Solid Liquid Fusion “endo” Solidification “exo” If enough energy is absorbed a solid becomes a liquid This is called FUSION (commonly called melting). What if the liquid is cooled (i.e. gives up energy). What eventually happens? The liquid becomes a solid. This is called SOLIDIFICATION, (commonly called freezing)

15. 14 Let’s next look at Solid to Gas phase changes Phase Change – Solid to Gas When certain solids absorb energy they go directly from the Solid to Gas phase, i.e. they skip the Liquid phase. There are only a few materials that do This. endothermic This process is known as SUBLIMATION. Is it an ENDO Or EXO phase change? Iodine (I 2 ) and dry Ice (CO 2 )

16. 15 Sublimation of Iodine Iodine solid Iodine Gas

17. 16 Sublimation is a unique Process. Certain criteria must Be met for it to occur. Phase Change – Solid to Gas endothermic So what kind of compounds SUBLIME? Compounds with Weak intermolecular forces of attraction between molecules and High Vapor Pressures CO 2 or I 2

18. 17 Let’s next look at the opposite of sublimation. In other Words the process of going from Gas to Solid. Phase Change – Solid to Gas Since this is the reverse process the same criteria of weak Molecular forces and high VP still apply. exothermic This process is known as Deposition. It’s an exo process. In other words, heat is released.

19. 18 Phase Change Quiz Gas Solid Liquid Energy Absorbed Energy Released Endothermic Exothermic a b c d e f ENDOTHERMIC a.Sublimation b.Fusion c.Vaporization EXOTHERMIC d.Deposition e.Condensation f.Solidification T  ?