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Phase Change

Phase Change. Mr. Shields Regents Chemistry U03 L03. Phase changes. Consider the following: Steam (G )  Water (L)  Ice cube (S) >100 deg C 0-100 deg C <0 deg C. This transition represents a change from one state of matter to another.

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Phase Change

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  1. Phase Change Mr. Shields Regents Chemistry U03 L03

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

  3. Phase Changes • Consider Ice (s) going through a phase change to water (l). • H20 (s)  H20 (l) • In both solid and liquid phases, the molecules are H2O. The identity of a substance during a phase change is not altered.

  4. Phase Changes 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  What’s happening to the molecules as we make these transitions?

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

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

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

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

  9. Gas Exothermic T increases Energy Released Energy Absorbed Liquid T decreases Endothermic Solid 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 Does the T of the phase increase or decrease? In an exothermic phase change the particles release energy Does the T of the phase increase or decrease?

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

  11. Gas Vaporization “endo” Condensation “exo” Liquid Solid Phase Change – Liquid / Gas If enough energy is absorbed a liquid becomes a gas. This is called VAPORIZATION. 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.

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

  13. Gas Fusion “endo” Liquid Solidification “exo” Solid Phase Change – Solid / Liquid 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)

  14. Phase Change – Solid to Gas Let’s next look at Solid to Gas phase changes 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. Iodine (I2) and dry Ice (CO2) endothermic This process is known as SUBLIMATION. Is it an ENDO Or EXO phase change?

  15. Iodine solid Iodine Gas Sublimation of Iodine

  16. Phase Change – Solid to Gas Sublimation is a unique Process. Certain criteria must Be met for it to occur. • So what kind of compounds SUBLIME? Compounds with Weak intermolecular forces of attraction between molecules and High Vapor Pressures endothermic CO2 or I2

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

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

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