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W H AT H A P P E N S W H E N T H I N G S G E T H O T
PHASE CHANGES
• Pick up a copy of the notes• Draw a solid, liquid, and gas
PHASE CHANGES
• The first couple you should know• Melting• Solid to liquid
• Freezing• Liquid to solid
• Vaporizing• Liquid to gas
• Condensing• Gas to liquid
• Now for the tricky ones• Sublimation• Solid to Gas
• Deposition• Gas to Solid
• All Blue ones are Endothermic• All Red ones are
Exothermic
ENDO/EXO-THERMIC
• Endothermic• Takes in energy• Energy transfers as
heat
• The molecules gain energy• The molecules are
now moving faster
• Exothermic• Lose energy• NOT LOOSE
• Heat leaves the system• Molecules are now
moving slower
• Before hot plate was 25 C• More heat = faster speed of molecules
• When speed changes enough, there is a phase change
DEMO
PHASE CHANGES
• Phase changes are determined by Intermolecular Forces• There’s that word again!
• Phase changes happen when intermolecular forces either dominate speed or the intermolecular forces are broken• Molecules with stronger intermolecular forces
have higher boiling/melting points
WHAT IS A PHASE CHANGE
• The kinetic energy (temperature) of a molecule becomes large enough to break intermolecular forces
• OR• The kinetic energy (temperature) becomes small
enough such that intermolecular forces take over• The distance between molecules changes• Further if IMFs are broken• Closer when IMFs dominate
• Temperature DOES NOT CHANGE during phase change• All energy goes into breaking/forming IMFs
• The temperature has reached the freezing or boiling point
MELTING
• Draw• You’ve seen it! Ice!• Solid to Liquid
• Molecules gain energy and move faster• Thus energy goes in• Endothermic
• Occurs at Melting Point
FREEZING
• Draw• You’ve seen it! Ice! (Again!)• Liquid to Solid
• Molecules lose energy and move slower• Thus energy goes out• Exothermic
• Occurs at Melting Point• The freezing point is the same as the melting point! 0 Celsius
SUBLIMATION
• Draw it• Solid to Gas (no liquid!)
• Molecules gain energy and move faster• Thus energy goes in• Endothermic
• Dry Ice! Very cold, -78 Celsius
CONDENSATION
• Draw• You’ve seen it every morning!• Tomorrow go outside and say ‘Look at all that phase change’
• Gas to Liquid
• Molecules lose energy and move slower• Thus energy goes out• Exothermic
• Occurs at Boiling point• The boiling point is the same as the condensing point!
VAPORIZATION
• Draw• You did this in lab. Why did the acetone boil before water?• Liquid to Gas
• Molecules gain energy and move faster• Thus energy goes in• Endothermic
• Occurs at Boiling point• The boiling point is the same as the condensing point!
QUICK ASIDE
• Anything can undergo a phase change• EVERY element/compound can vaporize, freeze,
or anything else• All at their own Temperature/Pressure• Copper can vaporize• Oxygen can freeze• Helium can condense
DEPOSITION
• Draw• You probably saw this this morning! Frost on the grass
• Frost on the car, frost on the window, frost everywhere
• Gas to Solid (No liquid phase!)
• Molecules lose energy and move slower• Thus energy goes out• Exothermic
DIFFERENT SPEEDS
• Not all molecules are going the same speed• Just like everyone in this room cannot run the same
speed• This means some molecules have more kinetic energy
VAPOR PRESSURE
• Some molecules have enough kinetic energy to enter the vapor phase even though most of the other molecules are still a liquid• This is vapor pressure
• The higher the temp the higher the Vapor Pressure• You already know this!• Green bottle
VOLATILITY
• Strong intermolecular forces = Low Vapor Pressure• Less volatile
• Weak Intermolecular Forces = High Vapor Pressure• More volatile
• Acetone is more volatile than water• It evaporated faster