Energy Changes

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Energy Changes Phase Changes

• Heating Cooling Curves

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energy needed to heat/cool matter!
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All P/C changes accompanied by energy changes

• phase changes are physical changes
• sometimes energy is absorbed (endothermic)
• sometimes energy is released (exothermic)
• energy change for given phase change can be
  measured/calculated

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• calculate amount E needed to change pure
  substance
• from one single phase to another
• Q mc?T
• Q energy in Joules
• m mass in grams
• C specific heat capacity
• ?T change in temperature Tf - Ti

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What about phase changes?

• more than one phase substance present
• melting ice
• both H2O(l) H2O(s) present at same time
• temperature is constant so ?T0
• even though ice is absorbing heat to change phase

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6 possible phase changes
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Potential Energy

• energy of relative position
• molecules are always attracted to one another
• energy put into system to pull molecules apart
  from one another
• farther apart, the higher their PE

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Melting Ice

• Ice on hot plate ice is melting
• ice absorbing heat from hot plate and using to
  change phase
• temperature of ice-water mix is constant
• heat energy from hot plate going into phase
  change
• heat energy not going to KE of particle is going
  to PE during phase change

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Identify a phase change as endothermic or
exothermic
endothermic
exothermic
gas
freezing condensation deposition
melting (fusion) vaporization sublimation
PE
liquid
solid
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Heating Cooling Curves

• way to illustrate energy changes
• measure temperature as function of time at
  constant heating/cooling rate

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III
IV
V
Liquid Gas
Solid Liquid
Solid
Liquid
Gas
KE?
KE?
boil pt.
KE?
KE?
PE?
PE?
Temperature
PE?
melt pt.
PE?
PE?
Time
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Melting Boiling Points

• Plateaus Phase changes PE changes
• 2 phases are present, temp is constant
• Melting point/Boiling point
• value always given as exact temp
  (number with unit C, K)

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What happens to temperature as heat is added at
boiling point?

• NOTHING
• temp doesnt change until only 1 phase present!

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To analyze a heating/cooling curve

• curve does it go uphill or downhill?
• label phases present in each region
• describe what happens to KE in each region
• describe what happens to PE in each region
• locate melting point and boiling point

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Heating curve of H2O
Why does it take longer for water to vaporize
than it does to melt?