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Energy Changes

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Energy Changes & Phase Changes Heating & Cooling Curves energy needed to heat/cool matter! calculate amount E needed to change pure substance (from one single phase ... – PowerPoint PPT presentation

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Title: Energy Changes


1
Energy Changes Phase Changes
  • Heating Cooling Curves

2
energy needed to heat/cool matter!
  • 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

3
Q mc?T
  • c specific heat capacity
  • (amount heat required to raise temp of 1g of pure
    substance by 1?C)
  • c is a physical constant
  • unique for every pure substance
  • cH2O 4.18 J/g?C

4
Heat Flow hot to cold
environment
5
What about phase changes?
  • sometimes more than one phase of substance is
    present
  • melting ice both liquid water solid water
    (ice) present at same time
  • temperature is constant ?T0
  • even though ice is absorbing heat to change phase

6
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

7
6 possible phase changes
8
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

9
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

10
Identify a phase change as endothermic or
exothermic
endothermic
exothermic
gas
freezing condensation deposition
melting (fusion) vaporization sublimation
PE
liquid
solid
11
Heating Cooling Curves
  • way to investigate energy changes
  • Measure temperature as function of time at
    constant heating/cooling rate

12
III
IV
V
Liquid Gas
Solid Liquid
Solid
Liquid
Gas
K.E.?
K.E.?
Boil pt.
K.E.?
K.E.?
P.E.?
P.E.?
Temperature
P.E.?
Melt pt.
P.E.?
P.E.?
Time
13
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 (C, K)

14
What happens to temperature as heat is added at
boiling point?
  • Nothing
  • temp doesnt change until only 1 phase present!

15
To analyze a heating/cooling curve
  • Does curve 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

16
Heating curve of H2O
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