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)

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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

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Heating curve of H2O