Calculations Involving Phase Changes Phase Change Diagram

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Calculations Involving Phase Changes
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Phase Change Diagram The Heating Curve of Water
(remember this from the lab Watch Water Melt and
Boil?!)
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• On your notes, draw a ½ page sized graph. Title
  your graph Temp, vs Heating Time/Heating Curve.
  Label the X/horizontal axis Time. Label the
  Y/vertical axis with the following temperatures.
  -25C, 0C, 25C, 50C, 75C, 100C, and 125C.

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• Draw the line that represents the Heating Curve
  of Water. Start from -25C to 125C.

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• Draw a 2nd graph. Label its axis like the first
  graph. Title your graph Temp vs Cooling Time/
  Cooling curve. Draw the line that represents the
  cooling curve. Start from 125C and end at -25C

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Latent Heat of Fusion - the amount of heat
absorbed when melting H2O OR the amount of heat
released when solidifying H2O

• ? H fus water - Amount of heat (q) required to
  change a mass (m) of a substance from a solid to
  a liquid or liquid to a solid q m ?HFus

? H fus water 334 Joules/ g
Melting/gaining heat
Freezing/losing heat
? H fus H2O - 334 Joules/g
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Latent Heat of Vaporization - the amount of heat
absorbed when evaporating H2O OR the amount of
heat released when condensing H2O

• Amount of heat (q) required to change a mass (m)
  of a substance from a liquid to a gas or gas to a
  liquid q m ?? Hvap

? H vap water 2260 Joules/ g
vaporizing/gaining heat
Condensing/losing heat
? H vap water - 2260 Joules/ g
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Fusion and Vaporization for Water
q m? H vap /- 2260 Joules/ g
q m ? H fus /- 334 Joules/ g

• For (a- b) (c-d) (e-f) on graph
• use q m ? c ? ? T
• To find energy needed to raise temperature

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Specific Heat Capacity of Water in 3 Phases

• Solid H2O Csp 2.1 j/g.C
• Liquid H2O Csp 4.18 j/g.C
• Gaseous H2O Csp 1.9 j/g.C

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E.g 1 How much heat in KJ is required to
change 50 grams of ice at -10 C to a liquid at
40 C?
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Sample Problem How much heat in KJ is required
to change 50 grams of ice at -10 C to a liquid
at 40 C?
Step 1 Draw a graph of the physical
process Step 2 Find heat it takes to raise ice
from -10 C to 0 C (q m ? c ? ? T)
q (50g) (2.1 J/g C ) (10 C ) 1050
J Step 2 Find heat to do a phase change from
solid- liquid (q m ? H Fus) q
(50 g) (334 J/ g) 16700 J Step 3 Find heat it
takes to raise temp from 0 C to 40 C (q m ?
c ? ? T) q (50 g) (4.18 J/g C ) (40 C )
8360 J Total (add steps 12 3) 26110 J or
26.110 KJ
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• E.g 2 - How much heat is lost when 1.43 grams of
  water is cooled from 74.3 C to -5.34 C?
• 1st I.D. Variables. 2nd Draw Graph
• Step 1 Draw a graph of the physical process
• Step 2 Find the amount of heat lost to cool the
  water from 74.3 C to 0 C
• qliq m ? Csp ? ? T
• q (1.43g) (4.18 J/g C ) (-74.3 C ) -444
  J

Step 3 Phase change How much heat is lost as
water changes from liquid to solid q m ? H
Fus q (1.43 g) (-334 J/ g) -478 J Step 4
How much heat does the ice lose as its temp drops
from 0 C to -5.34 C q m ? Csp ? ? T q(1.43
g)(2.1 J/g C )(-5.34 C ) -16.1 J Total (add
steps 12 3) 938 J