heat: phase change. 'change of phase' 'change of state'. the term 'change...
TRANSCRIPT
Heat: Phase Change
The term 'change of phase''change of phase' means the same thing as the term 'change of state'.'change of state'. o These changes of phase always occur
with a change of heat. o Heat (which is energy) either comes into
the material during a change of phase or heat comes out of the material during this change of phase.
o AAlthough the lthough the heat contentheat content of the material of the material changes, the changes, the temperaturetemperature does not. does not.
How could there be a change in heat during a phase change without a change in temperature?
If heat is coming into a substance during a phase change, ththee energy energy is used to break the bonds is used to break the bonds between the moleculesbetween the molecules of the substance.
What that means is… Heat enters a liquid to break apart the bonds
when the liquid boils (evaporates) into a gas, and heat enters a solid to break apart the bonds when it melts into a liquid.
Heat leaves a gas to form the molecular bonds when the gas condenses into a liquid, and heat leaves a liquid to form the molecular bonds when it freezes into a solid.
Therefore, Therefore, the temperature does not change the temperature does not change during a change in phaseduring a change in phase! The heat content ! The heat content does change.does change.
2.02
2.06
That means…
When calculating the heat absorbed or released during a phase change, use this formula:
Q = mQ = mHH
Hff (for fusion: ice water) = 333J/g
Hvv (for vaporization: water steam) = 2260J/g
Let’s try a problem like this…
50.0g of ice at 00C melts. a. How much energy does this involve? b. Is the energy given off or absorbed?c. Is this endothermic or exothermic?
Q = mQ = mHHff
Q = ?Q = ?
m = 50.0gm = 50.0g
HHff = 333 J/g = 333 J/g
Q = (50)(333)Q = (50)(333)
Q = 16650 JQ = 16650 J
Q = 16700JQ = 16700J
Heat must be absorbed to make the ice melt. The reaction is therefore, endothermic.
245g of water at 100.0oC boils and changes to steam. A. How much energy does this involve? B. Is the energy released or absorbed?C. Is this endothermic or exothermic?
Q = mQ = mHHvv
Q = ?Q = ?
m = 245 gm = 245 g
HHvv = 2260 J/g = 2260 J/g
Q = (245)(2260)Q = (245)(2260)
Q = 553700 JQ = 553700 J
Q = 554000JQ = 554000J
Heat must be absorbed to make the water evaporate. The reaction is therefore, endothermic.
How much energy does it take to convert 22g of ice at 0oC to water at 10.oC?
Ice to water
Q = mHf
Q = (22)(333)Q = 7300JIt takes 7300J of energy
to make the ice into water.
Water at 0oC to water at 10oC
Q = mct
Q = (22.0)(4.18)(10.)
Q = 920J
It takes 920J of energy to get the water to 10oC.
It takes 7300 + 920 = 8200J of energy to make the ice at 0oC into water at 10oC.
Two things have to happen. The ice has to change to water and then the water has to warm up to 10.oC.
How much energy does it take to make 125g of water at 50.0oC into steam at 140.0oC?
Water at 50.0oC to water at 100.oC
Q = mct
Q = (125)(4.18)(50.)
Q = 26100J
It takes 26100J of energy to get the water to 100oC.
Water to steam
Q = mHv
Q = (125)(2260)
Q = 283000J
It takes 283000J of energy to change the water into steam
Steam at 100oC to steam at 140oC
Q = mct
Q = (125)(2.02)(40)
Q = 10100J
It takes 10100J of energy to change the steam at 100oC to steam at 140oC.
The total: 26100 + 283000 + 10100 = 319000J
It takes 319000J of energy to change the water at 50oC to steam at 140oC.
How much heat does it take to make 175g of ice at -30.0oC into water at 45.0oC?
ice at -30.0oC to ice at 0oC
Q = mct
Q = (175)(2.06)(30.0)
Q = 10800J
It takes 10800J of energy to get the ice to 0oC.
ENDOTHERMIC
ice to water
Q = mHf
Q = (175)(333)
Q = 58300J
It takes 58300J of energy to change the ice into water
ENDOTHERMIC
water at 0oC to water at 45oC
Q = mct
Q = (175)(4.18)(45)
Q = 32900J
It takes 32900J of energy to change the water to 45.0oC.
ENDOTHERMIC
The total: 10800 + 58300 + 32900 = 102000J
It takes 102000J of energy to change the ice at –30.0oC to water at 45.0oC.
How much heat energy is released when 5.00g of steam at 100.oC changes to water at 80.0oC?
steam to water
Q = mHv
Q = (5.00)(-2260)Q = -11300J11300J of energy is
released when the steam changes to water.EXOTHERMIC
Water at 100.oC to water at 80oC
Q = mct
Q = (5.00)(4.18)(-20.)
Q = -418J
418J of energy is released.
EXOTHERMIC
-11300 + - 418 = -11700J of energy is developed when the steam changes to water.