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Heang Water: Heang Curve Demonstraon · 2018. 2. 24. · Water molar mass 18 g/mol boiling point...
Transcript of Heang Water: Heang Curve Demonstraon · 2018. 2. 24. · Water molar mass 18 g/mol boiling point...
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Hea$ngWater:Hea$ngCurveDemonstra$on
DepartmentofChemistry&BiochemistryUniversityofOregon
Eugene,OR97403USA
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Boiling vs. Evaporation Make a list: differences & similarities
Write a definition for boiling and for evaporation.
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Boiling – what is going on when we boil water?
Whichequa$onbestrepresentstheprocess?
A. H2O(l)àH2O(g)
B. 2H2O(l)à2H2(g)+O2(g)
C. H2O(g)àH2O(l)
D. 2H2(g)+O2(g)àH2O(l)
E. H2O(l)à2H(g)+O(g)
2pts
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Boiling represents the transition from liquid to gas.
• boiling water: H2O(l) à H2O(g) • boiling ethanol: CH3CH2OH(l) à CH3CH2OH(g)
If VP < external pressure, bubble collapses. If VP > external pressure, boiling can occur
The boiling point of a liquid is the temperature at which the vapor pressure equals the external pressure.
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Minimum escape kinetic energy
Average kinetic energy
All substances at the same temperature have the same average kinetic energy. However, there is always a distribution of
velocities, leading to a distribution of kinetic energies.
A certain fraction has enough energy to escape
into the gas phase.
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Evaporation Big Concept: molecules in a liquid state must have sufficient kinetic energy to overcome the intermolecular forces of attraction operating among the liquid molecules. Strong IMFs between molecules => Force of attraction is Strong
Gas phase
Liquid phase
H2O
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List the distinct steps involved in taking ice at -25°C to steam at 125°C .
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Energy Changes Associated with Changes of State
H2O(s) (ice)
H2O(l) (water)
H2O(g) (steam)
0°C melting
100°C boiling
heating heating heating
There is energy associated with each of these 5 processes.
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Tempe
rature(°C)
Time(minutes)
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For example, how much energy is associated with heating one mole of H2O from -25°C to 125°C?
Thereare5stepsinvolvedinthisprocess:1) hea$ngoftheicetoitsmel$ngpoint2) thephasechangefromsolidtoliquid3) hea$ngoftheliquidtoitsboilingpoint4) thephasechangefromliquidtogas5) hea$ngthegastothefinaltemperature
energy associated with heating: q = m×c×ΔT
energy associated with phase changes q = (ΔHphase change)(number of moles) q = (kJ/mol)(mol)
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The total energy is the sum of the energies for each step:
q1 + q2 + q3 + q4 + q5 = q total
Total Energy Required
Be careful with signs and units!
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Watermolarmass 18g/molboilingpoint 100°CFreezingpoint 0°CHeatofvaporiza$on 40.7kJ/molHeatoffusion 6.02kJ/molspecificheatcapacity(liquid) 4.18J/g°Cspecificheatcapacity(gas) 2.01J/g°Cspecificheatcapacity(solid) 2.09J/g°C
How much energy is associated with heating one mole of H2O from -25°C to 125°C?
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Region 1 solid ice at -25°C to solid ice at 0.0°C
• Heating 1.00 mole of ice at −25.0 °C up to the melting point, ice at 0.0 °C
• q = mass × cice × ΔT – Mass of 1.00 mole of H2O = 18.0 g – cice = 2.09 J/g ·°C
q = (18.0 g)(2.09 J/g°C)[0.0°C-(-25.0°C)]
q = 941 J q = 0.941 kJ
2pts
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• Melting 1.00 mole of ice at the melting point, 0.0 °C a phase change solid -> liquid
• q = n · ΔHfus n = 1.00 mole of ice ΔHfus = 6.02 kJ/mol
Region 2 solid ice at 0*C to liquid water at 0°C
q = (1.00 mol) (6.02 kJ/mol) = 6.02 kJ
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• Heating 1.00 mole of water at 0.0 °C up to the boiling point, 100.0 °C
• q = mass × cwater × ΔT mass of 1.00 mole of water = 18.0 g cwater = 4.18 J/g · °C
Region 3 water at 0°C to water at 100°C
q = (18.0 g)(4.18 J/g°C)(100.0°C-0.0°C) q = 7.52 x 103 J q = 7.52 kJ
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• Boiling 1.00 mole of water at the boiling point, 100.0 °C (phase change)
• q = n · ΔHvap n = 1.00 mole of ice ΔHvap = 40.7 kJ/mol
Region 4 water at 100°C to steam at 100°C
q = (1.00 mol) (40.7 kJ/mol) = 40.7 kJ
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• Heating 1.00 mole of steam at 100.0 °C up to 125.0 °C
• q = mass × csteam × ΔT mass of 1.00 mole of water = 18.0 g csteam = 2.01 J/g·°C
Region 5 steam at 100°C to steam at 125°C
q = (18.0 g)(2.01 J/g°C)(125.0°C-100.0°C) q = 904 J = 0.904 kJ
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The total energy is the sum of the energies for each step:
q1 + q2 + q3 + q4 + q5 = q total
0.941 kJ + 6.02 kJ + 7.52 kJ + 40.7 kJ + 0.904 kJ = 56.08 kJ
Total Energy Required
Be careful with signs and units!
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Energy Changes Associated with Changes of State
• The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other.
• The temperature of the substance does not rise during a phase change.
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Energy Changes Associated with Changes of State
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