why is water more stable as a solid below 0 o c but as a liquid above it?
TRANSCRIPT
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Why is water more stable as a solid below 0 oC but as a liquid
above it?
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What controls why only some things happen?
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Energy is neither created or destroyed during chemical or
physical changes, but it is transformed from one form
to another.
Euniverse = 0
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TYPES of ENERGY
Kinetic Potential
Mechanical Gravitational
Thermal Electrostatic
Electrical Chemical
Radiant
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Dropping a rock?
1 2 3
0% 0%0%
1. Gravitational Mechanical
2. Gravitational Thermal
3. Gravitational Gravitational
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Using a flashlight?
1 2 3 4
0% 0%0%0%
1. Thermal Radiant
2. Chemical Thermal
3. Chemical Radiant
4. Electrostatic Radiant
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Driving a Car?
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One turn of a giant windmill produces 0.26 kWh of electricity. Howmany kJ is this?
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Endo: heat added to system Exo: heat released by system
SYSTEM and SURROUNDINGSSystem: The thing under studySurroundings: Everything else in the universe
Energy transfer between system and surroundings:
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Dissolution of KNO3
HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)
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HEAT: What happens to thermal (heat) energy?
Three possibilities:
• Warms another object
• Causes a change of state
• Is used in an endothermic reaction
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Example 1: 5 g wood at 0 oC + 5 g wood at 100 oCExample 2: 10 g wood at 0 oC + 5 g wood at 100 oCExample 3: 5 g copper at 0 oC + 5 g copper at 100 oCExample 4: 5 g wood at 0 oC + 5 g copper at 100 oC
Clicker Choices: 1: 0 oC 2: 33 oC 3: 50 oC 4. 67 oC 5: 100 oC 6: other
Temperature Changes from Heat Exchange
Bill, go to the next slide. You know you want to.
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1 2 3 4 5 6
17% 17% 17%17%17%17%
Temperature Changes from Heat Exchange
5 g wood at 0 oC + 5 g wood at 100 oC
1. 0 oC
2. 33 oC
3. 50 oC
4. 67 oC
5. 100 oC
6. other
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1 2 3 4 5 6
17% 17% 17%17%17%17%
Temperature Changes from Heat Exchange
10 g wood at 0 oC + 5 g wood at 100 oC
1. 0 oC
2. 33 oC
3. 50 oC
4. 67 oC
5. 100 oC
6. other
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1 2 3 4 5 6
17% 17% 17%17%17%17%
Temperature Changes from Heat Exchange
5 g copper at 0 oC + 5 g copper at 100 oC
1. 0 oC
2. 33 oC
3. 50 oC
4. 67 oC
5. 100 oC
6. other
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1 2 3 4 5 6
17% 17% 17%17%17%17%
Temperature Changes from Heat Exchange
5 g wood at 0 oC + 5 g copper at 100 oC
1. 0 oC
2. 33 oC
3. 50 oC
4. 67 oC
5. 100 oC
6. other
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Conclusion I: What happens to thermal (heat) energy?
When objects of different temperature meet:
• Warmer object cools• Cooler object warms• Thermal energy is transferred
• qwarmer = -qcooler
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Conclusion II: What controls the magnitude of an object’s temperature change?
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Quantitative: Calculating Heat Exchange: Specific Heat Capacity
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Calculate the specific heat capacity of copper:
Calculate the specific heat capacity of wood:
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Which has the smallest specific heat capacity?
1 2 3 4
25% 25%25%25%1. Wood
2. Copper
3. Silver
4. Water
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Specific Heat Capacity
The energy required to heat one gram of a substance by 1 oC.
Usefulness: #J transferred = S.H. x #g x T
How much energy is used to heat 250 g water from 17 oC to 100 oC?
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What happens to thermal (heat) energy?
When objects of different temperature meet:
• Warmer object cools• Cooler object warms• Thermal energy is transferred
• qwarmer = -qcooler
specific heat x mass x T = specific heat x mass x T
warmer object cooler object
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Heat transfer between substances:
owood o
Jq = 1.8 5 g (-18 C)
g C
= -160 J
oCu o
Jq = 0.385 5 g (+82 C)
g C
= +160 J
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Conceptually Easy Example with Annoying Algebra:
If we mix 250 g H2O at 95 oC with 50 g H2O at 5 oC,
what will the final temperature be?
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Thermal Energy and Phase Changes
First: What happens?
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Thermal Energy and Phase Changes
First: What happens?
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Thermal Energy and Phase Changes
First: What happens?
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Warming:
• Molecules move more rapidly
• Kinetic Energy increases
• Temperature increases
Melting/Boiling:
• Molecules do NOT move more rapidly
• Temperature remains constant
• Intermolecular bonds are broken
• Chemical potential energy (enthalpy) increases
But what’s really happening?
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Energy and Phase Changes: Quantitative Treatment
Melting:
Heat of Fusion (DHfus) for Water: 333 J/g
Boiling:
Heat of Vaporization (DHvap) for Water: 2256 J/g
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Total Quantitative Analysis
Convert 40.0 g of ice at –30 oC to steam at 125 oC
Warm ice: (Specific heat = 2.06 J/g-oC)
Melt ice:
Warm water (s.h. = 4.18 J/g-oC)
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Total Quantitative Analysis
Convert 40.0 g of ice at –30 oC to steam at 125 oC
Boil water:
Warm steam (s.h. = 1.92 J/g-oC)
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Enthalpy Change and Chemical Reactions
H = energy needed to break bonds – energy released forming bonds
Example: formation of water:
H = ?
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Enthalpy Change and Chemical Reactions
H is usually more complicated, due to solvent and solid interactions.
So, we measure H experimentally.
Calorimetry
Run reaction in a way that the heat exchangedcan be measured. Use a “calorimeter.”
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Calorimetry Experiment
N2H4 + 3 O2 2 NO2 + 2 H2O
Energy released = E absorbed by water +E absorbed by calorimeter
Ewater =
Ecalorimeter =
Total E =
H = energy/moles =
0.500 g N2H4
600 g water
420 J/oC
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Hess’s Law
If reactions can be “added”so can their H values.