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Why is water more stable as a solid below 0 oC but as a liquid
above it?
What controls why only some things happen?
Energy is neither created or destroyed during chemical or
physical changes, but it is transformed from one form
to another.
Euniverse = 0
TYPES of ENERGY
Kinetic Potential
Mechanical Gravitational
Thermal Electrostatic
Electrical Chemical
Radiant
Dropping a rock?
1 2 3
0% 0%0%
1. Gravitational Mechanical
2. Gravitational Thermal
3. Gravitational Gravitational
Using a flashlight?
1 2 3 4
0% 0%0%0%
1. Thermal Radiant
2. Chemical Thermal
3. Chemical Radiant
4. Electrostatic Radiant
Driving a Car?
One turn of a giant windmill produces 0.26 kWh of electricity. Howmany kJ is this?
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:
Dissolution of KNO3
HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)
HEAT: What happens to thermal (heat) energy?
Three possibilities:
• Warms another object
• Causes a change of state
• Is used in an endothermic reaction
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.
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
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
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
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
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
Conclusion II: What controls the magnitude of an object’s temperature change?
Quantitative: Calculating Heat Exchange: Specific Heat Capacity
Calculate the specific heat capacity of copper:
Calculate the specific heat capacity of wood:
Which has the smallest specific heat capacity?
1 2 3 4
25% 25%25%25%1. Wood
2. Copper
3. Silver
4. Water
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?
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
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
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?
Thermal Energy and Phase Changes
First: What happens?
Thermal Energy and Phase Changes
First: What happens?
Thermal Energy and Phase Changes
First: What happens?
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?
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
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)
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)
Enthalpy Change and Chemical Reactions
H = energy needed to break bonds – energy released forming bonds
Example: formation of water:
H = ?
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.”
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
Hess’s Law
If reactions can be “added”so can their H values.