What happens to thermal (heat) energy?

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Thermodynamics
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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
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TYPES of ENERGY Kinetic Potential Mechanical
Gravitational Thermal Electrostatic Electrical
Chemical Radiant
Energy Conversion Examples 1. dropping a
rock 2. using a flashlight 3. driving a car
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SYSTEMS and SURROUNDINGS System The thing
under study Surroundings Everything else in the
universe Energy transfer between system and
surroundings
Endo heat added to system Exo heat released by
system
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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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Temperature Changes from Heat Exchange
Example 1 5 g wood at 0 oC 5 g wood at
100 oC Example 2 10 g wood at 0 oC 5 g
wood at 100 oC Example 3 5 g copper at 0 oC
5 g copper at 100 oC Example 4 5 g wood at 0
oC 5 g copper at 100 oC
Choices 1 0 oC 2 33 oC 3 50 oC 4.
67 oC 5 100 oC 6 other
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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

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Quantitative Calculating Heat Exchange Specific
Heat Capacity
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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
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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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But whats really happening?

• 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

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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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Energy and Chemical Reactions
Lots of different types of energy.We use
EnthalpyHeat exchanged under constant pressure.
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Energy/Enthalpy Diagrams
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Some Examples of Enthalpy Change
2 C(s) 2 H2(g) ? C2H4(g) ?H 52 kJ
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Enthalpy Change and Chemical Reactions
DH is usually more complicated, due to solvent
and solid interactions. So, we measure DH
experimentally. Calorimetry Run reaction in a
way that the heat exchanged can be measured. Use
a calorimeter.
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Bomb 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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Enthalpy Change and Bond Energies
DH energy needed to break bonds energy
released forming bondsExample formation of
water
DH 498 (2 x 436) 4 x 436 kJ -482 kJ
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General Rule
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SO2 ½ O2 ? SO3 dH -98.9 kJ2 SO3 ? 2 SO2
O2 dH ?
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Hesss Law
If reactions can be added so can their ?H
values.
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Thermochemistry Lab Calculations Goal What is
?H for the formation of MgO from Mg(s) and
O2(g)? Mg(s) ½ O2(g) ? MgO(s) ?H ?
kJ/mol Data From lab measurements Mg(s) 2
H(aq) ? Mg2(aq) H2(g) ?H1 ___________
kJ/mol MgO(s) 2 H(aq) ? Mg2(aq) H2O(l)
?H2 ___________ kJ/mol From a table
H2(g) ½ O2(g) ? H2O(l) ?H3 -285.8
kJ/mol Task Find a way to add these three
reactions to get the desired reaction. Manipulate
the ?H values as needed, and add them.
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Calculating Heat Production
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Heat of Formation
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Heat of Formation The general idea
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Find the enthalpy change for burning ethyl alcohol