Thermodynamics
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
Energy Conversion Examples:1. dropping a rock2. using a flashlight3. driving a car
Endo: heat added to system Exo: heat released by system
SYSTEMS and SURROUNDINGSSystem: The thing under studySurroundings: Everything else in the universe
Energy transfer between system and surroundings:
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
Choices: 1: 0 oC 2: 33 oC 3: 50 oC 4. 67 oC 5: 100 oC 6: other
Temperature Changes from Heat Exchange
What happens to thermal (heat) energy?
When objects of different temperature meet:
• Warmer object cools
• Cooler object warms
• Thermal energy is transferred
• qwarmer = -qcooler
Quantitative: Calculating Heat Exchange: Specific Heat Capacity
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 TreatmentMelting:
Heat of Fusion (Hfus) for Water: 333 J/g
Boiling:
Heat of Vaporization (Hvap) for Water: 2256 J/g
Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC
Warm ice: (Specific heat = 2.06 J/g-oC)
oo
J2.06 40.0 g (30.0 C) = 2470 J
g C
oo
J4.18 40.0 g (100.0 C) = 16720 J
g C
333 J40.0 g = 13320 J
g
Melt ice:
Warm water (s.h. = 4.18 J/g-oC)
Total Quantitative AnalysisConvert 40.0 g of ice at –30 oC to steam at 125 oC
Boil water:
oo
J1.92 40.0 g (25.0 C) = 1920 J
g C
2256 J40.0 g = 90240 J
g
Warm steam (s.h. = 1.92 J/g-oC)
Lots of different types of energy.
We use Enthalpy:
Heat exchanged under constant pressure.
Energy and Chemical Reactions
Energy/Enthalpy Diagrams
Some Examples of Enthalpy Change
2 C(s) + 2 H2(g) C2H4(g) H = +52 kJ
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.”
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
Enthalpy Change and Bond Energies
H = energy needed to break bonds – energy released forming bonds
Example: formation of water:
H = [498 + (2 x 436)] – [4 x 436] kJ = -482 kJ
General Rule:
SO2 + ½ O2 SO3 dH = -98.9 kJ
2 SO3 2 SO2 + O2 dH = ?
Hess’s Law
If reactions can be “added”so can their H values.
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/molData: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.
Calculating Heat Production
Heat of Formation
Heat of Formation: The general idea
Find the enthalpy change for burning ethyl alcohol
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