1. Chapter 7: Thermochemistry
Chemistry 140 Fall 2002
Chapter 7: Thermochemistry
Juana Mendenhall, Ph.D.
Assistant Professor
Lecture 1 March 29
General Chemistry: Chapter 20
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2. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Objectives
Define Terms involved in energy of chemical reactions.
Define, explain, and apply the first law of thermodynamics.
Define, explain, and apply the second law of thermodynamics.
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3. General Chemistry: Chapter 20
Definition of Terms
Every chemical reaction obeys two fundamental laws:
The law of conservation of mass
The law of conservation of energy
Energy (U): the capacity to do work
Work: directed energy chance resulting from a process i.e., force x distance
Kinetic Energy: movement
Radiant Energy: from sun (solar)
Thermal Energy: random motion of atoms & molecules
Chemical Energy: structural units of chemical substances
Potential Energy: stationary
Law of conservation of energy: energy can either be created nor destroyed, it is transferred from one object to another
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4. General Chemistry: Chapter 20
Definition of Terms
Heat: transfer of thermal energy b/w two bodies that are at different temperatures.
Thermochemistry: study of heat changes in chemical reactions
In order to analyze energy changes associated with chemical reactions, we first define the system & its surroundings.
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5. 7-1 Getting Started: Some Terminology
System
Surroundings
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6. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Heat Transfer
Energy transferred between a system and its surroundings as a result of a temperature difference.
Exothermic process: any process that gives off heat (transfers thermal energy to its surroundings)
Endothermic process: any process in which heat has to be applied to the system by the surroundings.
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7. General Chemistry: Chapter 20
Enthalpy (H)
Enthalpy (H): E + PV; heat flow into or out of a system.
We measure H: which is the difference between the enthalpies of the products and reactants.
H = H(products) - H(reactants)
The change in enthalpy, is equal to the heat given off or absorbed; value can be + or -.
If H > 0 process is endothermic
If H < 0 process is exothermic
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8. General Chemistry: Chapter 20
Units of Heat
Calorie (cal)
The quantity of heat required to change the temperature of one gram of water by one degree Celsius.
Joule (J)
SI unit for heat
1 cal = J
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9. General Chemistry: Chapter 20
Heat Capacity
The quantity of heat required to change the temperature of a system by one degree.
Molar heat capacity, m.
System is one mole of substance.
Specific heat capacity, s.
The amount of heat required to raise the temp. of one gram of the substance by one degree Cº
Heat capacity, C
The amount of heat required to raise the temperature of a given quantity of the substance by one degree Cº
Mass x specific heat.
C= ms
If we know the specific heat and the amount of sub
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10. General Chemistry: Chapter 20
Heat Capacity
If we know the specific heat & the amount of a
substance, then the change in the sample’s temp.
will give use the amount of heat (q)
q =
msT
q =
CT
Ex. A 466-g sample of water is heated from 8.50ºC to 74.60ºC.
Calculate the amount of heat absorbed by the water.
Answer: we use
q = msT
= (466g)(4.184J/gxºC)(74.60ºC ºC)
= 1.29 x 105 J
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11. Conservation of Energy
In interactions between a system and its surroundings the total energy remains constant— energy is neither created nor destroyed.
qsystem + qsurroundings = 0
qsystem = -qsurroundings
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12. 7-3 Heats of Reaction and Calorimetry
Chemistry 140 Fall 2002
7-3 Heats of Reaction and Calorimetry
Chemical energy.
Contributes to the internal energy of a system.
Heat of reaction, qrxn.
The quantity of heat exchanged between a system and its surroundings when a chemical reaction occurs within the system, at constant temperature.
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13. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Heats of Reaction
Exothermic reactions.
Produces heat, qrxn < 0.
Endothermic reactions.
Consumes heat, qrxn > 0.
Calorimeter
A device for measuring quantities of heat.
Add water
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14. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Bomb Calorimeter
qrxn = -qcal
qcal = q bomb + q water + q wires +…
Define the heat capacity of the calorimeter:
qcal = miciT = CT
all i
heat
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15. Coffee Cup Calorimeter Constant-Pressure Calorimetery
A simple calorimeter.
Well insulated and therefore isolated.
Measure temperature change.
qsystem = qsoln + qcalorimeter + qrxn
qrxn = -qsoln + qcalorimeter
Measures heat of neutralization, heat of fusion, heat of vaporization,
Heat of reactions. .
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16. General Chemistry: Chapter 20
Example 2
A quantity of 1.00 x 102 mL of M HCl is mixed with
1.00 x 102 mL of M NaOH in a styrofoam cup calorimeter having
heat capacity of 335 J/ºC. The initial temperature of the HCl and NaOH
solutions is the same, 22.50ºC, and the final temperature of the mixed
Solution is 24.90ºC. Calcualte the heat change for the neutralization
reaction.
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
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17. 7-5 The First Law of Thermodynamics
Internal Energy, U.
Total energy (potential and kinetic) in a system.
Translational kinetic energy.
Molecular rotation.
Bond vibration.
Intermolecular attractions.
Chemical bonds.
Electrons.
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18. First Law of Thermodynamics
A system contains only internal energy.
A system does not contain heat or work.
These only occur during a change in the system.
Law of Conservation of Energy
The energy of an isolated system is constant
U = q + w
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19. Free Energy and Free Energy Change
For the universe:
TΔSuniv. = TΔSsys – ΔHsys = -(ΔHsys – TΔSsys)
-TΔSuniv. = ΔHsys – TΔSsys
For the system:
G = H - TS
ΔG = ΔH - TΔS
ΔGsys = - TΔSuniverse
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20. General Chemistry: Chapter 20
Spontaneous Process
Potential energy decreases.
For chemical systems the internal energy U is equivalent to potential energy.
Berthelot and Thomsen 1870’s.
Spontaneous change occurs in the direction in which the enthalpy of a system decreases.
Mainly true but there are exceptions.
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21. General Chemistry: Chapter 20
Spontaneous Process
A process that occurs in a system left to itself.
Once started, no external actions is necessary to make the process continue.
A non-spontaneous process will not occur without external action continuously applied.
4 Fe(s) + 3 O2(g) → 2 Fe2O3(s)
H2O(s) H2O(l)
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22. General Chemistry: Chapter 20
19-4 Criteria for Spontaneous Change: The Second Law of Thermodynamics.
ΔStotal = ΔSuniverse = ΔSsystem + ΔSsurroundings
The Second Law of Thermodynamics:
ΔSuniverse = ΔSsystem + ΔSsurroundings > 0
All spontaneous processes produce an increase in the entropy of the universe.
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23. Criteria for Spontaneous Change
ΔGsys < 0 (negative), the process is spontaneous.
ΔGsys = 0 (zero), the process is at equilibrium.
ΔGsys > 0 (positive), the process is non-spontaneous.
J. Willard Gibbs
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24. Table 19.1 Criteria for Spontaneous Change
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25. General Chemistry: Chapter 20
7-4 Work
In addition to heat effects chemical reactions may also do work.
Gas formed pushes against the atmosphere.
Volume changes.
Pressure-volume work.
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26. General Chemistry: Chapter 20
Chemistry 140 Fall 2002
Pressure Volume Work
w = F  d
= (m  g)
 h
(m  g)
 A =
 h A
= PV
w = -PextV
Negative sign is introduced for Pext because the system does work ON the surroundings.
When a gas expands V is positive and w is negative.
When a gas is compressed V is negative and w is positive, indicating that energy (as work) enters the system.
In many cases the external pressure is the same as the internal pressure of the system, so Pext is often just represented as P
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27. 19-1 Spontaneity: The Meaning of Spontaneous Change
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28. 19-2 The Concept of Entropy
Entropy, S.
The greater the number of configurations of the microscopic particles among the energy levels in a particular system, the greater the entropy of the system.
ΔS > 0 spontaneous
ΔU = ΔH = 0
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29. General Chemistry: Chapter 20
Entropy Change
ΔS =
qrev T
For changes occurring at constant temperature
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30. 19-3 Evaluating Entropy and Entropy Changes
Phase transitions.
Exchange of heat can be carried out reversibly.
ΔS = ΔH
Ttr
H2O(s, 1 atm) H2O(l, 1 atm) ΔHfus = 6.02 kJ at K
ΔSfus =
ΔHfus
Ttr =
6.02 kJ mol-1 K
= 2.2010-2 kJ mol-1 K-1
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31. Free Energy and Free Energy Change
Chemistry 140 Fall 2002
Free Energy and Free Energy Change
Hypothetical process:
only pressure-volume work, at constant T and P.
qsurroundings = -qp = -ΔHsys
Make the enthalpy change reversible.
large surroundings, infinitesimal change in temperature.
Under these conditions we can calculate entropy.
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