Chapter 9 Energy, Enthalpy and Thermochemistry

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Chapter 9Energy, Enthalpy and Thermochemistry

• The study of energy and its interconversions is
  called thermodynamics.
• Kinetic Energy energy due to the motion of the
  object (1/2 mv2)
• Potential Energy energy due to position or
  composition
• Heat the transfer of energy between two objects
  due to a temperature difference
• Work a force acting over a distance

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Figure 9.1 (a) Initial position of balls
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Figure 9.1 (b) Final position of balls
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Energy of Matters
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State Function

• A property of the system depends only on its
  present state. A state function does not depend
  in any way on the systems past.
• Energy is a state function, but work and heat are
  not state function

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Combustion of methane
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Nitrogen/oxygen
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First Law of Thermodynamics

• The energy of the universe is constant

In closed system ?E ?Uqw (?????) q the
heat added to the system during the process w
the work done on the system during the
process qgt0 heat flows into the system from the
surroundings qlt0 an outflow of heat from the
system to the surroundings wgt0 work is done on
the system by the surroundings wlt0 the system
does work on the surroundings
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P-V Work
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P-V Work
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Enthalpy (?)

• The heat qp absorbed in a constant-pressure
• process equals the systems enthalpy change.

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• For a chemical reaction
• ?H?Hproducts-?Hreactants
• If ?Hreactantslt?Hproducts (endothermic)
• If ?Hreactantsgt?Hproducts (exothermic)
• Consider a constant-volume process
• dw-PdV0 (???????)
• ?Uqwqv
• ?Uqv

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Heat Capacity (???)
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Thermodynamics of Ideal Gases
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Heat Capacity of Heating an Ideal Monatomic Gas

• Under constant volume, the energy flowing into
  the gas is used to increase the translational
  energy of the gas molecules.

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Heat Capacity of Ideal Monatomic Gases
Gas Cv (J/K mol) Cp (J/K mol)
He 12.47 20.8
Ne 12.47 20.8
Ar 12.47 20.8
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Heat Capacity of Diatomic Gases
Gas Cv (J/K mol) Cp (J/K mol)
H2 20.54 28.86
N2 20.71 29.03
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Heat Capacity of Polyatomic Gases
Gas Cv (J/K mol) Cp (J/K mol)
N2O 30.38 38.70
CO2 28.95 37.27
C2H6 44.60 52.92
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Heat Capacity of Heating a Polyatomic Gas

• Polyatomic gases have observed values for Cv that
  are significantly greater than 3/2 R.
• This larger value for Cv results because
  polyatomic molecules absorb energy to excite
  rotational and vibrational motions in addition to
  translational motions.

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Cv and Cp of molecules
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Cv and Cp of Monatomic Gas
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Cv and Cp of H2O at 373K
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2 mol of monatomic ideal gas Calculate q, w, ?U
and ?H for both pathway
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• TA122K, TC366K, TB183K, TD61K
• Cv3/2R, Cp5/2R
• Path 1(A?C)
• w1-P?V-2atm(30-10)L101.3J/Latm-4.05103J
• q1qpnCp?T25/2(R)(366-122)1.01104J?H1
• ?U1nCv?T 23/2(R)(366-122)6.08103J
• Path 2(C?B)
• q2qvnCv?T23/2(R)(183-366)-4.56103J?U2
• ?H2nCp?T 25/2(R)(183-366)-7.6103J
• ?V0 w2 -P?V 0

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• Path 3(A?D)
• q3qvnCv?T23/2(R)(61-122)-1.52103J?U3
• ?H3nCp?T 25/2(R)(61-122)-2.53103J
• ?V0 w2 -P?V 0
• Path 4(D?B)
• w1-P?V-1atm(30-10)L101.3J/Latm-2.03103J
• q4qpnCp?T25/2(R)(183-61)5.08104J?H4
• ?U4nCv?T 23/2(R)(183-61)3.05103J

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Summary

• Path 1
• qpath1q1q25.5 103J
• wpath1w1w2 -4.05103J
• ?Hpath1 ?H1 ?H2 2.55103J
• ?Upath1 ?U1 ?U2 1.52103J
• Path 2
• qpath2q3q43.56103J
• wpath2w3w4 -2.03103J
• ?Hpath2 ?H3 ?H4 2.55103J
• ?Upath2 ?U3 ?U4 1.52103J

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Calorimetry

• Specific heat capacity with unit JK-1g-1
• Molar heat capacity with unit JK-1mol-1

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Coffee Cup Calorimeter

• A constant-pressure calorimetry is used in
  determining the change in enthalpy equals the
  heat.
• ?HqpnCp?T

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Bomb Calorimeter

• ?V0
• ?Uqv0qv
• ?UqvnCv ? T

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2SO2(g)O2(g)?2SO3(g) ?H-198 KJ 2 mol. 1
mol. 2 mol. Calculate ?H and ?U
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• P is constant, ?Hqp-198 KJ (energy flow out of
  system)
• ?U qp w
• w-P?V and ?V?n(RT/P)
• T and P are constant, ?nnfinal-ninitial-1 mol
• So w-P?V-P?n (RT/P)
• - ?nRT-(-1)(8.314)(298)2.48 kJ
• ?U qp w-198 kJ2.48 kJ-196 kJ

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Hesss Law

• If a reaction is carried out in a series of
  steps, ?H for the reaction will be equal to the
  sum of the enthalpy changes for the individual
  steps
• The overall enthalpy change for the process is
  independent of the number of steps or the
  particular nature of the path by which the
  reaction is carried out.

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• Consider the combustion reaction of methane to
• form CO2 and liquid H2O
• CH4(g) 2O2(g)? CO2(g) 2H2O(l)
• ?H1 -890KJ/mol
• This reaction can be thought of as occurring in
• two steps
• CH4(g) 2O2(g)? CO2(g) 2H2O(g)
• ?H2 -802 kJ/mol
• 2H2O(g)?2H2O(l)
• ?H3 -88KJ/mol
• ?H1 ?H2 ?H3

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Standard Enthalpies of Formation

• The change in enthalpy that accompanies the
  formation of 1 mole of a compound from its
  elements with all substances in their standard
  states.
• The superscript zero indicates that the
  corresponding process has been carried out under
  standard conditions.

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Present Sources of Energy

• Petroleum and Natural Gas Coal

New Energy Sources
Coal Conversion Hydrogen as a fuel
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CO2 capture
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