Thermodynamics and the End of the Universe

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Thermodynamics and the End of the Universe

• Featuring Your Host
• Dr. Mike Maddog Daniel

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Thermodynamics

• Concerned with energy changes and flow of energy.
• Chemical focus is on using Thermodynamic
  properties to determine if a reaction is
  spontaneous under a set of conditions (mostly T).
• Engineering focus on maximizing useful energy.

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Thermodynamics

• Spontaneous means the reaction is possible
  without outside help
• Thermochemistry - Study of energy given off or
  absorbed by a chemical reaction.

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Thermochemistry Review

• Potential Energy - Stored Energy
• due to natural attractions and repulsions
  between different objects
• Kinetic Energy-Energy of motion m v2/2

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Changes in matter require changes in energy
H2 1/2 O2?
Potential Energy
H2O (L)?
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Changes in matter require changes in energy
H2 1/2 O2?
- H2O has less potential energy than
elements. -Reaction converts potential energy
into kinetic energy
H2O (L)?
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1st Law of Thermodynamics

• Energy of universe is constant (energy is
  conserved)
• System Surroundings Universe
• If the system loses energy, then the surroundings
  gains an amount of energy

System
Surroundings
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1st Law of Thermodynamics

• ?Euniverse 0
• ?Euniverse ?Esystem ?Esurroundings
• ?Esystem -?E surroundings

System
Surroundings
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State Functions

• A function that only depends on the initial and
  final state of a system.
• State Function Final State - Initial State
• ?T Tf - Ti
• Functions will refer to the system unless noted.

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Enthalpy Change

• ?H change in enthalpy, change in energy at a
  constant pressure.
• For a reaction
• ?H H(reactants) - H(products)

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Enthalpy Change

• We cant measure absolute H, so we measure heat
  or enthalpy of formation
• ?Hfo Ho(compound) - Ho(elements in standard
  state)
• ?Hfo elements 0, (by definition)
• A B ? AB ?Hrxno
• ?Hrxno ?Hf,AB o

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Internal Energy

• Total Energy of System E
• (Internal Energy, U is also used)
• E PE KE
• Similar to H, only changes (?E), can be measured
• 1st Law of Thermodynamics often stated as
• ?E q w (heat work)

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?E q w (heat work)

• q and w are path functions, depend on how
  process is done
• The sum of q and w is a state function
• q gt 0 means heat is added to system
• w gt 0 means work is done to system. Physics
  uses wlt0 for work done to system

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?E q w (heat work)

• Engineers usually want to maximize w (we want
  our cars to move, not get hot)

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Work

• Here we consider only pressure volume work, not
  electrical or other kind.
• H2O(L) ? H2O(G), q gt0
• The volume expansion of water could push a piston
  against atmospheric pressure, doing work on the
  surroundings
• w lt 0

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More Work

• For a gas pushing a piston, x is the direction of
  expansion and A is the piston area.

x
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More Work

• For a gas pushing a piston, x will the the
  direction of expansion and A will be the piston
  area.
• w -? F dx
• Patm F/A ? Patm A F (substitute above)
• V Ax
• dV A dx ? dx dV / A (substitute above)

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More Work

• w -? F dx
• w -? Patm A dV
• A
• w -? Patm dV (assumes Patm constant)
• w -P ?V
• Units w Fx (kg m/s2) m
• w P ?V kg m/(m2 s2) m3

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Heat of Rxn at Constant Volume

• ?E q w q - P ?V
• In a closed container, ?V 0 so
• ?E q (heat of reaction at constant volume)

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Internal Energy and Enthalpy

• ?E q at constant volume
• ?E q w
• ?E ?H - P ?V (for constant P)
• ?H - ?E P ?V

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Volume Expansion Work

• ?E ?H - P ?V
• For Expansion ?E lt ?H

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Steam Expansion Work

• ?E q w
• ?H - P ?V (for constant P and T)
• H2O (L) heat ? H2O (G)
• ?Ho ? (at 25O)

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Steam Expansion Work

• ?Ho -241.8 - (-285.9) 44.1 kJ/mol
• ?Hvapo 44.1 kJ/mol (?Hvap is lower at higher T
• P ?V ?ngasRT (for an ideal gas)
• ?ngas nf - ni
• (1 mole) (8.314 j/K mol) (298 K)
• 2.48 kJ

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Steam Expansion Work

• ?Ho 44.1 kJ/mole
• P ?V 2.48 kJ
• ?E ?H - P ?V 41.6 kJ

-2.5 kJ
44.1 kJ
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Solid and Liquid Rxns.

• When reactants and products are all liquid or
  solid, ?V is very small and ?E ? ?H

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Even more Work

• Consider
• CaCO3 (s) energy ? CaO(s) CO2 (g)
• CaO(s) 2 HCl(g) ? CaCl2 (s) H2O (g)
• What is work and ?E?