The Thermodynamics of a Rubber Band

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The Thermodynamics of a Rubber Band

• Thinking about Positive and Negative Numbers
• With your host,
• Mike Daniel

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1st Assignment

• Watch Rubber Band to see if it will stretch
  spontaneously.
• A Spontaneous process is a process that occurs
  without outside intervention.

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Systems and Surroundings

• Thermodynamics divides the universe into
• A system, the part of the universe we are
  interested in
• And
• The Surroundings

System
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Spontaneous Processes

• Ball rolling down hill
• Heat flow from hotter object to cooler
• At T gt 0 C ice melts

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Continue Observing the Rubber Band
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Entropy

• Entropy (S) is a measure of Randomness or
  disorder
• Entropy is a thermodynamic function that
  describes the number of arrangements available to
  a system.
• Nature spontaneously proceeds to states that have
  the highest probability of existing.

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Second Law of Thermodynamics

• Any spontaneous process has an increase in the
  entropy of the universe
• ?S Sfinal S initial
• ?Suniverse gt 0 Is the second law in mathematical
  terms.

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Gibbs Free Energy

• J.W. Gibbs One of
• Americas Greatest
• Scientist and early
• Science nerds.
• Contributed to
• mathematics, optics,
• Statistical mechanics, and
• Thermodynamics.

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Gibbs Free Energy

• ?Suniverse hard to measure
• Gibbs developed a certain function, now called
  the Gibbs free energy
• For a spontaneous process, ?Gsystemlt 0
• ?Gsystem easier to measure than
• ?Suniverse

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Continue Observing the Rubber Band
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One last definition, Enthalpy

• The Change in Enthalpy,
• ?H the change in heat energy in a system at
  constant pressure.
• ?H gt 0, means heat is entering a system.
  Endothermic
• An ice cube in liquid water melts due to energy
  entering cube

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One last definition, Enthalpy

• The Change in Enthalpy,
• ?H the change in heat energy in a system at
  constant pressure.
• ?H lt 0, means heat is leaving a system.
  Exothermic.
• A fire is exothermic. You feel the heat leaving
  the fire.

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Review

• S, Entropy is a measure of disorder in a system.
• ?S gt 0 means a systems disorder increased
• H, Enthalpy is a measure of heat energy
• ?H gt 0 means energy entered system
• ?G lt 0 means a process is spontaneous

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Rubber Band Stretching

• Spontaneous or non-spontaneous?
• Stretching ?G gt 0 or lt 0 ?

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Rubber Band Experiment
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Endothermic (?H gt 0) or Exothermic (?H lt 0)?

• ?G ?H - T ?S
• For the Stretching Process
• ?G gt 0
• ?H lt 0
• Is ?S gt or lt 0?

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?G ?H - T ?S

• If ?H gt 0 and ?S gt 0 how will increasing T affect
  spontaneity of the reaction.

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2nd Rubber Band Experiment

• Stretch a Rubber Band with a Weight
• Heat with a Hair Dryer
• Does Rubber Band get longer or shorter?