Thermodynamics

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Thermodynamics

• Chapter 19 Brown-LeMay

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I. Review of Concepts

• Thermodynamics area dealing with energy and
  relationships
• First Law of Thermo law of conservation of
  energy - U q w
• the change in internal energy of a system
  equals q w
• Internal energy is a state function one that is
  independent of the pathway (altitude- state
  function, miles traveled is not a state function

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Concept review

• Heat (q) is the energy that moves into or out of
  a system because of temperature differences
  between the system and the surroundings
• Work (w) energy exchange that results when a
  force (f) moves an object through a distance (d)
  w f x d
• Work done to the system is positive work done by
  the system is negative

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Concept review

• Work in a chemical system can de described by the
  pressure and change in volume of a system
• W -p v
• u qp w cons. pressure
• u qp p v
• H is the reaction q at constant pressure
• H qp

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Delta H

• More precisely H is the quantity u pv
• H u pv
• H u pv
• H qp pv pv
• H qp
• All U,P,V are state functions so H is also a
  state function
• H n H0f(products) - n H0f
  (reactants)

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II. Entropy and the Second Law

• Spontaneous reactions are a physical or
  chemical change that occurs by themselves
• When delta H is neg exothermic the reaction
  tend to be spontaneous
• Spontaneous reactions are non-reversible
• When delta H is positive endothermic the
  reaction may or may not be spontaneous

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II. Entropy and the Second Law

• Endothermic reactions are/not spontaneous because
  of the naturally tendency to ether move towards
  or away from a random state
• Entropy S or S- is a measure of randomness or
  disorder of a system (J/K)
• state function (influenced by temp and
  pressure)

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The Second Law

• The entropy of a system and its surrounds always
  increases in a spontaneous process
• s s (final) s (initial)
• s increase in disorder
• s- decrease in disorder
• different from energy because it can be created

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Second Law

• Delta S entropy created q/T
• Delta S gt q/T spontaneous
• Delta S q/T equilibrium
• For a process to be spontaneous at a given
  temperature, the change in entropy of the system
  is greater than the heat of they system divided
  by the absolute temperature

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Second Law (spontaneous rxn)

• Delta s gt qp/T Delta H/T
• Delta H/T Delta S lt 0
• H T S lt0 spontaneous
• H T S gt0 non-spontaneous
• H T S 0 Equilibrium
• You can often look up Delta H and Delta S
  values form tables and determine if a reaction is
  spontaneous as written under certain conditions

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The Third Law Predicting Entropy Change

• Third Law a substance that is perfectly
  crystalline at 0 Kelvin has an entropy of zero
  as the temp of a substance is raised disorder
  increases as it absorbs heat

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Changes in Entropy

• In general Entropy increases as
• Liquids or solutions are formed from solids
• Gases are formed from either solids or liquids
• The number of molecules of gas increases
  during a chemical reaction
• the temperature of a substance in increased.

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• Ludwig Boltzmann developed a method to
  calculate entropies of substances based on the
  number of possible arrangements of the particles
  of a system
• Standard Entropy So the entropy value for the
  standard state of the species (indicated by the
  subscript sign)
• Gas pure sub at 1 atm tempt 250C or 298 k
  Solutions are one molar.
• Calc So Sum n So products - Sum n So reactants

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Free Energy and Spontaneity

• The Gibbs free energy of a substance combines its
  enthalpy and entropy into a single quantity that
  describes the total amount of energy available
  for use.
• G H TS
• At const temp G H T S
• If G is neg(-) the rxn is spontaneous
• If G is pos() the rxn non-spontaneous
• If G is zero the rxn is at equilibrium

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