7.7: Standard Molar Entropies

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7.7 Standard Molar Entropies

• Standard Molar Entropy Entropy of 1 mole of
  material at 298.15 K and 1 bar
• When we consider the standard molar entropies of
  molecules, we apply the following rule
• Standard Molar Entropies Increase as the
  Complexity of a Substance Increases
• and
• The Standard Molar Entropies of gases are higher
  than those of liquids or solids at the same
  temperature

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What Does this Mean?
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7.8 Standard Reaction Entropies

• Entropy and the tendency for spontaneous
  processes to increase the entropy of a system can
  be used to predict if/how reactions will occur
• We know from Sºm that one set of molecules will
  have lower entropy than another
• Taking the same logic a step further will allow
  us to predict the change in entropy of a reaction

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Standard Reaction Entropies

• Remember
• Molar entropy of a gas is much higher than that
  of a liquid or solid
• Increase in the number of gas molecules means
  (usually) a higher entropy after a reaction
• Vice versa for a decrease in the number of gas
  molecules
• Breaking larger molecules down into smaller
  molecules usually results in an increase in
  entropy
• More molecules means more disorder
• Dissolving a solid into a liquid usually
  increases the entropy

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Calculating the Entropy of a Reaction

• Sometimes we cant always use our judgment and we
  need to calculate the entropy
• In order to do this, we need the standard molar
  entropies of the products and the reactants as
  well as the number of moles of each

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Global Changes in Entropy

• We know now about the role of entropy in chemical
  reactions and spontaneous processes
• How can we use entropy to explain processes that
  spontaneously happen, but appear to go against
  the 2nd Law?
• Water freezing to ice
• Cold packs becoming cold in the summer
• Cells forming from the primordial seas
• The contradiction is just a matter of scale

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Isolated Systems

• The system may be doing something that seems to
  be contradictory to the 2nd Law, but when the
  surroundings are included, we form an Isolated
  System

Reactions will occur spontaneously if the TOTAL
entropy change is positive
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The Surroundings
?STot ?SSystem ?SSurr

• If ?STot is positive, the reaction is spontaneous
• If the ?Ssystem is negative, the reaction will
  still be spontaneous if ?SSurr is that much more
  positive

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?S and Enthalpy

• We can better understand the role of the
  surroundings by looking at the boiling of water

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Summary of System and Surroundings
Exothermic vs Endothermic Reactions
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