Why Chemical Reactions Occur

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Why Chemical Reactions Occur

• ENTHALPY - ENTROPY - GIBBS FREE ENERGY
• Silberberg Ch 20
• Because some endothermic reactions are
  spontaneous, lowering the enthalpy is not reason
  for reaction proceeding.

Mixing spontaneous but no energy change mixed
state more probable
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Recap

• The system - the portion of the universe being
  studied (usually the chemical reaction)
• The surroundings - everything that is outside the
  system (usually the immediate surroundings to the
  chemical reaction)
• The universe - the system the surroundings
• First Law of Thermodynamics
• The energy of the universe is constant.

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Entropy S

• Entropy is a thermodynamic quantity that
  describes the number of arrangements that are
  available to the system in a given state
• Theoretically
• S R ln W
• W number ways molecules can be arranged in
  that configuration
• R gas constant

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• The more ways a particular state can be achieved
  the greater is the likelihood of finding it in
  that state

• The black and white balls have more freedom if
  they can move around the entire system

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Entropy Qualitative

• More disordered higher S
• For given substance Sgas gt gt Sliquid gt
  Ssolid

Fig 20.5

• Same substance higher T higher S

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• Two similar substances more complex molecule has
  higher S
• eg at 298K

S / J K-1 mol-1
CH4 (g) 186
H2O (l) 70
H2O (g) 189
CH3(CH2)3CH3 (g) 388
H2S (g) 206
C(CH3)4 (g) 306
Fig 20.9
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Summary - Entropy

• For given substance Sgas gt gt Sliquid gt
  Ssolid
• Same substance higher T higher S
• Two similar substances more complex molecule
  has higher S
• In solution more dilute higher S
• Similar compounds higher mass higher S
  (eg H2S gt H2O)

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Entropy Of Reaction

• Entropies in reactions can be added subtracted
  in the same way as enthalpies (Hess Law)
• Eg calculate DSrxn for urea

S 2 ? 193 214 174
70 J mol1 K1 DSrxn
SSproducts S Sreactants DSrxn (174
70) (2 ? 193 214) 356 J mol1 K1
We would expect ?So lt 0 Why?
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Second Law Of Thermodynamics

• In a spontaneous process the entropy of the
  universe must increase
• DSsystem DSsurroundings gt 0
• universe becomes more disordered

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Third Law of Thermodynamics

• At 0 K, all substances have the same entropy
• (the baseline for entropy define this as S
  0)

Eg A perfect crystal has zero entropy at absolute
zero.
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Examples Of Entropy Change

• 1. Predict sign of DSsys for following processes
• Alcohol evaporating
• Goes from liquid to gas (vapour),
• DSsys S(gas) S(liquid) gt 0
• Lake freezing in winter
• Goes from liquid to solid,DSsys S(solid)
  S(liquid) lt 0
• 2. Arrange in order of decreasing standard molar
  entropy, S, ClO4, ClO2, ClO3
• More complex has higher entropy, so S values go
  
• ClO4 gt ClO3 gt ClO2

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Health Advice

• do the tutorial problem sheets and quizes
• past exam papers from 1st year Office(sold for
  3.30 for our copying costs)
• re-do ALL examples given in lectures without
  help from your notes
• ask a tutor or me for help
• university counselling service

• Reminder Quiz next week
• Ideal Gases - Kinetics - Equilibrium - Acids
  Bases
• Bring a calculator