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MHT-CET : Chemistry Entrance Exam

MHT - CET : Chemistry - Chemical Thermodynamics and Energetics Know More

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  • Spontaneous Change: A physical or chemical change that occurs on its own without help from external agency.
  • Entropy (S): The measure of the degree of disorder of a system. Entropy increases with increase in disorder. Total entropy change Ds is positive for a spontaneous change.
    Sgas > Sliquid > Ssolid
  • Second Law Of Thermodynamics: The total entropy change for a spontaneous process is positive. For a reversible change
    D
    s = qrev/T
  • Free Energy (G): It is a measure of the available energy for doing useful work.
  • Gibbs - Helmholtz Equation:

DG =

DH - TDS

- DG =

-DH + TDS

  • Useful or available work = Total available energy - Unavailable energy
  • Condition For Spontaneous Change: All changes for which DG is negative are spontaneous.
    Tendency for spontaneous change = tendency for system to give out heat + tendency for system to increase in entropy.
    .
  • Free Energy: A criterion for spontaneity
    DGreaction = Gproducts - Greactants
    DGreaction = -2.303 RT log K
    DG = -nFE
    DG = DH - TDS

  • Conditions For A Spontaneous Change:

  Sign For DH  

  Sign For DS  

  Occurrence Of Reaction  

+

+

Occurs TDS > DH

-

-

Occurs DH > TDS

+

-

Does not occur

-

+

Occurs

0

+

Occurs

0

-

Does not occur

  •  
  • Third Law Of Thermodynamics: The entropy of a perfect crystalline substance is zero at absolute zero.
  • Free Energy And Cell Potential: ECell = maximum work
    -
    DG = Maximum work
    DG = -nFE
    where n is the number of electrons in the balanced equation and F is Faraday constant (96500 coulomb/mol).
  • Limitations Of Thermodynamics
    a) It does not deal with internal structure of atoms and molecules.
    b) It does not tell anything about the time of completion of change or rate of reaction.
  • Bond Energy
    It is the average enthalpy change, DH per mole, for breaking one bond of the same type per molecule.

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