Enthalpy and Entropy

• What is entropy?


  A measure of disorder or randomness

  
• What are the units for entropy?

  
• Calculation for entropy change in a reaction

  
• A positive entropy change means...

  there is an increase in disorder (ΔS is more positive)

  
• A negative entropy change means...

  there is a decrease in disorder (ΔS is more negative)

  
• What is a feasibility?

  Whether a reaction is able to happen and is energetically feasible.

  -> A reaction which would go of its own accord at a particular temperature if there wasn't an activation energy barrier in the way

  
• Define standard entropy


  The entropy of one mole of a substance, under standard conditions (100kPa and 298K)

  Units --> JK-1mol-1

  Standard entropies are always positive

  
• Gibb's equation

  
• What is free energy change?

  The overall change in energy during a chemical reaction

  
• For a reaction or process to be feasible at a particular temperature

  ΔG must be negative or 0
• If ΔG is positive...

  the reaction/process is non-spontaneous and cannot occur at that temperature without the input of work from an external source
• If ΔG = 0

  T= ΔHx1000/ΔS

  This is when the reaction just becomes feasible
• When ΔH is positive and ΔS is positive...

  TΔS is more positive

  When TΔS>ΔH

  ΔG is less positive

  The reaction is more feasible when temperature is high enough
• When ΔH is negative and ΔS is positive...

  TΔS is more positive

  ΔG is more negative

  The reaction will always be feasible
• When ΔH is positive and ΔS is negative...

  TΔS is more negative

  ΔG is always more positive

  The reaction is never feasible
• When ΔH is negative and ΔS is negative...

  TΔS is more negative

  When T is really high TΔS>ΔH

  ΔG is less negative

  The reaction is usually feasible, but not feasible when T is really high
• What are limitations of predictions made for feasibility?

  If the reaction is slow and has a high activation energy
• A reaction that is thermodynamically feasible...

  is not necessarily kinetically feasible