22.5 -free energy

• Feasibility


  Whether a reaction is able to happen and is energetically feasible. The word spontaneous may also be used for energetically feasible.
• Free energy change


  The overall change in energy during a chemical reaction.
• Symbol of free energy change


  ∆G
• What two types of energy is free energy change made up of?


  1. The enthalpy change ∆H. This is the heat transfer between the chemical system and the surroundings.

  
  

  2. The entropy change at the temperature of the reaction T∆S. This is the dispersal of energy within the chemical system itself.
• Gibbs equation


  ∆G = ∆H - T∆S
• ∆G

  Free energy change
• ∆H

  Enthalpy change with surroundings
• T

  Temperature in K
• ∆S

  Entropy change of system
• What does ∆G tell us?

  If a reaction is spontaneous or not.
• Spontaneous reaction


  A reaction that occurs naturally and favors the formation of products at the specified conditions (doesn't require outside energy).
• ∆G less than 0

  Spontaneous (energetically feasible)
• ∆G more than 0


  Not spontaneous
• What must the unit of ∆S be in the Gibb's equation?


  KJ K⁻¹ mol⁻¹
• How can an endothermic reaction be feasible (occur spontaneously)?


  - If a reaction is endothermic, ∆H is positive.

  - For ∆G < 0 (spontaneous), T∆S must be > ∆H.

  - For T∆S > ∆H:

  - ∆S must be positive and

  - Temperature must be high enough so that T∆S is more than ∆H.
• Limitation of predictions made for feasibility


  The free energy change is useful for predicting feasibility, but many reactions have a negative ∆G and do not seem to take place.
• Why does hydrogen peroxide not decompose spontaneously when it has a ∆G < 0?


  Decomposition of hydrogen peroxide has a very large activation energy resulting in a very slow rate. If hydrogen peroxide is left for long enough, it would decompose.
• How can decomposition of hydrogen peroxide be sped up at room temperature?


  Addition of a trace of MgO₂ catalyst, allows the reaction to take place via an alternative route with a lower activation energy. The reduced energy barrier then allows the reaction to take place.
• Free energy change equation in terms of y=mx+c


  ∆G = -T∆S + ∆H

  y = ∆G

  m = ∆S

  x = T

  c = ∆H
• What does the y-intercept on a free energy change against temperature graph represent?


  Enthalpy change ∆H
• What does the x-intercept on a free energy change against temperature graph represent?


  Temperature at which the reaction stops being spontaneous.
• What does the gradient on a free energy change against temperature graph represent?


  Entropy change ∆S