Entropy, Gibbs Free Energy & Enthalpy

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Entropy ∆S

Low Entropy -> High Entropy

 (Ordered)         (Disordered)

Entropy, S, is a measure of disorder in a system.

Entropy Tends to a maximum.

If a system becomes more disordered the value of ΔS is positive.

ΔS = S final - S initial

Entropy is measured in Joules.

Entopy increase when:

  • Solids melt
  • Liquids boil
  • Solids dissolve in water
  • The temperature of solids, liquids and gases increase
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Gibbs Free Energy ∆G

 

Values of Gibbs Free Energy indicate whether a reaction is capable of taking place of its own accord.

 

Below are how the feasability of reactions differ with values of ΔG:

  • ΔG < 0 (Negative-) = Spontaneous reaction
  • ΔG > 0 (Positive+)  = Non-spontaneous reaction (spontaneous in the reverse direction)
  • ΔG = 0                    = The system is in equilibrium.
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Enthalpy ∆H

Enthalpy definitions:

Enthalpy of reaction - The enthalpy change when one mole of a substance reacts completely.

Enthalpy of formation - The enthalpy change when one mole of a compound is formed from its constituent elements.

Enthalpy of combustion - The enthalpy change when one mole of a substance burns completely in oxygen.

Enthalpy of hydrogenation - The enthalpy change when one mole of an unsaturated compound reacts completely with an excess of hydrogen to form a saturated compound.

Enthalpy of atomisation - The enthalpy change required to atomise one mole of a compound.

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Enthalpy ∆H (Cont.)

Enthalpy definitions:

Enthalpy of neutralization - The enthalpy change when one mole of water is produced from the reaction between an acid and a base.

Enthalpy of solution - The enthalpy change when one mole of a solute is dissolved completely in an excess of solvent.

Enthalpy of hydration - The enthalpy change when one mole of gaseous ions are completely dissolved in water forming one mole of aqueous ions.

Lattice dissociation enthalpy - The enthalpy change when one mole of an ionic compound separates into gaseous ions with infinite distance apart (no force of attraction).

Lattice formation enthalpy - The enthalpy change when one mole of an ionic compound is formed from gaseous ions with infinite distance apart.

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How They Link

Entropy, Gibbs Free Energy & Enthalpy are all linked together and with Temperature through this equation:  

∆G = ∆H - T∆S

If:  

H is Negative -       and       S is Positive +       then       G must be Negative -

H is Positive -        and       S is Negative -      then       G must be Positive +

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Calculations

If the question asks above which temperature will the reaction be feasible, G must be 0.

So the equation now reads: 0 = H - TS  Therefore  T = ∆H ∕∆S

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