Entropy, Gibbs Free Energy & Enthalpy

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

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.

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.

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.

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 +

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

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