Low Entropy -> High Entropy
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
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.
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 ∆H (Cont.)
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.
How They Link
Entropy, Gibbs Free Energy & Enthalpy are all linked together and with Temperature through this equation:
∆G = ∆H - T∆S
∆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