(a) Gibbs Energy: It is defined as the maximum amount of energy available to a system, during a process that can be converted into useful work.
Mathematical Expression of Gibbs Energy:
Gibbs energy is mathematically expressed as:
G = H – TS
Where G = Gibbs energy
H = enthalpy of system
S = entropy of system
T = absolute temperature
Change in free energy (ΔG) for the system:
ΔG = ΔΗ – TΔS
Where ΔΗ = change in enthalpy
ΔS = change in entropy
Gibbs Energy Criteria of Spontaneity:
For a spontaneous reaction, ΔG is negative i.e. ΔG < 0
(b) (i) Freezing of Water:
Here, entropy decreases (i.e. ΔS = −ve) because when liquid (water) freezes to form solid (ice), the molecules attain an ordered state.
(ii) C(graphite) → C(diamond)
Here, Entropy decreases (i.e. ΔS = −ve) because when C(graphite) is converted into C(diamond) , it attains more ordered state due to presence of covalent bonding in diamond.
