1.

(a) Define Gibbs energy. Give its mathematical expression. What is Gibb's energy criteria of spontaneity?(b) Predict the sign of AS for the following changes:(i) Freezing of water(ii) C(graphite) → C(diamond)

Answer»

(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.



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