OIA1011 SECOND & THIRD LAWS OF THERMODYNAMICS AND GIBBS FREE ENERGY

What does the second law of thermodynamics state?

The entropy of an isolated system increases in a spontaneous process and remains constant in a reversible process

Define entropy.

A measure of the dispersal of energy and matter, reflecting the degree of disorder in a system.

Provide an example of a spontaneous process.

Melting of ice at 20°C, where entropy increases

How is entropy change (ΔS) calculated during phase transitions?

ΔS=QT, where Q is heat absorbed and T is temperature in Kelvin

What is the entropy change for the fusion of 1 mol of water at 0°C?

ΔS=6.01 kJ/mol273.15 K=22.0 J K−1mol−1.

What happens to entropy when a system is heated from T1 to T2?

ΔS=Cpln⁡T2T1, where Cp is the heat capacity at constant pressure

How does entropy change for isothermal expansion of an ideal gas?

It increases as the gas expands, dispersing energy

Why can entropy not decrease in a spontaneous process?

It violates the second law of thermodynamics

What is the third law of thermodynamics?

The entropy of a perfect crystalline substance at absolute zero is zero

What are absolute entropies?

Entropies calculated from the third law, starting from S=0 at 0 K

How is the standard entropy change of a reaction calculated?

ΔS=ΣS∘(products)−ΣS∘(reactants).

What is Gibbs free energy (G)?

A thermodynamic potential used to predict the spontaneity of a process; G=H−TS.

How does ΔG indicate spontaneity?

ΔG < 0: Spontaneous; ΔG > 0: Non-spontaneous; ΔG = 0: Equilibrium

What is the formula for standard Gibbs free energy change?

ΔG∘=ΔH∘−TΔS∘.

Provide an example of a reaction with negative ΔG.

Combustion of glucose: C6H12O6+6O2→6CO2+6H2O.

How does temperature affect ΔG?

Higher temperatures favor processes where ΔS > 0, as ΔG=ΔH−TΔS.

What is the Gibbs-Helmholtz equation?

ΔG2−ΔG1=−ΔH(1T2−1T1), used to calculate ΔG at different temperatures

What is the effect of pressure on Gibbs free energy for gases?

ΔG=nRTln⁡P2P1 for ideal gases

Calculat ΔG for a gas compressed isothermally from 1 atm to 6.9 atm at 300 K.

ΔG=0.59×8.314×300ln⁡6.91=2840 J.

What happens to Gibbs free energy at phase equilibrium?

ΔG = 0, e.g., during melting or boiling

How does ΔG relate to freezing and melting?

Gsolid>Gliquid: Melting; Gsolid<Gliquid: Freezing.

Why is Gibbs free energy useful in predicting reactions?

It combines enthalpy and entropy to determine spontaneity and equilibrium

How is ΔG related to equilibrium constant (K)?

ΔG=−RTln⁡K, linking thermodynamics and chemical equilibria

What determines if a compound is thermodynamically stable?

Negative ΔG∘ for formation indicates stability

How do ΔS, ΔH, and ΔG work together?

They predict the feasibility of physical and chemical processes