Course: CHM-DSC-251
Institution: Assam University, Silchar
While the First Law focuses on energy conservation, the Second Law determines the direction of spontaneous change.
The First Law cannot explain why certain processes occur spontaneously in one direction but not the reverse (e.g., heat flowing from hot to cold).
Entropy is a measure of the molecular disorder or randomness of a system. It is a state function.
Mathematical expression for entropy change: dS = dq_rev / T
For a reversible process, the entropy change of the universe is zero. For an irreversible (spontaneous) process, the entropy of the universe increases.
The Clausius Inequality provides a mathematical criterion for the spontaneity of a process.
dS ≥ dq / T
To predict spontaneity without checking the entire universe, we use system-specific functions: Gibbs Free Energy (G) and Helmholtz Free Energy (A).
| Function | Definition | Condition |
|---|---|---|
| Helmholtz Energy (A) | A = U - TS | Constant Temperature and Volume |
| Gibbs Energy (G) | G = H - TS | Constant Temperature and Pressure |
The criteria for spontaneity based on free energy changes are vital for chemical reactions.
This equation relates the temperature dependence of Gibbs energy to enthalpy.
[d(G/T) / dT]_P = -H / T²
Maxwell relations allow us to relate non-measurable quantities (like entropy) to measurable ones (P, V, T).
Common Maxwell Relations derived from exact differentials:
The Joule-Thomson effect describes the temperature change of a real gas when it expands through a porous plug into a lower pressure region under adiabatic conditions.
μ_JT = (dT / dP)_H
For an ideal gas, μ_JT = 0 because there are no intermolecular forces to overcome.
Q: Why is the Second Law necessary if we have the First Law?
A: The First Law only tracks energy quantity. The Second Law tracks energy quality and directionality.
Q: What is the physical significance of ΔA?
A: It represents the maximum work (reversible work) a system can perform at constant T and V.