Laws of Thermodynamics | Physical Chemistry Class 11

Laws of Thermodynamics

Energy, Entropy & Spontaneity | Physical Chemistry

1. The Zeroth Law

                    Statement: If two systems (A and B) are each in thermal equilibrium with a third system (C), then they are in thermal equilibrium with each other.
                

Significance: This law defines the concept of Temperature. It is the basis for using thermometers.

Implies: If $T_A = T_C$ and $T_B = T_C$, then $T_A = T_B$.

2. The First Law (Law of Conservation of Energy)

                    Statement: Energy can neither be created nor destroyed, although it can be converted from one form to another. The energy of an isolated system is constant.
                

$$ \Delta U = q + w $$

Where:

$\Delta U$ = Change in Internal Energy.
$q$ = Heat exchanged (+ve if absorbed, -ve if released).
$w$ = Work done (+ve if work done on system, -ve if work done by system).

Limitation: It tells us about the energy balance but does not predict the direction or feasibility (spontaneity) of a process.

3. The Second Law (Entropy & Spontaneity)

                    Statement (Clausius): Heat cannot spontaneously flow from a colder body to a hotter body without external work.
                    
                    Entropy Statement: The entropy of the universe (System + Surroundings) increases in a spontaneous process.

$$ \Delta S_{total} = \Delta S_{sys} + \Delta S_{surr} > 0 \quad (\text{For Spontaneous}) $$

Gibbs Free Energy ($\Delta G$)

Combines Enthalpy ($H$) and Entropy ($S$) to predict spontaneity at constant T and P.

$$ \Delta G = \Delta H - T \Delta S $$

$\Delta G < 0$: Spontaneous
$\Delta G > 0$: Non-Spontaneous
$\Delta G = 0$: Equilibrium

4. The Third Law

                    Statement: The entropy of a perfectly crystalline substance approaches zero as the absolute temperature approaches zero (0 Kelvin).
                

$$ \lim_{T \to 0} S = 0 $$

Significance:

It allows the calculation of Absolute Entropies ($S^\circ$) of substances at room temperature.
Residual Entropy: Some substances (like $CO, NO, Ice$) do not have zero entropy at 0K due to crystal imperfections or disorder.

5. Summary Comparison

Law	Key Concept	Equation
Zeroth	Temperature	$T_A = T_B$
First	Internal Energy ($\Delta U$)	$\Delta U = q + w$
Second	Entropy ($S$)	$\Delta S_{univ} > 0$
Third	Absolute Zero	$S_{0K} = 0$

Practice Quiz

Test your knowledge on Thermodynamic Laws.

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