Thermodynamics: Laws Explained

• Introduction

Zeroth Law of Thermodynamics

• If two systems are individually in thermal equilibrium with a third system, they are in thermal equilibrium with each other.
• Establishes temperature as a fundamental property for determining thermal equilibrium.
• If A = C and B = C, then A = B.

First Law of Thermodynamics

• States that the increase in a system's internal energy equals the heat added to the system minus the work done by the system.
• Deals with the conservation of energy.
• Formula: $\Delta U = Q - W$
  • $\Delta U$ represents the change in internal energy.
  • $Q$ represents the heat added to the system.
  • $W$ represents the work done by the system.

Second Law of Thermodynamics

• The total entropy of an isolated system can only increase over time.
• Deals with the direction of natural processes.
• $\Delta S \geq 0$, where $\Delta S$ is the change in entropy.
• Heat cannot spontaneously flow from a cold body to a hot body.
• No heat engine can be more efficient than a Carnot engine operating between the same two temperatures.

Third Law of Thermodynamics

• As the temperature of a system approaches absolute zero, the entropy of the system approaches a minimum or zero value.
• Deals with the behavior of systems at very low temperatures.
• $lim_{T \to 0} S = S_0$
  • $S_0$ is the minimum entropy
  • Typically zero for a perfect crystalline substance.
• It is impossible to reach absolute zero in a finite number of steps.

Enthalpy

• Defined as the sum of the internal energy of a system and the product of its pressure and volume.
• $H = U + PV$
  • $H$ represents enthalpy.
  • $U$ represents internal energy.
  • $P$ represents pressure.
  • $V$ represents volume.

Gibbs Free Energy

• Measures the amount of energy available in a thermodynamic system to perform useful work at constant temperature and pressure.
• $G = H - TS = U + PV - TS$
  • $G$ is Gibbs Free Energy.
  • $H$ is Enthalpy.
  • $T$ is Absolute Temperature.
  • $S$ is Entropy.
  • $U$ is Internal energy.
  • $P$ is Pressure.
  • $V$ is Volume.
• $\Delta G < 0$: Indicates a spontaneous process.
• $\Delta G > 0$: Indicates a non-spontaneous process.
• $\Delta G = 0$: Indicates equilibrium.