Entropy is basically the measure of “disorder” in a system. It increases with every action and reaction. Entropy may be generated (or created), but it cannot be destroyed. Temperature and entropy are somewhat proportional. Entropy is measured in \frac{Joules}{Kelvin}\ (\frac{J}{K}).

  • 0^{th} Law of Thermodynamics: If two systems are in thermal equilibrium with a third system, then they must be in equilibrium with each other.
    • A=C,\ B=C,\ A=B
  • 1^{st} Law of Thermodynamics: Heat and work are forms of energy transfer.
  • 2^{nd} Law of Thermodynamics: The entropy of any isolated system not in thermal equilibrium will increase.
  • 3^{rd} Law of Thermodynamics: Entropy approaches a constant value as temperature approaches absolute zero. (A pure crystal would have zero entropy).

Example: A man running on a day with temperature of 32°C and he loses 2.5 kg of perspiration. In this case, the latent heat of vaporization for perspiration is 24.2\times 10^{5}\ \frac{J}{kg}. Estimate the change in entropy of the water in order to vaporize the water.


    • m=2.5\ kg
    • T=32\degree C+273=305\ K
    • L_{v}=24.2\times 10^{5}\frac{J}{kg}


    • change in entropy (Δs)

Relevant equations:

    • \Delta s=\frac{Q}{T},\ T=305\ K
    • Q=mL_{v}

Plug in numbers:

Q=(2.5\ kg)(24.2\times 10^{5}\frac{J}{kg})=6.05\times 10^{6}\ J

\Delta S=\frac{6.05\times 10^{6}\ J}{305\ K}=1.98\times 10^{4}\ \frac{J}{K}

\Delta S=19.8\ \frac{kJ}{K}