Introduction to Entropy

Introduction to Entropy

Understanding Entropy

  • Entropy is a central concept in thermodynamics, representing the degree of randomness or disorder in a system.
  • It is denoted by the symbol S and typically measured in joules per kelvin (J/K).
  • In any spontaneous change, the total entropy of a system and its surroundings always increases, as per the Second Law of Thermodynamics.
  • This concept introduces the idea that nature tends to move towards a state of greater disorder.

Microstates and Entropy

  • A microstate refers to a specific arrangement of particles in a system.
  • The more microstates a system has, the greater the system’s entropy.
  • More microstates mean more ways that energy can be distributed among the particles in a system, leading to a higher level of disorder.

Entropy and Phases of Matter

  • The entropy of a system depends on its physical state. Gases have the highest entropy, followed by liquids and then solids.
  • This is because the particles are arranged more randomly in gases and liquids compared to solids. Therefore, gases and liquids have more microstates and higher entropy.

Entropy in Chemical Reactions

  • In the context of chemical reactions, entropy increases when reactants transform into products with a higher degree of disorder.
  • A reaction where gases are produced from solids or liquids typically results in an increase in entropy.
  • Conversely, a reaction that generates solids from gases or liquids will usually lead to a decrease in entropy.

Predicting Change in Entropy

  • The reaction’s entropy change (∆S) can either be positive or negative, indicating an increase or decrease in entropy, respectively.
  • A negative ∆S implies a decrease in disorder which is, typically, a non-spontaneous reaction.
  • A positive ∆S implies an increase in disorder which, typically, signals a spontaneous reaction.

Entropy, Energy and Temperature

  • An increase in temperature usually leads to an increase in entropy, as particles have more energy to disrupt their existing arrangement.
  • When a substance is heated, its particles move faster, resulting in a more disordered arrangement and higher entropy.
  • However, note that energy distributed among many molecules results in a higher entropy than energy concentrated in a single molecule.