! S univ = ! S sys + ! S surr > 0 Note: Entropy is not energy! It is an index or measure of the dispersal of energy that occurs in all spontaneous processes passing from State 1 to State 2. ! S univ = ! S sys + ! S surr = 0 ! S univ = ! S sys + ! S surr < 0 2. No spontaneous change 3. Equilibrium condition 1. Criteria for Spontaneous change! While it appears that entropy focuses on the most statistically favored position distribution entropy is more concerned with the fact that energy levels become more closely spaced and more occupied. Reactions tend towards dispersing energy! There are many different ways we can Trans Roto Vibro Energy Spacing
Matter being dispersed into a larger number of statistical microstates disperses energy into a larger number of thermal energy levels increasing entropy. 0.5 atm evacuated 1.0 atm Gas expands spontaneously into larger volume Energy level Energy level Quantum mechancis dictates closer energy level spacing as V increases. Same amount of energy is dispersed or spread among more energy levels. Increasing entropy or the dispersal or spread of energy occurs in any spontaneous process. It provides humans with an “arrow of time” as the reverse situation never happens. Thermal energy flows from the higher occupied energy levels in the warmer object into the unoccupied levels of the cooler one until equal numbers of states are occupied. 100˚C 10˚C 100˚C 10˚C HOT COLD COMBINED Example: two block of different temperatures are brought together. 1 2 Be careful entropy is often illustrated incorrectly as a positionally a messy room or disordered cards and described as “disorder”. This is not correct in the thermodynamic sense! WRONG: Neither case represents thermodynamic entropy because rooms and cards do not exchange heat with its surroundings! This is a fundamental requirement. 1. Increasing temperature 2. Solid to Liquid or Liquid to Gas phase changes 3. Dissolution of a solid or liquid 5. Atomic size or molecular complexity 4. Dissolution of a gas decreases entropy. ! S increases as temperature rises as more energy states are filled ! S increases as phase changes to a more dispersed phase. Dissolving a solid or liquid into a solvent increases entropy. A gas becomes less dispersed when it dissolves in a liquid or solid. In similar substances, increases in mass relate directly to entropy. In allotropic substances, increases in complexity (e.g., bond flexibility) relate directly to entropy. There are many events that result in a higher number of microstates or higher entropy. ! S > 0 ! S > 0 ! S > 0 Phase Change Phase Change Dissolution Increasing T Qualitative Meaning of Entropy: Entropy refers to the degree to which energy is dispersed in a process. The higher the entropy the more energy is dispersed in that system.