Introduction to entropy: Difference between revisions

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At a microscopic level ''motional'' energy of molecules is responsible for the temperature of a substance or a system. “Heat” is the motional energy of molecules being transferred: when motional energy is transferred from hotter surroundings to a cooler system, faster moving molecules in the surroundings collide with the walls of the system and some of their energy gets to the molecules of the system and makes them move faster.

* (molecules in a gas like nitrogen at room temperature at any instant are moving at an average speed of nearly a thousand miles an hour, constantly colliding and therefore exchanging energy so that their individual speeds are always changing, even being motionless for an instant if two molecules with exactly the same speed collide head-on, before another molecule hits thenthem and they race off, as fast as 2500 miles an hour. At higher temperatures average speeds increase and motional energy becomes considerably greater.)

** Thus motional molecular energy (‘heat energy’) from hotter surroundings like faster moving molecules in a flame or violently vibrating iron atoms in a hot plate will melt or to boil a substance (the system) at the temperature of its melting or boiling point. That amount of motional energy from the surroundings that is required for melting or boiling is called the phase change energy, specifically the enthalpy of fusion or of vaporization, respectively. This phase change energy breaks bonds '''between''' the molecules in the system (not chemical bonds '''inside''' the molecules that hold the atoms together) rather than contributing to the motional energy and making the molecules move any faster – so it doesn’t raise the temperature, but instead enables the molecules to break free to move as a liquid or as a vapor.

** In terms of energy, when a solid becomes a liquid or a liquid a vapor, motional energy coming from the surroundings is changed to ‘ potential energy ‘ in the substance (phase change energy, which is released back to the surroundings when the surroundings become cooler than the substance's boiling or melting temperature, respectively). Phase change energy increases the entropy of a substance or system because it is energy that must be spread out in the system from the surroundings so that the substance can exist as a liquid or vapor at a temperature above its melting or boiling point. When this process occurs in a ‘universe’ that consists of the surroundings plus the system, the total energy of the universe becomes more dispersed or spread out as part of the greater energy that was only in the hotter surroundings transfers so that some is in the cooler system. This energy dispersal increases the entropy of the 'universe'.