22.4 Entropy

• Entropy S can be used to explain things that occur naturally:

  • a gas spreading through a room
  • heat from a fire spreading through a room
  • ice melting in a hot room

  in the examples energy is being dispersed and becoming more spread out. There is always a natural tendency for energy to spread out rather than be concentrated in one place.
• the greater the entropy, the greater the dispersal of energy and the greater the disorder
• the term entropy is used for the dispersal of energy within the chemicals making up the chemical system
• the units of entropy are JK^-1mol^-1. the greater the entropy value, the greater the energy is spread out per kelvin per mole. in general;

  • solids have the smallest entropies
  • liquids have greater entropies
  • gases have the greatest entropies

  each state has substances that can have very different entropies
• At 0K, there would be no energy and all substances would have an entropy value of zero. Above 0K energy become dispersed amongst the particles and all substances have a positive energy.
• systems that are more chaotic have a higher entropy value:

  • if a system changes to become more random, energy can be spread out more - there will be an entropy change delta S which will be more positive.
  • if a system changes to become less random, energy become more concentrated - the entropy change delta S will be negative.

  in an equation you can predict wether entropy decreases or increases by comparing the physical states and amounts of gas molecules on either side.
• entropy increases during changes in state that give more random arrangements of particles:

  1. solid
  2. liquid
  3. gas
• when any substance changes from solid to liquid to gas, its entropy increases:

  • melting and boiling points increase the randomness of particles
  • energy is spread out more and delta S is positive
• reactions that produce gases result in an increase in entropy, for example calcium carbonate reacting with HCl.

  • production of gas increases and the disorder of particles
  • energy is spread out more and delta S is positive

  
• you can predict the sign of entropy change for reactions where the reactants and products have different number of gas molecules. for example the reaction of nitrogen and hydrogen gases to produce ammonia.

  • there is a decrease in the randomness of particles
  • the Ernst is spread out less and delta S is negative

  
• the standard entropy of a substance in the entropy of one mole of a substance, under standard conditions (100kPA and 298K)

  • standard entropies have units JK^-1mol^-1
  • standard entropies are always positive
• standard entropies can be used to calculate the entropy change of a reaction:

  delta S = sum of standard entropies (Products)- sum of standard entropies (reactants)