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 all 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 of energy within the chemicals making up the chemical system
• units of entropy are JK-1mol-1
• the greater the enthalpy value the greater that energy is spread out per Kelvin per mole
• in general:

  • solids have the smallest entropies
  • liquids have greater entropies
  • gases have the greatest entropies
• at 0K there would be no energy and all substances would have an entropy value of zero
• above 0K, energy becomes dispersed amongst the particles and all substances have positive entropy
• 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 positive
  • if a system changes to become less random, energy becomes more conenctrated - the entropy change delta S will be negative
• in an equation for a physical change, or chemical change, you can predict whether entropy increases or decreases by comparing the physical states and amount of gas molecules on either side of an equation
• entropy increases during changes of state that give a more random arrangement of particles:

  solid - liquid - gas
• so when any substance changes from solid to liquid to gas - entropy increases:

  • melting and boiling 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:

  • production of a gas increases the disorder of particles
  • energy is spread out more and delta S is positive
• you can predict the sign of the entropy change for reactions where the reactants and products have different numbers of gas molecules:

  e.g. N2 (g) + 3H2 (g) = 2NH3(g)

  4 moles of gas 2 moles of gas

  • there is a decrease in the randomness of particles
  • the energy is spread out less and delta S is negative
• every substance has a standard entropy which can be found in data books
• the standard entropy of a substance is the entropy of one mole of a substance, under std conditions:

  • units J K-1 mol-1
  • always positive