Enthalpy and Entropy

• What is entropy?

  measure of the dispersal of energy in a system which is greater, the more disordered a system
• entropy change will be positive when disorder has increased

  • e.g gas formation
• what are the units of entropy?

  J K ⁻¹ mol ⁻¹
• the greater the entropy value the greater that energy is spread out per kelvin mole
• Entropy values

  entropy increases due to changes in state that give a more random arrangement of particles

  • solids = smallest
  • liquids = greater
  • gases = greatest
• at K= 0 there will be no energy and so all substances have an entropy value of 0
• above K= 0 energy becomes dispersed among particles and all substances have positive entropy
• If a system changes to become more random:

  • energy can be spread out more
  • positive entropy change
• If a system changes to become less random:

  • energy becomes more concentrated
  • negative entropy change
• disorder increases from

  solid to liquid to gas
• changes in the number of gaseous molecules

  reactions that produce gas results in an increase in entropy

  • increases disorder
  • positive enthalpy change
  • energy is more spread out
• Changes in the number of gaseous molecules

  reactions that lose moles of gas result in a decrease in entropy

  • decrease in disorder and randomness of particles
  • Energy is less spread out
  • negative entropy change
• what is free energy change?

  overall energy change during a chemical reaction
• what is feasibility?

  describes whether a reaction is able to happen and if it is energetically feasible/possible
• spontaneous

  tends to describe reactions that go off on their own accord
• entropy change has to be divided by 1000 when put into the gibbs free energy equation
• △G = KJmol ⁻¹
• △H = KJmol ⁻¹
• feasibility of a reaction depends on the balance between △H and T△S
• feasibility

  to be feasible △G must be less than 0

  • must be negative
  • must be a decrease in free energy
• feasibility

  a reaction/ process is not feasible if △G is greater than 0

  • positive free energy change
• Feasibility and temperature

  Exothermic reaction ( △H is negative)

  • if △S is positive = △G is negative so the reaction is feasible
  • if △S is negative = △G is large positive so the reaction is not feasible
• Feasibility and temperature

  Endothermic reaction ( △H is positive)

  • if △S is positive = △G is positive so the reaction is not feasible
  • if △S is negative = △G is positive so the reaction is not feasible
• endothermic reactions are never feasible
• Limitations of Gibbs free energy

  -only used for reactions under standard conditions

  -doesn’t take into account the kinetics (rate of reaction) as there might be a large activation energy to overcome before a reaction can take place

  -some reactions are feasible but kinetically too slow
• limitation example

  decomposition of hydrogen peroxide at 298K

  • very high activation energy, needs to be catalysed by MnO2
  • if reaction was left for log enough H2O2 would eventually decompose
• Entropy change

  △S°=ΣS° (products) −ΣS° (reactants)
• Gibbs free energy

  △G= △H − T△S