Entropy

• Magnesium ribbon and sulfuric acid will start fizzing straight away as hydrogen is produced and heat is evolved as the reaction is exothermic - it is a spontaneous reaction.
• Solid sodium hydrogencarbonate and hydrochloric acid will also instantly start fizzing as carbon dioxide is produced. The temperature of the mixture will drop as the reaction is endothermic - it is a spontaneous reaction.
• Gold and dilute hydrochloric acid will never react, no matter how much you heat it or how much time you wait - the reaction is not spontaneous or feasible.
• Diamond in oxygen will not immediately start burning - but it would if you heated it enough to get the reaction going and overcome the activation energy. This can still be called a spontaneous reaction.
• What is entropy?

  The measure of the number of ways particles can be arranged and the number of ways that the energy can be shared out between the particles.

  The more dispersed energy is the greater the entropy.

  The more randomisation or disorder in a structure, the more entropy it will have.

  For our purposes, we can define entropy as 'an increase in disorder or randomness'.
• Qualitatively, entropy is a measure of how much the energy of atoms and molecules becomes more spread out in a process.

  Entropy can be defined in terms of:

  • statistical probabilities of a system
  • the other thermodynamic quantities of the system

  Entropy change is the measure of how more widely a specific quantity of molecular energy is dispersed in a process.
• Entropy is given the symbol S.

  Standard entropy is given the symbol S delta (298K and 1 atm)
• The standard entropy of a substance is the entropy of 1 mole of that substance under standard conditions.
• Entropy is measure in units of J K-1 mol-1 (remember to convert to kJ in calculations when needed)
• An increase is disorder and entropy will lead to a positive entropy change.
• Entropy is determined by:

  • Temperature, and consequently, physical state
  • Number of particles - the more particles, the more ways they can be arranged. Entropy will increase with the number of moles.
• • The more possible arrangement of molecules the more stable the substance will be. Therefore, particles will try to increase their entropy.
  • Some reactions are feasible, even when their enthalpy change is endothermic because of an increase in entropy.
• Theoretically, at 0K (absolute zero) a perfectly ordered, pure crystal will have an entropy of zero.
• Explain, in terms of molecules, why the entopry is zero when the temperature is 0K.

  • particles are stationary
  • no disorder
• Which state of matter has the highest entropy?

  Gases, because there are more ways that the particles can be arranged, hence greatest 'disorder'.
• Entropy change = total entropy of products - total entropy of reactants (Note: if the balanced equation requires multiple moles remember to multiply the entropy values by the number of moles)
• If the entropy change is positive, the reaction is feasible, however other conditions might prevent it from taking place (enthalpy, temperature and kinetics)
• If entropy must always increase, why can water vapour form liquid water?

  Because heat will transfer to the surrounding, resulting in a net increase in entropy.