Back to Table of Contents


 Entropy and chemical processes

  • DS in chemical reactions
  • The use of DS to predict chemical reactivity

DS in chemical reactions

The standard reaction entropy,  is defined as the difference between the molar entropies of the pure, separated products and the pure separated reactants, all substances being in their standard states at the specified temperature (=298.15 K unless otherwise stated). Thus for a generalised reaction we have:

The use of DS to predict chemical reactivity

For a reaction to occur, the second law of thermodynamics must be satisfied, i.e.: 


Consider the reaction:

This means that:

>> Does this mean that since  the process does not occur at 298K (25oc)? 
NO - The entropy condition that has to be satisfied is , i.e. we need to have:

may be calculated from the enthalpy of formation of water, , which meant that the enthalpy change for the reaction above is -571.6 kJ mol-1. This means that:

NOTE: Although this is an excellent way to predict whether a reaction is going to take place or not, it is not ideal to have to consider the 'surroundings' all the time. It would be ideal to have a property which would be solely dependent on the system. Such a property is called the free energy, and is considered in the next section.