A2-Level Enthalpy and Entropy

  • The standard entropy of a substance refers to the entropy of that substance measured in standard conditions (temperature of 298K and a pressure of 101Kpa).

  • Units for standard entropy are JK mol

  • A substance has different entropies depending upon its state and the conditions it is in.

  • Water has a low entropy below 0°C (as it’s a solid), but a high entropy at over 100°C (as it’s a gas).

  • Standard entropies, So, enable the entropies of different substances to be compared and the overall entropy change that occurs in a reaction can be found.

  • Entropy change, ∆S = (sum of standard entropies of products) – (sum of standard entropies of reactants)







Calculating Entropy Change


If entropy is a way of measuring disorder, then that means every substance has its own value of entropy.  For example, the particles in solid sodium chloride have a low entropy, as they are in a fixed position and have little room to move. The particles in a sample of chlorine gas, however, are not in a fixed position and can be very disordered – chlorine would be described here as having a high entropy.


The problem with describing a substance’s entropy is that entropy changes with temperature. This means that the entropy of a given substance will change if we change the temperature. Think about simple kinetic theory – the more energy you give a substance, the more the particles move. More movement of particles means more disorder, thus a higher level of entropy.


Solid water (ice) has a very low entropy; gaseous water (steam) has a very high entropy. In both cases the particles are still water! The substance is the same chemically, but the temperature change has caused a change in state and entropy. This means we need a ‘standard’ temperature and pressure to describe the entropy of a particular substance.


Standard conditions = 298 K, 101 kPa


Under standard conditions, water is a liquid and the standard entropy of water refers to the entropy content of 1 mole of water at 298 K and 101 kPa – ‘the standard entropy of H O’.


By using the standard entropies (S ) of substances, we can calculate the change of entropy in a reaction.

Values taken from https://www.chem.wisc.edu/deptfiles/genchem/netorial/modules/thermodynamics/table.htm(2019)

One important thing to consider here is that standard entropies have units of J K mol . This becomes important when using entropy values alongside enthalpy values (the standard units for enthalpies are kJ mol  ). See ‘Free Energy’.