Quick Notes Entropy Change
- Entropy change, ΔS⦵ = (sum of standard entropies of products) – (sum of standard entropies of reactants)
- Standard entropies, S⦵, enable the entropies of different substances to be compared and the overall entropy change that occurs in a reaction can be found.
- The standard entropy of a substance refers to the entropy of 1 mole of the substance, measured at standard conditions (298K and 101kPa).
- Units for standard entropy are JK-1mol-1
- A substance has different entropies depending upon its state and conditions.
- Water has a low entropy below 0oC (as it’s a solid), but a high entropy at over 100oC (as it’s a gas).
Full Notes Entropy Change
Entropy is a way of measuring disorder, which means every substance has its own value of entropy as every substance is ‘ordered’ in a slightly different way.
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 molecules in a sample of chlorine gas, however, are not in a fixed position and can be very disordered, so chlorine gas has a high entropy.
The problem with describing a substance’s entropy is that entropy changes with 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 if we want to compare the entropies of different substances, we need a ‘standard’ temperature and pressure to use for measurements.
Standard conditions = 298 K, 101 kPa
Under standard conditions, water is a liquid. 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 H2O’.
By using the standard entropies (S⦵) of other substances, we can calculate the change of entropy in a reaction.
Exam style question
‘Using the data given, calculate the entropy change for the following reaction’.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
|Species||Standard Entropy, S⦵(J K-1 mol-1)|
values taken from
One important thing to consider here is that standard entropies have units of J K-1 mol-1. This becomes important when using entropy values alongside enthalpy values (the standard units for enthalpies are kJ mol-1). See ‘Free Energy’.