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.

equation for enetropy change of products reactants

For example:

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)
CH4(g) 186.3
O2(g) 205.1
H2O(g) 69.9
CO2(g) 213.7

values taken from

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

calculating entropy change using standard entropies

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.