IB Chemistry HL

Revision Notes

15.2.1 Entropy



  • You may have wondered why it is that endothermic reactions occur at all, after all, what can be the driving force behind endothermic reactions if the products end up in a less stable, higher energy state?
  • Although the majority of chemical reactions we experience everyday are exothermic,  ΔHꝋ alone is not enough to explain why endothermic reactions occur

Reaction Kinetics Endothermic Reaction Activation Energy, downloadable AS & A Level Chemistry revision notes

The driving force behind chemical reactions cannot be explained by enthalpy changes alone as it does not sense for chemical to end up in a less stable higher energy state in endothermic reactions

  • The answer is entropy

Predicting Entropy Changes

Chaos in the universe

  • The entropy (Sof a given system is the number of possible arrangements of the particles and their energy in a given system
    • In other words, it is a measure of how disordered or chaotic a system is
  • When a system becomes more disordered, its entropy will increase
  • An increase in entropy means that the system becomes energetically more stable
  • For example, during the thermal decomposition of calcium carbonate (CaCO3) the entropy of the system increases:

CaCO3(s) → CaO(s) + CO2(g)

    • In this decomposition reaction, a gas molecule (CO2) is formed
    • The CO2 gas molecule is more disordered than the solid reactant (CaCO3), as it is constantly moving around
    • As a result, the system has become more disordered and there is an increase in entropy
  • Another typical example of a system that becomes more disordered is when a solid melts
    • For example, melting ice to form liquid water:

H2O(s) → H2O(l)

    • The water molecules in ice are in fixed positions and can only vibrate about those positions
    • In the liquid state, the particles are still quite close together but are arranged more randomly, in that they can move around each other
    • Water molecules in the liquid state are therefore more disordered
    • Thus, for a given substance, the entropy increases when its solid form melts into a liquid
  • In both examples, the system with the higher entropy will be energetically favourable (as the energy of the system is more spread out when it is in a disordered state)

Entropy change, downloadable AS & A Level Chemistry revision notes

Melting a solid will cause the particles to become more disordered resulting in a higher entropy state


Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top