IBChemistry: HLDPRevision Notes15. HL Energetics / Thermochemistry15.2 Entropy & Spontaneity15.2.1 Entropy

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First teaching 2014

Last exams 2024

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Entropy (DP IB Chemistry: HL)

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Entropy

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

5-1-2-endo-reaction-profile

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 (S) of 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

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    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.