Entropy (HL) (DP IB Chemistry)

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 every day are exothermic,  ΔH^ꝋ alone is not enough to explain why endothermic reactions occur

Endothermic 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

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)

Low entropy to high entropy 

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