Entropy (HL) (DP IB Chemistry): Revision Note

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 answer is entropy

Entropy and the dispersal of energy

• Entropy, S, is a measure of the distribution of matter and/or energy in a system

• It tells us how many possible ways the particles and their energy can be arranged

  • In other words, it is a measure of how disordered or chaotic a system is

• The more possible arrangements there are, the higher the entropy

• A more disordered system (higher entropy) is usually more energetically stable

Gas formation

• 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, resulting in an increase in entropy

Melting

• Another 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, system with higher entropy is more energetically favourable, because the energy is more widely dispersed

Predicting entropy change

• Under the same conditions, entropy increases in the order:

solid < liquid < gas

• You can often predict whether entropy will increase or decrease by considering:

  • Are gases formed or used up in the reaction?

  • Is a solid becoming a liquid or gas?

  • Are the particles becoming more mobile or disordered?