9701 · 23.3
Entropy change, ΔS — FAQ
Frequently asked questions for 9701 Entropy change, ΔS. Direct answers first, then deeper explanation — then practise with marking.
Can the absolute entropy (S) of a substance be negative?
No. The absolute entropy (S°) of a substance is always a positive value. This is based on the Third Law of Thermodynamics, which states that the entropy of a perfect crystal at absolute zero (0 K) is zero. As temperature increases or the substance becomes less ordered, entropy can only increase from this zero point.
Can the entropy change (ΔS) for a reaction be negative?
Yes, absolutely. A negative ΔS means the system has become more ordered. For example, in the reaction 2H₂(g) + O₂(g) → 2H₂O(l), three moles of gas turn into two moles of liquid. This is a significant decrease in disorder, so ΔS is negative.
If the universe tends towards disorder (positive entropy), how can ordered structures like living organisms exist?
This is a classic question! Living organisms are highly ordered systems (low entropy). They maintain this order by taking in energy from their surroundings (e.g., food or sunlight) and, in the process of using that energy, they release heat and waste products, causing a much larger increase in the entropy of their surroundings. The total entropy of the universe (organism + surroundings) still increases, satisfying the Second Law of Thermodynamics.