Entropy change, ΔS — common mistakes

The number one mistake students make is with units. Enthalpy changes ($ΔH$) are almost always in kJ mol⁻¹, but entropy changes ($ΔS$) are in J K⁻¹ mol⁻¹. When you use these together in the Gibbs free energy equation ($ΔG = ΔH - TΔS$), you MUST convert them to the same energy unit. The easiest way is to multiply the $ΔH$ value by 1000 to convert it to J mol⁻¹.

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.

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.

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.