Q.Explain the reversible transition between rhombic and monoclinic sulphur, stating the transition temperature.

[!TLDR] Rhombic and monoclinic sulphur interconvert reversibly at 369 K: rhombic is stable below it, monoclinic above it up to melting. [!ANSWER] The transition temperature is 369 K (95.5 …

Rhombic and monoclinic sulphur, though built from the identical $\text{S}_8$ ring packed into different crystal lattices, are each thermodynamically stable only over their own temperature range, and interconvert reversibly into one another at a sharp transition temperature of $369\ \text{K}$ ($95.5^\circ\text{C}$). Below $369\ \text{K}$, rhombic sulphur is the stable form, and any monoclinic sulphur present slowly converts into it; above $369\ \text{K}$ (and up to sulphur's melting point, around $392\ \text{K}$), monoclinic sulphur becomes the stable form, and rhombic sulphur converts into it instead. At exactly $369\ \text{K}$, the two solid forms coexist in equilibrium with each other. This kind of reversible interconversion between two solid forms, each stable over its own defined temperature range, is called **enantio …

State the transition temperature and explain which form is stable on each side of it, then name …

Getting the stable-below/stable-above assignment backwards; treating the interconversion as one-directional (irreversible) rather than …