Question

Schematic of phase diagrams for a pure gas hydrate system of methane (CH$_4$), carbon dioxide (CO$_2$), hydrogen sulphide (H$_2$S) and nitrogen (N$_2$) between the lower and upper quadruple points are shown in the figure. Arrowheads indicate the stable hydrate region for a particular gas hydrate system. Match the phase diagram with the corresponding pure gas hydrate.

• A. I – CH$_4$; II – N$_2$; III – CO$_2$; IV – H$_2$S
• B. I – H$_2$S; II – CH$_4$; III – CO$_2$; IV – N$_2$
• C. I – N$_2$; II – CH$_4$; III – H$_2$S; IV – CO$_2$
• D. I – N$_2$; II – CH$_4$; III – CO$_2$; IV – H$_2$S

Hint:

Relative hydrate stability (from least to most stable): N$_2$<CH$_4$<CO$_2$<H$_2$S.

Answer 1

The Correct Option is D

- The stability of hydrates depends on the nature of the guest molecule.
- Nitrogen (N$_2$) forms hydrates under the least stable conditions (requires highest pressure at a given temperature), hence its stability curve lies farthest to the left (I).
- Methane (CH$_4$) is more stable than N$_2$ but less than CO$_2$ and H$_2$S, hence curve (II).
- Carbon dioxide (CO$_2$) hydrates are more stable than methane, hence curve (III).
- Hydrogen sulphide (H$_2$S) hydrates are the most stable (form at the lowest pressure for a given temperature), hence curve (IV).
Therefore, the correct matching is (D).