JEE Main 2022 · 26 June · morning · Single correct · +4 / −1
The correct order of melting points of hydrides of group 16 elements is :
- (a) H₂S < H₂Se < H₂Te < H₂O
- (b) H₂O < H₂S < H₂Se < H₂Te
- (c) H₂S < H₂Te < H₂Se < H₂O
- (d) H₂Se < H₂S < H₂Te < H₂O
Answer and solution
Answer (a) H₂S < H₂Se < H₂Te < H₂O
Solution
Chemistry ma’am · 3 steps
Water sits on top because of hydrogen bonding. The other three line up by molar mass.
- Water is special. Oxygen is small and very electronegative, so H₂O molecules hold each other by strong hydrogen bonds. Breaking these needs a lot of energy, so H₂O melts at the highest temperature.
- The rest by size. S, Se and Te are larger and much less electronegative, so their hydrides form no real hydrogen bonds. Only van der Waals forces hold them together, and these grow stronger as molar mass and size increase: H₂S < H₂Se < H₂Te.
Hydride Molar mass (g mol⁻¹) Main attraction Melting point (K) H₂S 34 van der Waals 188, lowest H₂Se 81 van der Waals 208 H₂Te 130 van der Waals 222 H₂O 18 hydrogen bonding 273, highest - So. The order is H₂S < H₂Se < H₂Te < H₂O, which is option (a).
The idea to keep. For hydrides of a group, check hydrogen bonding first: the N, O or F hydride breaks the trend and sits much higher than expected. The rest rise with molar mass as van der Waals forces get stronger.
Answer from the NTA final answer key. Solution by PrepWiser.