H₂S is less acidic than H₂Te because the S–H bond is stronger (higher bond dissociation enthalpy) than the Te–H bond. Down Group 16 of the periodic table, as atomic size increases, the M–H bond length increases, bond dissociation enthalpy decreases, and H⁺ (proton) is released more easily — increasing acidity. The order of acidity: H₂O < H₂S < H₂Se < H₂Te.
H₂S is less acidic than H₂Te — acidity increases down Group 16.
Order: H₂O < H₂S < H₂Se < H₂Te (increasing acidity).
Reason: S–H bond (363 kJ/mol) is stronger than Te–H bond (238 kJ/mol).
Stronger M–H bond → H⁺ released less easily → weaker acid.
Down the group: atomic size increases → bond length increases → bond dissociation enthalpy decreases.
Weaker bond → easier H⁺ release → stronger acid.
Group 16 hydrides: H₂O, H₂S, H₂Se, H₂Te
Acidity order (increasing): H₂O < H₂S < H₂Se < H₂Te
So H₂S is LESS acidic than H₂Se and H₂Te.
Reason: Acid strength depends on ease of releasing H⁺ (proton): • The easier it is to break the M–H bond → the more H⁺ is released → the stronger the acid • M–H bond strength (bond dissociation enthalpy) decreases down the group
Bond parameters down Group 16: Bond | Bond Length (pm) | Bond Dissociation Enthalpy O–H | 96 | 497 kJ/mol S–H | 134 | 363 kJ/mol Se–H | 146 | 276 kJ/mol Te–H | 169 | 238 kJ/mol
Conclusion: • O–H bond is the strongest → H₂O releases H⁺ least easily → weakest acid • Te–H bond is the weakest → H₂Te releases H⁺ most easily → strongest acid • Therefore: H₂S is less acidic than H₂Te ✓
Why does M–H bond strength decrease from O to Te?
Increasing atomic size: • Going down Group 16: O < S < Se < Te (increasing atomic radius) • Larger atom → longer M–H bond → weaker overlap between orbitals
Decreasing orbital overlap: • S uses 3s and 3p orbitals; O uses 2s and 2p orbitals • 3p orbitals of S are larger → less effective overlap with 1s of H • Orbital overlap decreases down the group → bond becomes weaker
Electronegativity: • Electronegativity decreases down the group: O > S > Se > Te • Higher electronegativity → O attracts the shared electrons more strongly • But despite this, H₂O is the weakest acid because the O–H bond is very strong (the bond doesn't break easily) • The bond dissociation enthalpy effect dominates over electronegativity
Note on H₂O: • H₂O is anomalously weak as an acid despite O being highly electronegative • Because the O–H bond is very short and strong (hydrogen bonding also strengthens it) • In aqueous solution, pKa of H₂O ≈ 15.7 (very weak acid)
Exam-expected answer for 'Why H₂S is less acidic than H₂Te': The S–H bond is stronger than Te–H bond because S is smaller. When the bond is stronger, it releases H⁺ less easily. H₂Te has a weaker Te–H bond (longer, weaker overlap), so it releases H⁺ more readily → H₂Te is more acidic than H₂S.
H₂S is less acidic than H₂Te because the S–H bond (363 kJ/mol) is stronger than the Te–H bond (238 kJ/mol). Down Group 16, atomic size increases, bond length increases, and bond dissociation enthalpy decreases. A weaker Te–H bond releases H⁺ more easily, making H₂Te a stronger acid than H₂S.
Acidity order: H₂O < H₂S < H₂Se < H₂Te. Acidity increases down the group because M–H bond dissociation enthalpy decreases (bond becomes weaker), making H⁺ release progressively easier.
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