For each of the properties below, state whether H2O or H2S has the greater value, and justify your a — Bonding / Intermolecular Forces Chemistry Question
Problem Context
For each of the properties below, state whether H2O or H2S has the greater value, and justify your answer.
Heat of fusion
Model Answer
H2O has the greater heat of fusion. O-H---O hydrogen bonds are possible in water, while the S–H bonds in H2S are not polar enough to support hydrogen bonding. This leads to stronger intermolecular forces in water and hence a larger heat of fusion.
Bond angle
Model Answer
H2O has the larger bond angle. The 2s orbital is significantly involved in bonding in H2O, which makes the angle close to the tetrahedral angle of 109.5º (actually 105º). In H2S, the 3s orbital is much smaller than the 3p orbital, so the latter dominates the bonding, and the angle is close to the angle between p orbitals, 90º (actually 92º).
Acid dissociation constant (Ka)
Model Answer
H2S has the larger Ka. The larger S has less electron-electron repulsion, stabilizing the anionic conjugate base. (Explanations involving bond strength are possible as well.)
Stretching frequency in the infrared spectrum
Model Answer
H2O has the larger stretching frequency. ν is proportional to , where k is the force constant of the bond and µ is the reduced mass of the bond. H2O has both (slightly) smaller reduced masses and (significantly) stronger bonds, so its stretching frequencies are higher than those in H2S.