The boiling point of HCl (188 K) is lower than that for HI (238 K) because
A. HI is more polarizable, dispersion forces compensate for the lower dipole forces in HI.
B. HCl is more polar therefore the intermolecular dipole–dipole forces are stronger in HI.
C. HI is capable of hydrogen bonding, HCl is not.
D. the molecular mass of HI is greater than that of HCl.
E. HI is more polar therefore the intermolecular dipole–dipole forces are stronger in HI.
The Correct Answer and Explanation is :
The correct answer is: A. HI is more polarizable, dispersion forces compensate for the lower dipole forces in HI.
Explanation:
Boiling points are primarily determined by the strength of intermolecular forces. For molecules like hydrogen halides (HX), the relevant forces include:
- Dipole-Dipole Interactions: HCl is more polar than HI because chlorine (Cl) is more electronegative than iodine (I). This means that HCl has stronger dipole-dipole interactions than HI.
- London Dispersion Forces: These forces arise from temporary dipoles induced in molecules. Dispersion forces are generally stronger in molecules with higher polarizability, which depends on the size and number of electrons in the molecule. HI, with a larger iodine atom and more electrons, is far more polarizable than HCl.
Despite HCl having stronger dipole-dipole interactions, the boiling point of HI is higher due to the significantly stronger London dispersion forces in HI. The larger size of the iodine atom and its greater polarizability enhance the temporary dipoles, leading to stronger overall intermolecular attractions in HI compared to HCl.
Why the Other Options Are Incorrect:
- B. HCl is more polar therefore the intermolecular dipole–dipole forces are stronger in HI: This is contradictory. While HCl is more polar, it does not mean the dipole-dipole forces in HI are stronger.
- C. HI is capable of hydrogen bonding, HCl is not: Neither HI nor HCl forms hydrogen bonds, as hydrogen bonding requires highly electronegative atoms like N, O, or F.
- D. The molecular mass of HI is greater than that of HCl: While true, molecular mass alone does not determine boiling point. It indirectly affects dispersion forces, but polarizability is the key factor.
- E. HI is more polar therefore the intermolecular dipole–dipole forces are stronger in HI: This is incorrect because HI is less polar than HCl.
In conclusion, the higher boiling point of HI is attributed to its greater polarizability, leading to stronger London dispersion forces that outweigh HCl’s stronger dipole-dipole interactions.