The boiling points of the hydrogen halides are listed below.
HF 19°C
HCl –85°C
HBr –67°C
HI –35°C
Which of the following accounts for the relatively high boiling point of HF?
1. HF has the strongest H—X bond.
2. HF molecules form hydrogen bonds.
3. HF molecules have the strongest va der Waals forces.
Answer
You need to read about hydrogen bonding.
The correct answer is 2. HF molecules form hydrogen bonds.
To determine this, we need to understand the intermolecular forces present in the hydrogen halides. Intermolecular forces are responsible for holding molecules together in a liquid or solid state and are directly related to the boiling points of substances.
1. HF has the strongest H—X bond: This option refers to the bond strength between hydrogen (H) and the halogen atom (X) in each hydrogen halide. While it is true that HF has the highest bond strength compared to the other hydrogen halides, the boiling point is not solely determined by the strength of the H—X bond.
2. HF molecules form hydrogen bonds: This option correctly identifies the reason for the relatively high boiling point of HF. Hydrogen bonds are a special type of intermolecular force that occurs when hydrogen is bonded to an element with high electronegativity, such as fluorine (F). The hydrogen atom in HF is attracted to the electronegative fluorine atom of another HF molecule, creating a strong intermolecular force. These hydrogen bonds contribute to the higher boiling point of HF compared to the other hydrogen halides.
3. HF molecules have the strongest van der Waals forces: Van der Waals forces are the weak intermolecular forces caused by temporary fluctuations in electron density. While all hydrogen halides have van der Waals forces, HF has the additional significant contribution from hydrogen bonds, making this option incorrect.
Therefore, the correct answer is 2. HF molecules form hydrogen bonds, which account for the relatively high boiling point of HF compared to the other hydrogen halides.