Question

The relative strength of the hydrogen bond between the molecules in liquid hydrogen halides can be best ascertained by a comparison of their (1) bond dissociation energies (2) enthalpy changes of vapourisation (3) freezing points (4) enthalpy changes of formation

Short answer

Enthalpy changes of vapourisation

Step-by-step solution

Understanding the Context

Hydrogen bonds are interactions between a hydrogen atom and an electronegative atom. The strength of these bonds affects various physical properties of compounds, including those of liquid hydrogen halides.

Identify the Provided Options

The options given are: (1) bond dissociation energies (2) enthalpy changes of vapourisation (3) freezing points (4) enthalpy changes of formation. Each provides different information about the compounds.

Analyze Bond Dissociation Energies

Bond dissociation energy refers to the energy required to break a bond in a molecule. While important, it pertains more to the strength of the individual molecular bonds rather than the intermolecular hydrogen bonds.

Examine Enthalpy Changes of Vapourisation

The enthalpy change of vapourisation indicates the amount of energy required to convert a liquid into its gaseous phase. This involves breaking intermolecular forces, including hydrogen bonds. Hence, it reflects the strength of hydrogen bonds.

Consider Freezing Points

The freezing point is the temperature at which a liquid becomes solid. It is influenced by intermolecular forces, but it does not directly measure the strength of hydrogen bonds.

Evaluate Enthalpy Changes of Formation

The enthalpy change of formation relates to the energy change when a compound forms from its elements in standard states. It does not directly reflect the strength of intermolecular hydrogen bonds.

Conclusion

Comparing the strengths of hydrogen bonds in liquid hydrogen halides can be best achieved by examining enthalpy changes of vapourisation, as this measure involves overcoming intermolecular hydrogen bonds during the phase change from liquid to gas.

Key concepts

Bond Dissociation Energy

Bond dissociation energy is the energy required to break a specific bond in a molecule. Think of it as the strength needed to pull apart two bonded atoms within a molecule. This can tell us a lot about how strongly the atoms are held together.
However, when we're talking about liquid hydrogen halides, we're actually more interested in the interactions between different molecules (intermolecular forces) rather than within a single molecule. This is because hydrogen bonds, which hold the molecules together in the liquid phase, are the primary concern here.
While bond dissociation energy is important, it's measuring something slightly different: the strength of the bonds between the atoms in a single molecule rather than the hydrogen bonds between molecules.

This means that bond dissociation energy is useful for understanding molecular stability but not as useful for comparing the strengths of hydrogen bonds between different molecules.

Enthalpy Change of Vapourisation

Enthalpy change of vapourisation is the energy required to turn a liquid into a gas. This process needs energy because it involves breaking the hydrogen bonds holding the liquid molecules together.
When you heat a liquid until it becomes a gas, you have to overcome the forces that are keeping the molecules close together. This includes the hydrogen bonds in the liquid hydrogen halides.
Therefore, comparing the enthalpy changes of vapourisation for different hydrogen halides can give us a good idea of the strength of these hydrogen bonds.

Simply put, the more energy needed to turn a liquid into a gas, the stronger the hydrogen bonds are in that liquid.

Freezing Points

The freezing point is the temperature at which a liquid turns into a solid. This transition involves the formation of intermolecular forces, including hydrogen bonds.
While the freezing point can tell us something about the strength of intermolecular forces, it doesn't directly measure the strength of hydrogen bonds. Other factors also influence freezing points, like molecular structure and size.
For example, even if two substances have similar hydrogen bond strengths, their freezing points can be different due to variations in molecular arrangement or additional types of intermolecular forces at play.

Because of these complexities, freezing points are not the best standalone measure for comparing the strength of hydrogen bonds between different hydrogen halides.

Enthalpy Change of Formation

Enthalpy change of formation is the energy change when a compound forms from its elements in their standard states. It gives us a broader picture of how stable a compound is.
This value encompasses all sorts of energy changes, including bond strengths within the molecule and intermolecular interactions. However, it doesn't zero in on the hydrogen bonds specifically.
When we want to compare hydrogen bond strengths specifically between different hydrogen halides, the enthalpy change of formation doesn't provide the focused information we need.

Thus, while important for understanding overall stability, enthalpy change of formation is not ideal for comparing the specific strength of hydrogen bonds between molecules in liquid hydrogen halides.