How does the molar mass of molecules affect the boiling points?
e.g MOlar masses
HI-->127.9
HF--->20.07
HCl---> 36.45
Boling points
HI-->-35.55
HF---> 20
HCl--->-114.17
can it be said that except for HF molecule, as molar mass increases the boling points alos increases
I think the boiling point you have for HCl is out. See, for example, http://www.transtutors.com/chemistry-homework-help/S-and-P-block-elements/hydrides-of-halogens.aspx
However, your conclusion is correct that boiling point increases, generally, as molar mass increases. HF is the one out of line because of H bonding.
Yes, it can generally be said that as the molar mass of molecules increases, the boiling points also tend to increase. This trend can be observed in the examples you provided.
To understand why this happens, we need to consider the intermolecular forces between molecules. Boiling occurs when the intermolecular forces are overcome and the molecules transition from a liquid to a gas phase.
One major type of intermolecular force is known as van der Waals forces, which include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. These forces arise from the interactions between electron clouds or polar molecules.
In general, larger and more complex molecules have stronger intermolecular forces because they have more electrons and a larger surface area for interaction. As a result, more energy is required to break these stronger forces, leading to higher boiling points.
Let's analyze the examples given:
1. HI (hydrogen iodide): HI has a molar mass of 127.9 g/mol and a boiling point of -35.55°C. This low boiling point can be attributed to the fact that HI molecules are primarily held together by weak London dispersion forces.
2. HF (hydrogen fluoride): HF has a molar mass of 20.07 g/mol and a boiling point of 20°C. Despite its relatively low molar mass, HF has a higher boiling point compared to HI due to hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom is bonded to a highly electronegative atom (such as fluorine) and is attracted to a lone pair of electrons on a nearby molecule. This stronger intermolecular force elevates the boiling point of HF.
3. HCl (hydrogen chloride): HCl has a molar mass of 36.45 g/mol and a boiling point of -114.17°C. Similar to HI, HCl is primarily held together by London dispersion forces, but its boiling point is significantly lower due to its smaller size and lower mass.
Therefore, while the trend of increasing boiling points with increasing molar mass holds true for HI and HCl, HF deviates from this pattern due to the additional factor of hydrogen bonding.