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Which of the substances has the lowest boiling point?

a) $\ce{H2O}$ b) $\ce{H2S}$ c) $\ce{H2Se}$ d) $\ce{H2Te}$

I've been searching for any formula or rule to determine this, but I read that it didn't exist. Do I have to remember some boiling points by heart?

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    $\begingroup$ Lookup Hard and Soft (Lewis) acids and bases. $\endgroup$ – MaxW Apr 6 at 4:01
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    $\begingroup$ Molecules don't have boiling points: BP is a bulk property when lots of molecules get together. $\endgroup$ – matt_black Apr 6 at 9:40
  • $\begingroup$ The key here is to figure out what factors affect boiling point. Specifically, anything that affects intermolecular forces. You should consider the molecular weight (which relates to dispersion forces) and the ability to form hydrogen bonds. $\endgroup$ – Zhe Apr 7 at 23:51
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While it may be difficult to determine the exact boiling point of a substance, many factors play to compare if a substance has a higher boiling point than the others.

1) Intermolecular force. Notice that it is "intermolecular force" instead of "intramolecular force". The reason being when you boil something you want to transform the substance from liquid state into gas state, and that can only be achieved by weakening the connection of molecules, not by breaking the intramolecular bond of a molecule. There are generally three types of intermolecular force : London dispersion force [weakest], dipole-dipole, and hydrogen bonds [strongest]. The stronger the force, the larger the amount of energy needed to break off the connection between the molecules, thus the boiling point is higher.

  1. London dispersion force (LDF/Van der Waals) usually occurs when there exists no significant dipole in the molecule (propane, hexane)
  2. Dipole-dipole occurs when there exist an observable amount of charge difference between atoms in a molecule. This can be seen in the case of hydrogen chloride. The chlorine atom is more electronegative, thus it attracts the more electropositive hydrogens of other molecules too (but doesn't bond with them).
  3. The hydrogen bond is the strongest one. It occurs if a molecule has a hydrogen atom bonded to either F (fluorine), O (oxygen), or N (nitrogen). These atoms are highly electronegative. This happens in the case of water $H_2O$. I reccomend you to think why the hydrogen bond is the strongest and Van der Waals (London dispersion force) is the weakest intermolecular force.

2). Molecular mass. Suppose you have two substances, both of which happen to interact through LDF. Another thing to consider is the mass of the molecule. If the molecule is bigger, then the surface area is larger, which results in a bigger LDF. This leads us to the conclusion that the mass of the molecule is proportional to the boiling point.

3). Branches. In alkanes (consisting of C and H only), generally a straight chain alkane has a higher boiling point than similar branched alkanes due to the surface area between two adjacent molecules. This would be harder if the alkane has branches. Compare : 2,3-dimethylbutane and hexane. Both have 6 carbons, but the boiling point of 2,3-dimethylbutane is 331.15 K whereas hexane is 341.15 K.

Take a look at your case. $H_2O$ has oxygen in it, so hydrogen bond. Although the electronegativity of hydrogen, selenium, sulfur, and tellurium are around 0.1 - 0,48 difference, it doesn't really contribute much to the overall dipole of the molecule. So these three must interact with LDF. We can see that the molecular mass of sulfur < selenium < tellurium (S < Se < Te). From what we know, the bigger the mass, the larger the boiling point.

From this point we can tell that : $H_2S < H_2Se < H_2Te < H_2O$

And a quick Google search will show us that the boiling point of these molecules are : $H_2S (-60) < H_2Se(-41,25) < H_2Te(-2.2) < H_2O$ (100)

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Boiling point or melting points of compounds depend on the intermolecular forces acting between them. Now here we can see hydrogen bonding is present in $H_2O $ resulting in highest boiling point among them. In the other compounds dispersion forces act between the molecules which goes on increasing as the molecular mass increases. So $H_2Te$ has strongest dispersion force so among the rest three it has high boiling point followed by $H_2Se$ and $H_2S$. Hence the order of increasing boiling point is $$ H_2S<H_2Se<H_2Te<H_2O $$ And to Answer your question H2S has the lowest boiling point.

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