Video: Identifying the Cause of the Higher Boiling Point of H₂O Compared to H₂S in a Set of Causes

Which of the following contribute to the high boiling point of H₂O compared to that of H₂S? [A] Network covalent bonds [B] Dipole-dipole interactions [C] London dispersion forces [D] Hydrogen bonds [E] Ionic bonds

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Video Transcript

Which of the following contribute to the high boiling point of H₂O compared to that of H₂S? A) Network covalent bonds, B) Dipole-dipole interactions, C) London dispersion forces, D) Hydrogen bonds, or E) Ionic bonds.

In this question, we’re comparing the boiling points of two different chemical species — H₂O, water, and H₂S, which is dihydrogen sulfide. So before we compare these two, let’s review what’s going on when a substance boils. Boiling is, of course, the process of going from a liquid to a gas. The particles in both a liquid and a gas move around in their containers. But the particles in a liquid are kind of stuck together because of the intermolecular attractions between molecules. Gas particles, on the other hand, have enough energy to overcome these attractions between the particles. This means that the process of boiling is putting enough energy into a liquid so that all of the liquid particles have enough energy to overcome the intermolecular attractions.

So if a substance has a high boiling point, that means that the substance needs more energy to overcome those intermolecular attractions between the liquid particles so that it can be a gas. So this means that if H₂O has a higher boiling point than H₂S, H₂O must have stronger intermolecular attractions between molecules than H₂S does. So let’s take a look at water and dihydrogen sulfide. So we can go through our answer choices to figure out why water has stronger intermolecular attractions than dihydrogen sulfide does. If we compare water to dihydrogen sulfide, their structures are pretty similar. Which makes sense as sulphur is directly below oxygen on the periodic table.

Now, let’s look through our answer choices. We’re looking for something that will reflect the fact that water has stronger intermolecular attractions than H₂S. The first answer choice is network covalent bonds, which, like the name suggests, is a network of covalently bonded atoms. Two common examples of network covalent bonds are: graphite, which is a network composed of covalently bonded carbons where each carbon is bonded to three other carbons. And diamond, which is a network of covalently bonded carbons where each carbon is bound to four other carbons. Network covalent bonds doesn’t describe water or dihydrogen sulfide, as neither exists in a large network of covalently bonded atoms. So answer choice A is not the correct answer.

Our next answer choice, dipole-dipole interactions, occur due to differences in electronegativity between atoms in a molecule. The difference in electronegativity will cause one atom in the molecule to become partially positively charged and the other to become partially negatively charged. If we have many of these molecules, the partially negative end of one molecule will be attracted to the partially positive end of the other. This would occur in both water and dihydrogen sulfide, as both oxygen and sulphur are more electronegative than hydrogen. So let’s keep this one in mind but move on to our other answer choices.

Our next answer choice is London dispersion forces. London dispersion forces are due to the movement of electrons in a molecule. We normally think of electrons being evenly distributed around the nucleus, like the picture on the left. But electrons can move around a molecule almost instantaneously. This means that sometimes they’ll be unevenly distributed around the nucleus, like the picture on the left. Here, all of the electrons are on one side of the nucleus. Which causes one end of the molecule to become positively charged and the other end to become negatively charged. This instantaneous dipole that is formed as a result of electrons moving can cause attractions to occur between molecules.

In general, London dispersion forces are stronger for larger molecules. As in larger molecules, the electrons are further from the nucleus. So it’s easier for them to move around. Sulphur is a larger atom than oxygen is. Which means that we would expect hydrogen disulfide to have stronger London dispersion forces than water. However, we’re looking for something that will contribute to the fact that water has stronger intermolecular attractions overall than hydrogen sulfide does. So London dispersion forces cannot be the correct answer choice.

Our next answer choice, hydrogen bonds, is a special kind of intermolecular attraction that occurs between hydrogen and highly electronegative atoms like oxygen, nitrogen, or Fluorine. The electronegativity difference between hydrogen and the other atom — in this case, for my example, is fluorine — will cause the hydrogen to become partially positively charged and the fluorine to become partially negatively charged. Similar to what occurs and dipole-dipole interactions, the partially positively charged hydrogen will then become attracted to the negatively charged fluorine in another molecule. Unlike the dipole-dipole interactions, now there’s a lone pair on the already quite negatively charged atom.

Lone pairs have a high density of negative charge. So the attraction between the negatively charged lone pair and the positively charged hydrogen can be quite strong. Hydrogen bonds are often considered to be the strongest intermolecular attraction. Since the oxygen in water is so electronegative and has a lone pair, water would have hydrogen bonds. Dihydrogen sulfide, on the other hand, wouldn’t because sulphur is not electronegative enough. Since hydrogen bonds are such a strong intermolecular attraction and water has them but dihydrogen sulfide does not, answer choice D must be the answer that we’re looking for. But let’s look at our last answer choice, just to be sure.

Ionic bonds typically occur in a compound containing a nonmetal and a metal. In these type of compounds, the nonmetal is so much more electronegative than the metal that the nonmetal steals the electron from the metal, causing there to be negatively and positively charged ions. This does not describe water or dihydrogen sulfide. So wouldn’t make sense that Ionic bonds contribute to the higher boiling point of water. Like we’ve discussed, hydrogen bonds are extremely strong intermolecular attractions. And they’re present in water but not present in dihydrogen sulfide. So hydrogen bonds contribute to the high boiling point of H₂O compared to H₂S.

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