For statements I and II, state for each if they’re true or false. I) The boiling point of H₂S is lower than the boiling point of H₂O. II) H₂O has a greater molecular mass than H₂S. If both are true, state if II is a correct explanation for I.
Statement I is asking us to compare the boiling points of H₂S, which is called dihydrogen sulfide, and H₂O, which is, of course, water. So to answer this part of the question, let’s review the process of boiling. And why certain substances might have a higher boiling point than others. Boiling is, of course, the process of going from a liquid to a gas. In both the liquid phase and the gas phase, the particles are moving around constantly in their container. But in the liquid phase, the particles are more stuck together than they are as a gas. This is because as a liquid, the particles experience stronger attractions between molecules. We call those attractions intermolecular forces.
In the gas phase, these particles have enough energy to overcome these intermolecular attractions between molecules. So the gas particles are more free to move around their container because they have more energy. This means that the process of boiling is adding enough energy to a liquid so that all of the liquid particles can overcome the intermolecular forces between them. So they become a gas. This means that if a substance has stronger intermolecular forces between the molecules, the substance will need more energy to overcome those intermolecular forces. Which means that the substance will have a higher boiling point.
In this question, we’re being tasked with determining whether H₂S or H₂O will have a higher boiling point. So what we really need to figure out is whether H₂S or H₂O has stronger intermolecular forces. Whichever substance has stronger intermolecular forces will have the higher boiling point. So now, let’s compare H₂S and H₂O to try to figure out which one will have stronger intermolecular forces, so we can determine which one will have a higher boiling point. As we can see from looking at their structures, these two molecules look pretty similar. This makes sense as oxygen is directly above sulfur on the periodic table. Let’s think about the differences between these two molecules.
Sulfur has a higher atomic mass than oxygen does. So H₂S will be heavier than H₂O. The other difference between these two molecules is that oxygen is far more electronegative than sulfur is. There’s a special kind of intermolecular force that occurs when hydrogen is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. This type of intermolecular force is called hydrogen bonding.
Water is an excellent example of hydrogen bonding. Since oxygen is more electronegative than hydrogen, it will pull more on the electrons on the bond between the oxygen and the hydrogen. Causing the oxygen to be partially negatively charged and the hydrogen to be partially positively charged. Another feature of hydrogen bonding is that the atom that hydrogen is bonding to, in addition to being highly electronegative, has a lone pair. Lone pairs have a high density of negative charge. So this means that the atom that the hydrogen is bonded to has a lot of negative charge in one area. So if we have many of these molecules, the partially positive hydrogen from one of the molecules will be attracted to the partially negative lone pair from another molecule.
This attraction between the partially positive hydrogen in one molecule and the partially negative lone pair on the electronegative atom of another is incredibly strong. Hydrogen bonding is often considered to be the strongest intermolecular force. Since hydrogen bonding is such a strong attraction between molecules and H₂O has it but H₂S doesn’t. This means that H₂O will have much stronger intermolecular attractions than H₂S. Since stronger intermolecular forces between molecules takes more energy to overcome, which means a higher boiling point, H₂O will have a higher boiling point than H₂S does.
Statement I says that the boiling point of H₂S is lower than the boiling point of water. As we’ve just determined, H₂O has a higher boiling point than H₂S, due to the stronger intermolecular forces between molecules. So statement I is true. Statement II is asking us to compare the molecular masses of H₂O and H₂S. So let’s calculate the molecular mass for each of these molecules.
Hydrogen has an atomic mass of one unified atomic mass unit. Which you might sometimes see as amu or atomic mass units. Oxygen has an atomic mass of 16, and sulfur has an atomic mass of 32. This means that H₂O will have a molecular mass of 18, and H₂S will have a mass of 34. Statement II says that H₂O has a greater molecular mass than H₂S. As we’ve just determined, this is false.
Since statement II is false, we don’t have to state if II is a correct explanation for I. Besides, as we’ve discussed, the fact that H₂S has a lower boiling point than H₂O has nothing to do with the molecular mass of either molecule. It has everything to do with the fact that H₂O has stronger intermolecular forces between molecules due to the presence of hydrogen bonding in water.