Explain the observed trend in the melting points of the hydrogen halides.

HI -50.8 °C

HBr -88.5 °C

HCl -114.8 °C

HF -83.1 °C

Video transcript

Welcome everyone. Our next problem says, explain the observed trend in the melting points of hyd rides. And then we have H two te negative 49.0 °C, H two se negative 75.7 °C, H two S negative 85.5 °C h2o 0 °C. And our answer choices are a, the melting point increases as the molar mass increases because electrons are held more loosely as the size of the molecule increases. Water is an exception because it can exhibit ionic interactions. B the melting poise increases as the molar mass increases because electrons are held more loosely as the size of the molecule increases. Water is an exception because it can exhibit hydrogen bonding. C the melting point decreases as the molar mass increases because electrons are held more tightly as the size of the molecule increases. Water is an exception because it can exhibit ionic interactions or d the melting point decreases as the molar mass increases because electrons are held more tightly as the size of the molecule increases. Water is an exception because it can exhibit hydrogen bonding. Well, first of all, without even analyzing our compounds, I can eliminate choices A and C because water generally does not exhibit ionic interactions as a rule. So we'll go ahead and cross that out. That's not going to be our explanation. Much more likely water an exception because an exhibit hydrogen bonding. When we talk about melting point, we talk about how much energy needs to be put in to cau break those bonds between the atoms in or the molecules in a solid state. And so as we know, hydrogen bonding is a strong intermolecular or stronger intermolecular force, so that would mean more energy has to be put into it. So we go between B and D we just need to look at is the melting point increasing or decreasing as molar mass increases. So let's look at the atoms in our hydride. Well, we've got Tellurium selenium sulfur and oxygen. These are all in the same column. They're all in group six A. So we're talking about going in a vertical trend in the periodic table and we are in decreasing moller mouse here because Tellurium is the lowest on the periodic table. Selenium's above it sulfur above it and then oxygen above it. So we see that our melting point is decreasing as our molar mass decreases or vice versa, our melting point is increasing as our molar mass increases. So when looking at our answer choices, we see that corresponds to choice B, the melting point increases as the molar mass increases choice D is incorrect since it says the melting point decreases as the molar mass increases. And as the question says, this occurs because electrons are held more loosely as the size of the molecule increases. That allows more dipole moment to occur, meaning that there is a stronger intermolecular interaction. So this would be stronger intermolecular interactions. And of course, water is the exception to this. It has the lowest smaller mass exception, lowest molar mass, but highest melting point. And again, that's because there's hydrogen bonding that happens in water. And that's an even stronger intermolecular force than so a stronger force than dipole interactions. So to explain these trends in the melting points of hydris, our answer is choice b the melting point increases as the molar mass increases because electrons are held more loosely as the size of the molecule increases. Water is an exception because it can exhibit hydrogen bonding. See you in the next video.