Which substance in each of the following pairs would you...

Key Concepts

Molecular Size and Polarizability

Molecular size and polarizability refer to a molecule's mass and how easily its electron cloud can be distorted. Larger or more polarizable molecules tend to have stronger London dispersion forces since their electron clouds are more easily shifted, leading to stronger temporary dipoles. This contributes to higher boiling points in substances where dispersion forces are the dominant intermolecular force.

Ionic Bonding

Ionic bonding involves the electrostatic attraction between oppositely charged ions and is typically much stronger than the intermolecular forces acting in molecular compounds. Substances with ionic bonds generally exhibit very high melting and boiling points, because the energy needed to separate the ions is considerably greater than that required to overcome molecular interactions.

London Dispersion Forces

London dispersion forces are a type of weak intermolecular attraction present in all molecules, but particularly important in non-polar compounds. They arise from instantaneous fluctuations in electron density and increase in strength with greater molecular size and polarizability. This means that larger, heavier, and more easily polarizable molecules generally experience stronger dispersion forces and, consequently, have higher boiling points.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many physical properties, including boiling points. Stronger intermolecular attractions require more energy to overcome, resulting in higher boiling points. These forces include hydrogen bonds, dipole-dipole interactions, London dispersion forces, and ionic attractions, each of which plays a role in how readily a substance transitions from liquid to gas.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. These bonds are significantly stronger than typical dipole interactions and can greatly elevate a substance's boiling point by requiring substantially more energy to break the network of interactions.

Transcript

00:01 Starting with a, we're going to consider xenon and neon.

00:07 So the big difference between both of these, despite the fact they're both noble gases, is that xenon has atomic number 54, which means it's going to have 54 electrons, and it's going to have a much bigger electron cloud versus neon, which only has 10 electrons and a much smaller electron cloud.

00:28 As a result of this, we're going to see much stronger dispersion forces in xenon.

00:36 So we're going to have more, more dispersion.

00:42 And as a result, xenon is going to have a higher boiling point.

00:50 So thus, we see that xenon will have a higher boiling point out of these two.

00:57 Now, moving on to b.

01:00 For b, we are considering between co2 and cs2.

01:13 Very similar compounds.

01:15 One key difference here, though.

01:17 And this is that sulfur sits right below oxygen on the periodic table.

01:23 So sulfur is going to have a larger electron cloud.

01:26 The same that we saw with neon and xenon.

01:33 So, as a result, we're going to see, well, let's just write this out here, that cs2 has more electrons, and that's going to give it higher dispersion forces, and as a result, we're going to see a higher boiling point.

02:05 Moving on to c now.

02:07 For c, we are considering between ch4 and cl2.

02:17 Ch4, methane, or chlorine in its molecular form.

02:26 Cl2 is going to have a lot more electrons right off the bat than ch4.

02:33 Ch4 is sitting over here with, it's going to have carbons, electrons plus hydrogen's electrons.

02:46 And if we take a look at carbon's electrons, which are related to its atomic number, that's six.

02:54 And hydrogen has that atomic number of one.

02:57 So add it all up, six plus four, ten electrons compared to chlorine.

03:05 And the atomic number, chlorine, is 17...