The table shown here lists the molar heats of vaporization...

The table shown here lists the molar heats of vaporization for several organic compounds. Use specific examples from this list to illustrate how the heat of vaporization varies with (a) molar mass, (b) molecular shape, (c) molecular polarity, (d) hydrogen-bonding interactions. Explain these comparisons in terms of the nature of the intermolecular forces at work. (You may find it helpful to draw out the structural formula for each compound.) $$ \begin{array}{ll} \text { Compound } & \begin{array}{l} \text { Heat of Vaporization } \\ \mathbf{( k J / m o l )} \end{array} \\ \hline \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} & 19.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 27.6 \\ \mathrm{CH}_{3} \mathrm{CHBrCH}_{3} & 31.8 \\ \mathrm{CH}_{3} \mathrm{COCH}_{3} & 32.0 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} & 33.6 \\ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 47.3 \end{array} $$

The table shown here lists the molar heats of vaporization for several organic compounds. Use specific examples from this list to illustrate how the heat of vaporization varies with
(a) molar mass, (b) molecular shape, (c) molecular polarity,
(d) hydrogen-bonding interactions. Explain these comparisons in terms of the nature of the intermolecular forces at work. (You may find it helpful to draw out the structural formula for each compound.)
$$
\begin{array}{ll}
\text { Compound } & \begin{array}{l}
\text { Heat of Vaporization } \\
\mathbf{( k J / m o l )}
\end{array} \\
\hline \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} & 19.0 \\
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 27.6 \\
\mathrm{CH}_{3} \mathrm{CHBrCH}_{3} & 31.8 \\
\mathrm{CH}_{3} \mathrm{COCH}_{3} & 32.0 \\
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Br} & 33.6 \\
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH} & 47.3
\end{array}
$$

Key Concepts

Molecular Shape

Molecular shape affects how efficiently molecules can pack together and interact. Elongated or linear molecules have more surface area for contact and can often experience stronger dispersion forces compared to compact or branched molecules. This increased interaction surface area leads to a higher heat of vaporization because more energy is needed to separate the molecules.

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of dipole-dipole interaction that occurs when a hydrogen atom is covalently bonded to a highly electronegative atom such as oxygen, nitrogen, or fluorine. Hydrogen bonds significantly increase the energy required to vaporize a substance because they create strong attractions between molecules. This is why molecules capable of hydrogen bonding typically exhibit notably high heats of vaporization.

Intermolecular Forces

Intermolecular forces are the various attractions between molecules that determine their physical properties, such as boiling points and heats of vaporization. They include London dispersion forces, dipole-dipole interactions, and hydrogen bonding. The strength and type of these forces directly affect the energy needed for a molecule to change phases—stronger intermolecular attractions lead to higher heats of vaporization.

Molar Mass

Molar mass is a key factor affecting the strength of London dispersion forces. As molar mass increases, there are more electrons present in the molecule, which enhances the induced dipoles and temporary attractive interactions between molecules. These stronger dispersion forces require more energy to overcome during vaporization, resulting in higher heats of vaporization.

Molecular Polarity

Molecular polarity results from asymmetrical charge distribution within a molecule, leading to permanent dipole moments. Polar molecules engage in dipole-dipole interactions, which are generally stronger than the dispersion forces found in nonpolar molecules. This means that polar molecules require more energy to transition into the vapor phase, thereby having higher heats of vaporization.

Transcript

00:01 This is a messy problem, but it's not too hard.

00:04 We are given six substances.

00:07 And actually, we were given the chemical formula for each of these, which you can find in your text.

00:12 But i've sketched each one.

00:16 And let me see if i can find the problem again.

00:20 So we've got the molar heats of vaporization.

00:23 I've done those in red.

00:26 And we've got several examples.

00:28 We are going to use specific examples to illustrate.

00:58 Illustrate how heat of vaporization varies with different things.

01:14 And then we're going to do several different things.

01:17 So i'll do the first one on this page, and i think i'll go to a different color.

01:22 Let's go with a light blue.

01:25 Our first thing that we're going to do is molar mass.

01:36 So we're going to use specific examples to explain how this all varies with molar mass.

01:43 So for molar mass, let's look at compounds.

01:45 One and two, or a and b.

02:06 So a and b have similar, similar, what i'm going to call a non -branched structure, and it only contains hydrogen and carbon.

02:36 B has a longer chain, which means a greater molar mass.

02:59 The greater molar mass leads to stronger lunder dispersion forces.

03:31 And that means a higher heat of vaporization.

03:42 And that's the answer for a.

03:47 The answer for b.

03:49 And b asks us to talk about b molecular shape.

04:17 Okay, so for molecular shape, let's pick, we're going to pick, i think we'll pick, c and d.

04:30 So i'm going to highlight these quick and then i'm just going to erase that highlighter.

04:36 So we're going to do c and c and d.

04:41 These are two we're going to look at.

04:43 You notice these are both branch chains.

04:49 No, let's not do that one.

04:53 Let's do c.

04:54 Let's do these two.

04:56 Okay.

04:57 Now these both are, they both have the same molar mass.

05:06 Their heats aren't terribly different, but they are the same chemical formula, but they're just different structures.

05:19 Get back to that page, shoot.

05:22 I should have before i went to my other page, i should have deleted those marks.

05:28 Oh, well...

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