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Chapter 11: Problem 21

Which member in each pair has the greater dispersion forces? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) or $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH},\( (b) \)\mathrm{NH}_{3}$ or \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\), (c) $\mathrm{CH}_{2} \mathrm{Cl}_{2}\( or \)\mathrm{CH}_{2} \mathrm{Br}_{2}$

Short Answer

Expert verified
In each pair, the molecule with greater dispersion forces is: (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol), (b) \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\) (Trimethylamine), and (c) \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (Dibromomethane). This is because these molecules are larger and have more electrons, resulting in a more polarizable electron cloud and stronger dispersion forces.

Step by step solution

01

Identify the molecular structures

First, let's draw the molecular structures for both \(\mathrm{CH}_{3} \mathrm{OH}\) (Methanol) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol). Methanol: H3C-O-H Ethanol: H3C-CH2-O-H
02

Compare the molecular sizes

It can be observed that ethanol has an extra CH2 group compared to methanol, which makes it a larger molecule with more electrons.
03

Determine the greater dispersion forces

Since ethanol has a larger molecular size and more electrons compared to methanol, it has greater dispersion forces. Therefore, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (Ethanol) has the greater dispersion forces in Pair A. #Pair B: \(\mathrm{NH}_{3}\) and \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)#
04

Identify the molecular structures

First, let's draw the molecular structures for both Ammonia (\(\mathrm{NH}_{3}\)) and Trimethylamine (\(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\)). Ammonia: H3N Trimethylamine: (H3C)3N
05

Compare the molecular sizes

It can be observed that trimethylamine has three CH3 groups attached to the central nitrogen atom, making it a larger molecule with more electrons compared to ammonia.
06

Determine the greater dispersion forces

Since trimethylamine is a larger molecule and has more electrons compared to ammonia, it has greater dispersion forces. Therefore, \(\mathrm{N}\left(\mathrm{CH}_{3}\right)_{3}\) (Trimethylamine) has the greater dispersion forces in Pair B. #Pair C: \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) and \(\mathrm{CH}_{2} \mathrm{Br}_{2}\)#
07

Identify the molecular structures

First, let's draw the molecular structures for both Dichloromethane (\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\)) and Dibromomethane (\(\mathrm{CH}_{2} \mathrm{Br}_{2}\)). Dichloromethane: H2C-Cl2 Dibromomethane: H2C-Br2
08

Compare the molecular sizes

It can be observed that the only difference between dichloromethane and dibromomethane is the halogen substituent; one has chlorine, and the other has bromine. Bromine atoms are larger and have more electrons compared to chlorine atoms.
09

Determine the greater dispersion forces

Since dibromomethane has larger halogen substituents (bromine atoms) with more electrons compared to dichloromethane, it has greater dispersion forces. Therefore, \(\mathrm{CH}_{2} \mathrm{Br}_{2}\) (Dibromomethane) has the greater dispersion forces in Pair C.

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Most popular questions from this chapter

One of the attractive features of ionic liquids is their low vapor pressure, which in turn tends to make them nonflammable. Why do you think ionic liquids have lower vapor pressures than most room-temperature molecular liquids?

(a) What is the relationship between surface tension and temperature? (b) What is the relationship between viscosity and temperature? (c) Why do substances with high surface tension also tend to have high viscosities?

A number of salts containing the tetrahedral polyatomic anion, \(\mathrm{BF}_{4}^{-}\), are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}{ }^{2-}\) do not form ionic liquids. Explain this observation.

Suppose the vapor pressure of a substance is measured at two different temperatures. (a) By using the Clausius-Clapeyron equation (Equation 11.1) derive the following relationship between the vapor pressures, \(P_{1}\) and \(P_{2}\), and the absolute temperatures at which they were measured, \(T_{1}\) and \(T_{2}\) : $$ \ln \frac{P_{1}}{P_{2}}=-\frac{\Delta H_{\mathrm{vap}}}{R}\left(\frac{1}{T_{1}}-\frac{1}{T_{2}}\right) $$ (b) Gasoline is a mixture of hydrocarbons, a component of which is octane $\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\right)$. Octane has a vapor pressure of \(1.86 \mathrm{kPa}\) at \(25^{\circ} \mathrm{C}\) and a vapor pressure of \(19.3 \mathrm{kPa}\) at \(75^{\circ} \mathrm{C}\). Use these data and the equation in part (a) to calculate the heat of vaporization of octane. \((\mathbf{c})\) By using the equation in part (a) and the data given in part (b), calculate the normal boiling point of octane. Compare your answer to the one you obtained from Exercise 11.81 . (d) Calculate the vapor pressure of octane at \(-30^{\circ} \mathrm{C}\).

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (a) Iodine solid turns to iodine gas when it is heated. (b) Snowflakes turn into water when they fall on an open palm. (c) Droplets of water appear on grass in a cold humid morning. (d) Dry ice gradually disappears when left at room temperature for some period of time.
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