Ch.11 - Liquids and Intermolecular Forces

Brown - Chemistry: The Central Science 15th Edition

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Brown 15th Edition

Ch.11 - Liquids and Intermolecular Forces

Problem 84

Chapter 11, Problem 84

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?

Name: Chemistry: The Central Science
Author: Brown
ISBN: 9780137542970

Verified step by step guidance

Step 1: Understand the nature of ionic liquids. Ionic liquids are composed of ions, which are charged particles. These ions are held together by strong electrostatic forces known as ionic bonds.

Step 2: Compare the forces in ionic liquids to those in molecular liquids. In molecular liquids, the molecules are held together by weaker intermolecular forces such as hydrogen bonds, dipole-dipole interactions, or London dispersion forces.

Step 3: Consider the energy required to vaporize the liquid. For a liquid to vaporize, the particles must overcome the forces holding them together. Since ionic bonds are much stronger than the intermolecular forces in molecular liquids, more energy is required to vaporize ionic liquids.

Step 4: Relate the energy requirement to vapor pressure. Vapor pressure is a measure of a liquid's tendency to evaporate. Because ionic liquids require more energy to overcome their strong ionic bonds, they have a lower tendency to evaporate, resulting in a lower vapor pressure.

Step 5: Conclude with the implications of low vapor pressure. The low vapor pressure of ionic liquids contributes to their nonflammability, as there are fewer vapor molecules available to ignite.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Ionic Liquids

Ionic liquids are salts that are liquid at room temperature, composed entirely of ions. Their unique structure, which includes large, asymmetric cations and anions, leads to strong electrostatic interactions. This results in a high degree of ion pairing and limited molecular mobility, contributing to their distinctive properties, such as low vapor pressure.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of molecules to escape from the liquid phase into the gas phase. Substances with strong intermolecular forces, like ionic liquids, have lower vapor pressures because more energy is required to overcome these forces and allow molecules to vaporize.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring particles (atoms, molecules, or ions). In ionic liquids, the strong ionic bonds and interactions between charged particles create significant forces that hold the liquid together. This results in lower volatility compared to molecular liquids, where weaker van der Waals forces allow for easier vaporization.

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Related Practice

Textbook Question

Use the normal boiling points propane (C3H8) -42.1 °C butane (C4H10) -0.5 °C pentane (C5H12) 36.1 °C hexane (C6H14) 68.7 °C heptane (C7H16) 98.4 °C to estimate the normal boiling point of octane (C8H18). Explain the trend in the boiling points.

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Textbook Question

(b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

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Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives.

(a) How many of these compounds exhibit dispersion interactions?

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Textbook Question

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, 𝑃1 and 𝑃2, and the absolute temperatures at which they were measured, 𝑇1 and 𝑇2:

ln𝑃1𝑃2=−Δ𝐻vap𝑅(1𝑇1−1𝑇2)

b. Gasoline is a mixture of hydrocarbons, a component of which is octane (CH3CH2CH2CH2CH2CH2CH2CH3). Octane has a vapor pressure of 13.95 torr at 25°C and a vapor pressure of 144.78 torr at 75°C. Use these data and the equation in part (a) to calculate the heat of vaporization of octane.

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 11.83.

d. Calculate the vapor pressure of octane at −30°C.

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Textbook Question

The following table gives the vapor pressure of hexafluorobenzene (C₆F₆) as a function of temperature: (a) By plotting these data in a suitable fashion, determine whether the Clausius–Clapeyron equation (Equation 11.1) is obeyed. If it is obeyed, use your plot to determine ∆H_vap for C₆F₆.

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Textbook Question

The table below shows the normal boiling points of benzene and benzene derivatives. (e) Why is the boiling point of phenol the highest of all?

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