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Chapter 11: Problem 82

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?

Short Answer

Expert verified
Ionic liquids have lower vapor pressures than most room-temperature molecular liquids because they have stronger intermolecular forces, primarily due to the significant Coulombic forces between their cations and anions. In contrast, room-temperature molecular liquids have weaker forces, such as dipole-dipole, hydrogen bonding, and van der Waals forces. Stronger intermolecular forces require more energy for evaporation, resulting in lower vapor pressures and making ionic liquids nonflammable.

Step by step solution

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1. Understanding the composition of ionic liquids

Ionic liquids are composed of ions, specifically cations and anions. These ions are not well-coordinated, meaning that they are not highly ordered in their structure. In ionic liquids, the electrostatic forces between cations and anions, called Coulombic forces, are a significant contributor to the overall intermolecular forces present in the liquid.
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2. Understanding the composition of room-temperature molecular liquids

Room-temperature molecular liquids consist of neutral molecules which interact through various types of intermolecular forces including dipole-dipole, hydrogen bonding, and van der Waals forces. These forces tend to be weaker than the forces present in ionic liquids.
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3. The relationship between intermolecular forces and vapor pressure

Vapor pressure is influenced by the strength of the intermolecular forces present in a substance. A substance with stronger intermolecular forces requires more energy to evaporate, leading to a lower vapor pressure. Conversely, a substance with weaker intermolecular forces requires less energy to evaporate, resulting in a higher vapor pressure.
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4. Comparing intermolecular forces in ionic and molecular liquids

As discussed earlier, ionic liquids have significant Coulombic forces acting between the ions that contribute to the overall intermolecular forces present in the liquid, making them stronger compared to the forces in room-temperature molecular liquids. Room-temperature molecular liquids have weaker intermolecular forces such as dipole-dipole, hydrogen bonding, and van der Waals forces, which are not as strong as the ionic forces in ionic liquids.
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5. Connecting the strength of intermolecular forces to the vapor pressure of ionic liquids

The stronger intermolecular forces in ionic liquids mean that more energy is required for these substances to evaporate, or transition to their gas phase. As a result, ionic liquids have lower vapor pressures compared to room-temperature molecular liquids, which have weaker intermolecular forces and, therefore, higher vapor pressures. This explains why ionic liquids have lower vapor pressures and tend to be nonflammable compared to most room-temperature molecular liquids.

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Most popular questions from this chapter

Indicate whether each statement is true or false: (a) The critical pressure of a substance is the pressure at which it turns into a solid at room temperature. (b) The critical temperature of a substance is the highest temperature at which the liquid phase can form. (c) Generally speaking, the higher the critical temperature of a substance, the lower its critical pressure. (\boldsymbol{d} ) In general the more intermolecular forces there are in a substance, the higher its critical temperature and pressure.

Carbon tetrachloride, \(\mathrm{CCl}_{4},\) and chloroform, \(\mathrm{CHCl}_{3},\) are common organic liquids. Carbon tetrachloride's normal boiling point is \(77^{\circ} \mathrm{C} ;\) chloroform's normal boiling point is \(61^{\circ} \mathrm{C} .\) Which statement is the best explanation of these data? (a) Chloroform can hydrogen-bond, but carbon tetrachloride cannot. (b) Carbon tetrachloride has a larger dipole moment than chloroform. (c) Carbon tetrachloride is more polarizable than chloroform.

The boiling points, surface tensions, and viscosities of water and several alcohols are as shown below:(a) From ethanol to propanol to \(n\) -butanol the boiling points, surface tensions, and viscosities all increase. What is the reason for this increase? (b) How do you explain the fact that propanol and ethylene glycol have similar molecular weights (60 versus 62 amu), yet the viscosity of ethylene glycol is more than 10 times larger than propanol? (c) How do you explain the fact that water has the highest surface tension but the lowest viscosity?

Ethylene glycol \(\left(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) and pentane \(\left(\mathrm{C}_{5} \mathrm{H}_{12}\right)\) are both liquids at room temperature and room pressure, and have about the same molecular weight. (a) One of these liquids is much more viscous than the other. Which one do you predict is more viscous? (b) One of these liquids has a much lower normal boiling point \(\left(36.1^{\circ} \mathrm{C}\right)\) compared to the other one \(\left(198^{\circ} \mathrm{C}\right) .\) Which liquid has the lower normal boiling point? (c) One of these liquids is the major component in antifreeze in automobile engines. Which liquid would you expect to be used as antifreeze? (d) One of these liquids is used as a "blowing agent" in the manufacture of polystyrene foam because it is so volatile. Which liquid would you expect to be used as a blowing agent?

Compounds like \(\mathrm{CCl}_{2} \mathrm{F}_{2}\) are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of \(\mathrm{CCl}_{2} \mathrm{F}_{2}\) is 289 \(\mathrm{J} / \mathrm{g}\) . What mass of this substance must evaporate to freeze 200 \(\mathrm{g}\) of water initially at \(15^{\circ} \mathrm{C} ?\) (The heat of fusion of water is \(334 \mathrm{J} / \mathrm{g} ;\) the specific heat of water is \(4.18 \mathrm{J} / \mathrm{g}-\mathrm{K}\) .
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