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Chapter 11: Problem 69

For a given substance, the liquid crystalline phase tends to be more viscous than the liquid phase. Why?

Short Answer

Expert verified
The liquid crystalline phase is more viscous than the liquid phase because it has a long-range ordered molecular arrangement and stronger intermolecular forces. This ordered arrangement restricts the movement of the molecules, making it more difficult for them to flow past one another compared to the disordered molecular arrangement in the liquid phase.

Step by step solution

01

Understand the Terms

Liquid Phase: The phase in which the substance's molecules have enough thermal energy to move freely and flow past each other, but are still linked together by intermolecular forces that prevent them from escaping the substance's surface. Liquid Crystalline Phase: An intermediate phase between the solid and liquid phases, in which the substance's molecules are partially ordered in a regular, predictable pattern (like in a solid), but can still flow and change their positions (like in a liquid). Viscosity: The resistance of a fluid to flow or deformation due to the intermolecular forces and long-range ordered molecular arrangements.
02

Understand the Intermolecular Forces

In both the liquid crystalline and liquid phases, intermolecular forces, such as those due to van der Waals forces, hydrogen bonding, and dipole-dipole interactions, are responsible for the resistance to flow. The extent of the intermolecular forces among the molecules determines the viscosity of the fluid. Stronger intermolecular forces result in a higher viscosity.
03

Contrast the Molecular Arrangements

In the liquid phase, the molecules have a random and disordered arrangement, which allows them to move past each other more easily. In the liquid crystalline phase, the molecules exhibit long-range ordered molecular arrangements. This means that the molecules have a more organized structure, which can resemble layers or columns. Due to this ordered arrangement, the molecules in the liquid crystalline phase have more restricted movement and are less able to flow past one another compared to the molecules in the liquid phase.
04

Relate Viscosity to Molecular Arrangements and Intermolecular Forces

The viscosity of a fluid depends on both its molecular arrangement and the strength of its intermolecular forces. Since the liquid crystalline phase has a more structured, long-range ordered arrangement compared to the liquid phase, there is a higher resistance to flow among the molecules, which results in an increased viscosity. Additionally, the relatively stronger intermolecular forces in the liquid crystalline phase, due to the molecular order, contribute to its higher viscosity compared to the liquid phase.
05

Conclusion

The liquid crystalline phase of a substance tends to be more viscous than the liquid phase due to a combination of its long-range ordered molecular arrangement and stronger intermolecular forces. This ordered arrangement restricts the movement of the molecules, which makes it more difficult for them to flow past one another compared to the disordered molecular arrangement in the liquid phase.

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Most popular questions from this chapter

You are high up in the mountains and boil water to make some tea. However, when you drink your tea, it is not as hot as it should be. You try again and again, but the water is just not hot enough to make a hot cup of tea. Which is the best explanation for this result? (a) High in the mountains, it is probably very dry, and so the water is rapidly evaporating from your cup and cooling it. (b) High in the mountains, it is probably very windy, and so the water is rapidly evaporating from your cup and cooling it. (c) High in the mountains, the air pressure is significantly less than \(101.3 \mathrm{kPa}\), so the boiling point of water is much lower than at sea level. (d) High in the mountains, the air pressure is significantly less than \(101.3 \mathrm{kPa}\), so the boiling point of water is much higher than at sea level.

(a) How does the average kinetic energy of molecules compare with the average energy of attraction between molecules in solids, liquids, and gases? (b) Why does increasing the temperature cause a solid substance to change in succession from a solid to a liquid to a gas? (c) What happens to a gas if you put it under extremely high pressure?

Hydrazine \(\left(\mathrm{H}_{2} \mathrm{NNH}_{2}\right),\) hydrogen peroxide \((\mathrm{HOOH}),\) and water \(\left(\mathrm{H}_{2} \mathrm{O}\right)\) all have exceptionally high surface tensions compared with other substances of comparable molecular weights. (a) Draw the Lewis structures for these three compounds. (b) What structural property do these substances have in common, and how might that account for the high surface tensions?

(a) Two pans of water are on different burners of a stove. One pan of water is boiling vigorously, while the other is boiling gently. What can be said about the temperature of the water in the two pans? (b) A large container of water and a small one are at the same temperature. What can be said about the relative vapor pressures of the water in the two containers?

In terms of the arrangement and freedom of motion of the molecules, how are the nematic liquid crystalline phase and an ordinary liquid phase similar? How are they different?
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