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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 tends to be more viscous than the liquid phase because of the increased molecular order in the liquid crystalline phase. In this phase, the molecules exhibit some degree of long-range order, typically forming layered or aligned structures, which restricts their freedom of movement and makes it harder for them to slide past each other. This results in stronger intermolecular forces and reduced mobility, leading to higher viscosity compared to the liquid phase, where molecules are randomly arranged and can easily slide past one another.

Step by step solution

01

Definition of Liquid and Liquid Crystalline Phase

A liquid is a state of matter characterized by its ability to flow and adapt to the shape of its container, with short-range molecular order but no long-range order. On the other hand, a liquid crystalline phase is an intermediate phase between a liquid and a crystal, where the molecules exhibit some degree of long-range order while still maintaining fluidity.
02

Definition of Viscosity

Viscosity (\(\eta\)) is a measure of a fluid's resistance to flow, or deformation by shear stress or tensile stress. It refers to the internal friction between adjacent layers of fluid that are moving at different velocities. Higher viscosity means that a fluid has more resistance to flow and appears thicker. Conversely, lower viscosity indicates a thinner fluid that flows more easily.
03

Molecular Structure in Liquid and Liquid Crystalline Phase

In the liquid phase, the molecules are randomly arranged and can easily slide past one another, which allows the liquid to flow easily. In the liquid crystalline phase, the molecules are more ordered, typically forming layered or aligned structures. These ordered structures reduce the freedom of movement of the molecules, making it harder for them to slide past each other.
04

Comparing Viscosity in Liquid and Liquid Crystalline Phase

Due to the increased molecular order in the liquid crystalline phase, the molecules experience more resistance to flow. This results in a higher viscosity in the liquid crystalline phase compared to the liquid phase. The ordered molecular arrangements in the liquid crystalline phase create stronger intermolecular forces and reduce the mobility of the molecules, thus increasing the viscosity.

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Most popular questions from this chapter

(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?

Ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) melts at \(-114^{\circ} \mathrm{C}\) and boils at \(78^{\circ} \mathrm{C}\) . The enthalpy of fusion of ethanol is \(5.02 \mathrm{kJ} / \mathrm{mol},\) and its enthalpy of vaporization is 38.56 \(\mathrm{kJ} / \mathrm{mol}\) . The specific heats of solid and liquid ethanol are 0.97 and \(2.3 \mathrm{J} / \mathrm{g}-\mathrm{K},\) respectively. (a) How much heat is required to convert 42.0 \(\mathrm{g}\) of ethanol at \(35^{\circ} \mathrm{C}\) to the vapor phase at \(78^{\circ} \mathrm{C} ?(\mathbf{b})\) How much heat is required to convert the same amount of ethanol at \(-155^{\circ} \mathrm{C}\) to the vapor phase at \(78^{\circ} \mathrm{C} ?\)

Based on the type or types of intermolecular forces, predict the substance in each pair that has the higher boiling point:(a) propane \(\left(\mathrm{C}_{3} \mathrm{H}_{8}\right)\) or \(n\) -butane \(\left(\mathrm{C}_{4} \mathrm{H}_{10}\right),(\mathbf{b})\) diethyl ether \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\right)\) or 1 -butanol \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH}\right)\) (c) sulfur dioxide \(\left(\mathrm{SO}_{2}\right)\) or sulfur trioxide \(\left(\mathrm{SO}_{3}\right),(\mathbf{d})\) phosgene \(\left(\mathrm{Cl}_{2} \mathrm{CO}\right)\) or formaldehyde \(\left(\mathrm{H}_{2} \mathrm{CO}\right)\)

Suppose you have two colorless molecular liquids, one boiling at \(-84^{\circ} \mathrm{C},\) the other at \(34^{\circ} \mathrm{C},\) and both at atmospheric pressure. Which of the following statements is correct? For each statement that is not correct, modify the statement so that it is correct. (a) The higher-boiling liquid has greater total intermolecular forces than the lower- boiling liquid. (b) The lower-boiling liquid must consist of nonpolar molecules. (c) The lower- boiling liquid has a lower molecular weight than the higher-boiling liquid. (d) The two liquids have identical vapor pressures at their normal boiling points. (e) At \(-84^{\circ}\) both liquids have vapor pressures of 760 \(\mathrm{mm} \mathrm{Hg}\) .

Which type of intermolecular force accounts for each of these differences? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) boils at \(65^{\circ} \mathrm{C} ; \mathrm{CH}_{3} \mathrm{SH}\) boils at \(6^{\circ} \mathrm{C} .(\mathbf{b}) \mathrm{Xe}\) is a liquid at atmospheric pressure and \(120 \mathrm{K},\)whereas Ar is a gas under the same conditions. (c) Kr, atomic weight 84 amu, boils at \(120.9 \mathrm{K},\) whereas \(\mathrm{Cl}_{2},\) molecular weight about 71 amu, boils at 238 \(\mathrm{K}\) . (d) Acetone boils at \(56^{\circ} \mathrm{C},\) whereas 2 -methylpropane boils at \(-12^{\circ} \mathrm{C}\) .
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