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Ch.11 - Liquids and Intermolecular Forces
All textbooksBrown 14th EditionCh.11 - Liquids and Intermolecular ForcesProblem 9
Chapter 11, Problem 9

In Table 11.3, we saw that the viscosity of a series of hydrocarbons increased with molecular weight, doubling from the six-carbon molecule to the ten-carbon molecule. (c) The surface tension of the hydrocarbon liquids in Table 11.4 does increase from hexane to decane, but only by a rather small amount (20% overall, compared to the doubling of viscosity). Which of the statements below is the most likely explanation for this phenomenon? (i) The flexibility of the molecules has a much larger effect on viscosity than on surface tension. (ii) Viscosity only depends on molecular weight, but surface tension depends on molecular weight and on intermolecular forces. (iii) Larger molecules can make larger liquid droplets and therefore have lower surface tension.

Verified step by step guidance
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Step 1: Begin by understanding the concepts of viscosity and surface tension. Viscosity is a measure of a fluid's resistance to flow, while surface tension is the energy required to increase the surface area of a liquid due to intermolecular forces.
Step 2: Consider the relationship between molecular weight and viscosity. As molecular weight increases, the size and complexity of the molecules also increase, which can lead to greater resistance to flow due to entanglement and interactions between molecules.
Step 3: Analyze the effect of molecular weight on surface tension. Surface tension is influenced by intermolecular forces such as van der Waals forces. While larger molecules may have stronger intermolecular forces, the increase in surface tension is not as pronounced as the increase in viscosity.
Step 4: Evaluate the statements provided in the problem. Statement (i) suggests that molecular flexibility affects viscosity more than surface tension, which is plausible because flexible molecules can entangle more easily, increasing viscosity.
Step 5: Consider statement (ii), which posits that viscosity depends solely on molecular weight, while surface tension depends on both molecular weight and intermolecular forces. This statement aligns with the understanding that surface tension is influenced by the nature of intermolecular forces, not just molecular weight.

Key Concepts

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

Viscosity

Viscosity is a measure of a fluid's resistance to flow, which is influenced by the size and shape of its molecules. Larger molecules typically have greater intermolecular interactions, leading to increased viscosity. In hydrocarbons, as molecular weight increases, the viscosity tends to double due to these enhanced interactions, making it harder for the fluid to flow.
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Surface Tension

Surface tension is the energy required to increase the surface area of a liquid, arising from cohesive forces between liquid molecules. While larger molecules can contribute to surface tension, it is also significantly affected by the strength of intermolecular forces. In hydrocarbons, the increase in surface tension from hexane to decane is relatively small, indicating that other factors, such as molecular flexibility, play a role.
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Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence physical properties like viscosity and surface tension. These forces include van der Waals forces, dipole-dipole interactions, and hydrogen bonding. In the context of hydrocarbons, the balance between molecular weight and the strength of these forces helps explain why viscosity increases significantly while surface tension changes only modestly.
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Related Practice
Textbook Question

At three different temperatures, T1, T2, and T3, the molecules in a liquid crystal align in these ways:

(a) At which temperature or temperatures is the substance in a liquid crystalline state? At those temperatures, which type of liquid crystalline phase is depicted?

Textbook Question

At three different temperatures, T1, T2, and T3, the molecules in a liquid crystal align in these ways:

(b) Which is the highest of these three temperatures? [Section 11.7]

Textbook Question

List the three states of matter in order of (a) increasing molecular disorder

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Textbook Question

List the three states of matter in order of (b) increasing intermolecular attraction.

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Textbook Question

(a) How does the average kinetic energy of molecules com- pare with the average energy of attraction between mole- cules in solids, liquids, and gases?

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