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Ch.11 - Liquids and Intermolecular Forces
Chapter 11, Problem 33b

(b) What is the relationship between viscosity and temperature?

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1
Viscosity is a measure of a fluid's resistance to flow. It describes how 'thick' or 'sticky' a fluid is.
As temperature increases, the kinetic energy of the molecules in a fluid also increases.
With higher kinetic energy, the molecules move more rapidly and can overcome intermolecular forces more easily.
This increased molecular movement at higher temperatures generally leads to a decrease in viscosity for liquids.
Therefore, the relationship between viscosity and temperature is typically inverse for liquids: as temperature increases, viscosity decreases.

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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. It quantifies how thick or sticky a fluid is, with higher viscosity indicating a greater resistance to flow. For example, honey has a higher viscosity than water, meaning it flows more slowly. Viscosity is influenced by intermolecular forces and the temperature of the fluid.
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Temperature's Effect on Viscosity

Temperature significantly affects the viscosity of liquids and gases. Generally, as temperature increases, the viscosity of liquids decreases because the increased kinetic energy allows molecules to move more freely, reducing resistance to flow. Conversely, for gases, viscosity tends to increase with temperature due to increased molecular collisions.
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Intermolecular Forces

Intermolecular forces are the attractive forces between molecules that influence their physical properties, including viscosity. Stronger intermolecular forces result in higher viscosity, as molecules are held together more tightly, making it harder for them to move past one another. Understanding these forces helps explain why different substances exhibit varying viscosities at different temperatures.
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Related Practice
Open Question
Look up and compare the normal boiling points and normal melting points of H2O and H2S. Based on these physical properties, which substance has stronger intermolecular forces? What kinds of intermolecular forces exist for each molecule?
Textbook Question

A number of salts containing the tetrahedral polyatomic anion, BF4-, are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion SO42- do not form ionic liquids. Explain this observation.

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Textbook Question

The generic structural formula for a 1-alkyl-3-methylimid- azolium cation is where R is a -CH2(CH2)nCH3 alkyl group. The melting points of the salts that form between 1-alkyl-3-methylimidazolium cation and the PF6- anion are as follows: R = CH2CH3 (m.p. = 60 °C), R = CH2CH2CH3 (m.p. = 40 °C), r = CH2CH2CH2CH3 (m.p. = 10 °C), and R = CH2CH2CH2CH2CH2CH3 (m.p. = -61 °C). Why does the melting point decrease as the length of alkyl group increases?

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Textbook Question

Based on their composition and structure, list CH2Cl2, CH3CH2CH3, and CH3CH2OH in order of (c) increasing surface tension

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Textbook Question

Liquids can interact with flat surfaces just as they can with capillary tubes; the cohesive forces within the liquid can be stronger or weaker than the adhesive forces between liquid and surface:

(b) Which of these diagrams, i or ii, rep- resents what happens when water is on a nonpolar surface?

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Open Question
Hydrazine (H2NNH2), hydrogen peroxide (HOOH), and water (H2O) all have exceptionally high surface tensions compared with other substances of comparable molecular weights. What structural property do these substances have in common, and how might that account for the high surface tensions?