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Chapter 11: Problem 33

(a) Explain why surface tension and viscosity decrease with increasing temperature. (b) Why do substances with high surface tensions also tend to have high viscosities?

Short Answer

Expert verified
Surface tension and viscosity both decrease with increasing temperature because higher temperatures weaken the intermolecular forces within a fluid. Weaker intermolecular forces result in lower surface tension (reduced surface particle attraction) and in lower viscosity (reduced internal friction). Substances with high surface tensions tend to have high viscosities because both properties are related to the strength of the intermolecular interactions within the fluid. Strong intermolecular forces result in greater surface elasticity (surface tension) and resistance to flow (viscosity).

Step by step solution

01

Define Surface Tension and Viscosity

Surface tension can be thought of as an elastic property of the surface of a liquid that causes it to behave like a stretched elastic membrane. It is due to the unbalanced attractive forces between the molecules at the surface of a liquid. In other words, surface tension is the force that acts upon a liquid's surface to prevent liquid particles from spreading. Viscosity, on the other hand, is a measure of a fluid's resistance to flow. It is essentially the internal friction of a fluid, and it describes how thick or sticky it is. Viscosity is influenced by the forces between fluid particles and how easily they can move past each other.
02

Temperature Effect on Surface Tension

As temperature increases, the kinetic energy of molecules also increases, and they move more vigorously. This increased movement weakens the attractive forces between the molecules at the surface of a liquid, resulting in a decrease in surface tension. The decreased attractive forces between liquid particles at higher temperatures make it easier for particles to break away from the surface, lowering the surface tension. In other words, surface tension decreases with increasing temperature because the intermolecular forces become weaker as temperature rises.
03

Temperature Effect on Viscosity

Viscosity also tends to decrease when a liquid's temperature is increased. As mentioned earlier, viscosity is a measure of a fluid's resistance to flow, and it depends on the internal forces and interactions between its particles. When temperature increases, the kinetic energy of a fluid's particles also increases. This leads to weaker molecular cohesion and, therefore, less fluid resistance to flow. As a result, viscosity decreases with increasing temperature as the internal friction between a fluid's particles becomes weaker with increasing molecular energy.
04

Relationship between Surface Tension and Viscosity

Both surface tension and viscosity are influenced by the intermolecular forces within a fluid. Substances with strong intermolecular forces will tend to have higher surface tensions, as these forces work to hold the surface particles together more tightly. Such substances will also tend to have higher viscosities, as strong intermolecular forces make it more difficult for particles to flow past each other. Therefore, substances with high surface tensions often have high viscosities because both properties are related to the strength of the intermolecular interactions within the fluid. These strong interactions contribute to more resistance to flow (viscosity) and greater surface elasticity (surface tension).

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Most popular questions from this chapter

(a) What is the significance of the triple point in a phase dia gram? (b) Could you measure the triple point of water b measuring the temperature in a vessel in which water vapo liquid water, and ice are in equilibrium under one atmospher of air? Explain.

Using this graph of \(\mathrm{CS}_{2}\) data, determine (a) the approximate vapor pressure of \(\mathrm{CS}_{2}\) at \(30^{\circ} \mathrm{C}\), (b) the temperature at which the vapor pressure equals 300 torr, (c) the normal boiling point of \(\mathrm{CS}_{2}\). [Section 11.5]

The relative humidity of air equals the ratio of the partial pressure of water in the air to the equilibrium vapor pressure of water at the same temperature times \(100 \% .\) If the relative humidity of the air is \(58 \%\) and its temperature is \(68^{\circ} \mathrm{F}\), how many molecules of water are present in a room measuring \(12 \mathrm{ft} \times 10 \mathrm{ft} \times 8 \mathrm{ft} ?\)

The critical temperatures \((\mathrm{K})\) and pressures \((\mathrm{atm})\) of a series of halogenated methanes are as follows: $$ \begin{array}{lcccc} \text { Compound } & \mathbf{C C l}_{3} \mathbf{F} & \mathbf{C C l}_{2} \mathbf{F}_{2} & \mathbf{C C I F}_{3} & \mathbf{C F}_{4} \\ \hline \text { Critical temperature } & 471 & 385 & 302 & 227 \\ \text { Critical pressure } & 43.5 & 40.6 & 38.2 & 37.0 \end{array} $$ (a) List the intermolecular forces that occur for each compound. (b) Predict the order of increasing intermolecular attraction, from least to most, for this series of compounds. (c) Predict the critical temperature and pressure for \(\mathrm{CCl}_{4}\) based on the trends in this table. Look up the experimentally determined critical temperatures and pressures for \(\mathrm{CCl}_{4}\), using a source such as the CRC Handbook of Chemistry and Physics, and suggest a reason for any discrepancies.

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?
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