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Chapter 12: Problem 11

Silicone oils, such as \(\mathrm{H}_{3} \mathrm{C}\left[\mathrm{SiO}\left(\mathrm{CH}_{3}\right)_{2}\right]_{\mathrm{n}} \mathrm{Si}\left(\mathrm{CH}_{3}\right),\) are used in water repellents for treating tents, hiking boots, and similar items. Explain how silicone oils function.

Short Answer

Expert verified
Silicone oils serve as water-repellents because of their largely non-polar nature due to their chemical structure: they have Si-O bonds and methyl (CH3) groups. The non-polarity and hence hydrophobic properties of the methyl groups make silicone oils repel water. When applied to a surface, these oils create a barrier that prevents water from penetrating or soaking the material, causing it to roll off instead.

Step by step solution

01

Understand Silicone Oil Structure

The silicone oil mentioned in the exercise, \(H_{3}C[SiO(CH_{3})_{2}]_{n}Si(CH_{3})\), is a polymer composed of repeating units of silicon atoms bound to oxygen atoms (forming Si-O bonds), with each silicon atom also bound to two methyl groups. This gives silicone oils a unique structure that contributes to its water-repelling properties.
02

Identify Hydrophobic Properties

Methyl groups (CH3) are hydrocarbon groups, which are non-polar and hydrophobic (water-repelling). The presence of the methyl groups in the silicone oil makes the oil largely non-polar, and hence water-repellent, since water is a polar molecule and tends to interact poorly with non-polar substances. This is the main reason silicone oils are used as water repellents.
03

Recall the 'Like Dissolves Like' Principle

According to this general principle in chemistry, substances tend to mix or miscible with other substances that have similar polarity. As a polar molecule, water tends to mix with other polar substances but not with non-polar ones. Therefore, when a surface, like a tent or a hiking boot, is treated with a silicone oil, water would not mix with the oil but would rather roll off the oil-coated surface.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Water Repellents
Water repellents are materials or coatings applied to surfaces to make them resistant to water. They are often used on items that need protection from moisture, like tents and hiking boots. These repellents create a barrier that prevents water from soaking into the material. By providing a water-repellent layer, they help protect against water damage, mold, and rot.
Silicone oils are a popular choice for water-repellent products because their unique molecular structure naturally resists interactions with water. This property is crucial for keeping surfaces dry and protected in various weather conditions.
Polymer Structure
The polymer structure of silicone oils plays a key role in their functionality as water repellents. Polymers are large molecules composed of repeating smaller units, known as monomers. In silicone oils, the polymer chain is predominantly made up of silicon and oxygen atoms, with two methyl groups attached to each silicon.
  • Si-O Polymer Chains: The silicon-oxygen backbone, represented as dashed lines, provides the structural framework.
  • Methyl Groups: The presence of methyl ( CH extsubscript{3} ) groups attached to the silicon atoms makes the surface predominantly non-polar.
Together, these elements construct a flexible yet robust structure that deflects water, enhancing the surface's water-repelling ability.
Hydrophobic Properties
The hydrophobic properties of silicone oils stem from their molecular structure, particularly the non-polar methyl groups. Hydrophobic literally means "water-fearing," and these properties in silicone oils make them ideal candidates for water repellency. Here's why:
  • Non-Polar Nature: The non-polar bonds within methyl groups lack the partial charges needed to interact with polar water molecules. This prevents water from sticking to or being absorbed by the surface.
  • Water Beading: On treated surfaces, water tends to form droplets that "bead" up, rolling off easily without soaking into the material.
These hydrophobic interactions ensure that when silicone oils are applied to a surface, they create a protective shield that significantly reduces water penetration.

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Most popular questions from this chapter

Are the fullerenes network covalent solids? What makes them different from diamond and graphite? It has been shown that carbon can form chains in which every other carbon atom is bonded to the next carbon atom by a triple bond. Is this allotrope of carbon a network covalent solid? Explain.

The enthalpy of vaporization of benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}(\mathrm{l}),\) is \(33.9 \mathrm{kJmol}^{-1}\) at \(298 \mathrm{K}\). How many liters of \(\mathrm{C}_{6} \mathrm{H}_{6}(\mathrm{g})\) measured at \(298 \mathrm{K}\) and \(95.1 \mathrm{mmHg}\), are formed when \(1.54 \mathrm{kJ}\) of heat is absorbed by \(\mathrm{C}_{6} \mathrm{H}_{6}(1)\) at a constant f 298 K?

We have learned that the enthalpy of vaporization of a liquid is generally a function of temperature. If we wish to take this temperature variation into account, we cannot use the Clausius-Clapeyron equation in the form given in the text (that is, equation 12.2 ). Instead, we must go back to the differential equation upon which the Clausius-Clapeyron equation is based and reintegrate it into a new expression. Our starting point is the following equation describing the rate of change of vapor pressure with temperature in terms of the enthalpy of vaporization, the difference in molar volumes of the vapor \(\left(V_{g}\right),\) and liquid \(\left(V_{1}\right),\) and the temperature. $$\frac{d P}{d T}=\frac{\Delta H_{\mathrm{vap}}}{T\left(V_{\mathrm{g}}-V_{1}\right)}$$ Because in most cases the volume of one mole of vapor greatly exceeds the molar volume of liquid, we can treat the \(V_{1}\) term as if it were zero. Also, unless the vapor pressure is unusually high, we can treat the vapor as if it were an ideal gas; that is, for one mole of vapor, \(P V=R T\). Make appropriate substitutions into the above expression, and separate the \(P\) and \(d P\) terms from the \(T\) and \(d T\) terms. The appropriate substitution for \(\Delta H_{\text {vap }}\) means expressing it as a function of temperature. Finally, integrate the two sides of the equation between the limits \(P_{1}\) and \(P_{2}\) on one side and \(T_{1}\) and \(T_{2}\) on the other. (a) Derive an equation for the vapor pressure of \(\mathrm{C}_{2} \mathrm{H}_{4}(\mathrm{l})\) as a function of temperature, if \(\Delta H_{\mathrm{vap}}=\) \(15,971+14.55 T-0.160 T^{2}\left(\text { in } J m o l^{-1}\right)\) (b) Use the equation derived in (a), together with the fact that the vapor pressure of \(\mathrm{C}_{2} \mathrm{H}_{4}(1)\) at \(120 \mathrm{K}\) is 10.16 Torr, to determine the normal boiling point of ethylene.

Because solid \(p\) -dichlorobenzene, \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2},\) sublimes rather easily, it has been used as a moth repellent. From the data given, estimate the sublimation pressure of \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2}(\mathrm{s})\) at \(25^{\circ} \mathrm{C} .\) For \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2} ; \mathrm{mp}=\) \(53.1^{\circ} \mathrm{C} ;\) vapor pressure of \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Cl}_{2}(1)\) at \(54.8^{\circ} \mathrm{C}\) is \(10.0 \mathrm{mmHg} ; \Delta H_{\text {fus }}=17.88 \mathrm{kJ} \mathrm{mol}^{-1} ; \Delta H_{\text {vap }}=\) \(72.22 \mathrm{k}] \mathrm{mol}^{-1}\)

For each of the following substances describe the importance of dispersion (London) forces, dipoledipole interactions, and hydrogen bonding: (a) \(HCl;\) (b) \(\mathrm{Br}_{2} ;\) (c) ICl; (d) \(\mathrm{HF} ;\)\ (e) \(\mathrm{CH}_{4}\)
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