Ch.11 - Liquids and Intermolecular Forces

Brown - Chemistry: The Central Science 14th Edition

Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232당신이 사용하는 게 아니라요?교과서 변경

Brown 14th Edition

Ch.11 - Liquids and Intermolecular Forces

문제 86

11장, 문제 86

The following data present the temperatures at which certain vapor pressures are achieved for dichloromethane (CH2Cl2) and methyl iodide (CH3I): (c) The order of volatility of these two substances changes as the temperature is increased. What quantity must be different for the two substances for this phenom- enon to occur?

검증된 단계별 안내

Step 1: Understand the concept of volatility. Volatility is the ability of a substance to vaporize. It is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with lower vapor pressure.

Step 2: Recall that the vapor pressure of a substance depends on the temperature and the heat of vaporization (the energy required to convert a given quantity of a substance from a liquid into a gas at a given pressure). This relationship is given by the Clausius-Clapeyron equation: ln(P2/P1) = -ΔHvap/R (1/T2 - 1/T1), where P1 and P2 are the vapor pressures at temperatures T1 and T2 respectively, ΔHvap is the heat of vaporization, and R is the gas constant.

Step 3: Understand that if the order of volatility of two substances changes as the temperature is increased, it means that the temperature dependence of their vapor pressures is different. This can only happen if their heats of vaporization (ΔHvap) are different.

Step 4: Therefore, for the order of volatility of dichloromethane (CH2Cl2) and methyl iodide (CH3I) to change as the temperature is increased, their heats of vaporization must be different.

Step 5: In conclusion, the quantity that must be different for the two substances for this phenomenon to occur is the heat of vaporization.

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주요 개념

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Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase at a given temperature. It reflects the tendency of molecules to escape from the liquid phase into the gas phase. As temperature increases, the kinetic energy of the molecules also increases, leading to higher vapor pressures. Understanding vapor pressure is crucial for analyzing the volatility of substances.

Volatility

Volatility refers to the tendency of a substance to vaporize. A more volatile substance has a higher vapor pressure at a given temperature, meaning it evaporates more readily. The order of volatility between substances can change with temperature due to differences in intermolecular forces, such as hydrogen bonding or van der Waals forces, which affect how easily molecules can escape into the vapor phase.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. These forces, including hydrogen bonds, dipole-dipole interactions, and London dispersion forces, significantly influence the physical properties of substances, including boiling points and vapor pressures. For the order of volatility to change with temperature, the strength of these intermolecular forces must differ between the two substances, affecting their vapor pressures at elevated temperatures.

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(b) A flask of water is connected to a vacuum pump. A few moments after the pump is turned on, the water begins to boil. After a few minutes, the water begins to freeze. Explain why these processes occur.

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교과서 질문

Naphthalene (C₁₀H₈) is the main ingredient in traditional mothballs. Its normal melting point is 81 °C, its normal boiling point is 218 °C, and its triple point is 80 °C at 1000 Pa. Using the data, construct a phase diagram for naphthalene, labeling all the regions of your diagram.

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교과서 질문

Suppose the vapor pressure of a substance is measured at two different temperatures. (a) By using the Clausius–Clapeyron equation (Equation 11.1) derive the following relationship between the vapor pressures, P1 and P2, and the absolute temperatures at which they were measured, T1 and T2: (b) Gasoline is a mixture of hydrocarbons, a component of which is octane (CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃). Octane has a vapor pressure of 13.95 torr at 25 °C and a vapor pressure of 144.78 torr at 75 °C. Use these data and the equation in part (a) to calculate the heat of vaporization of octane. (c) By using the equation in part (a) and the data given in part (b), calculate the normal boiling point of octane. Compare your answer to the one you obtained from Exercise 11.81. (d) Calculate the vapor pressure of octane at - 30 °C.

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교과서 질문

The following table gives the vapor pressure of hexafluorobenzene (C₆F₆) as a function of temperature: (a) By plotting these data in a suitable fashion, determine whether the Clausius–Clapeyron equation (Equation 11.1) is obeyed. If it is obeyed, use your plot to determine ∆H_vap for C₆F₆.

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교과서 질문

A particular liquid crystalline substance has the phase diagram shown in the figure. By analogy with the phase diagram for a nonliquid crystalline substance, identify the phase present in each area.

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