Ch.11 - Liquids and Intermolecular Forces

Problem 44b

Chapter 11, Problem 44b

Ethyl chloride (C₂H₅Cl) boils at 12 °C. When liquid C₂H₅Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably. (b) Assume that the heat lost by the surface is gained by ethyl chloride. What enthalpies must you consider if you were to calculate the final temperature of the surface?

Verified step by step guidance

Step 1: Identify the processes involved. In this case, we have the boiling of ethyl chloride (C2H5Cl) and the cooling of the surface.

Step 2: Recognize that the heat lost by the surface is gained by the ethyl chloride. This is an application of the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred or transformed.

Step 3: Understand that the enthalpy of vaporization of ethyl chloride is involved. This is the energy required to convert a substance from a liquid to a gas at its boiling point. It is absorbed by the ethyl chloride as it vaporizes.

Step 4: Consider the enthalpy of cooling of the surface. This is the energy released by the surface as it cools down. It is transferred to the ethyl chloride.

Step 5: Note that the final temperature of the surface will be determined by the balance between these two enthalpies. If the enthalpy of vaporization of ethyl chloride is greater than the enthalpy of cooling of the surface, the surface will cool down. If it's less, the surface will heat up.

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Key Concepts

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

Enthalpy

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is defined as the sum of the internal energy and the product of pressure and volume. In chemical processes, changes in enthalpy (ΔH) indicate the heat absorbed or released during reactions or phase changes, which is crucial for understanding heat transfer in the context of ethyl chloride cooling the surface.

Heat Transfer

Heat transfer refers to the movement of thermal energy from one object or substance to another due to a temperature difference. In this scenario, the heat lost by the warmer surface is transferred to the ethyl chloride, causing the surface to cool. Understanding the mechanisms of heat transfer—conduction, convection, and radiation—is essential for calculating the final temperature of the surface after contact with the ethyl chloride.

Phase Change

Phase change is the transition of a substance from one state of matter to another, such as from liquid to gas. In the case of ethyl chloride, when it is sprayed, it may evaporate, absorbing heat from the surface. The enthalpy of vaporization, which is the heat required to convert a unit mass of a liquid into vapor without a change in temperature, plays a critical role in determining how much heat is absorbed and how it affects the final temperature of the surface.

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Related Practice

Textbook Question

Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (d) Molten lava from a volcano turns into solid rock.

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Open Question

(a) What phase change is represented by the "heat of fusion" of a substance? (b) Is the heat of fusion endothermic or exothermic? (c) If you compare a substance’s heat of fusion to its heat of vaporization, which one is generally larger?

Textbook Question

Ethyl chloride (C₂H₅Cl) boils at 12 °C. When liquid C₂H₅Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably. (a) What does this observation tell us about the specific heat of C₂H₅Cl(g) as compared with that of C₂H₅Cl(l)?

For many years drinking water has been cooled in hot climates by evaporating it from the surfaces of canvas bags or porous clay pots. How many grams of water can be cooled from 35 to 20 °C by the evaporation of 60 g of water? (The heat of vaporization of water in this temperature range is 2.4 kJ/g. The specific heat of water is 4.18 J/g-K).

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Open Question

Compounds like CCl2F2 are known as chlorofluorocarbons, or CFCs. These compounds were once widely used as refrigerants but are now being replaced by compounds that are believed to be less harmful to the environment. The heat of vaporization of CCl2F2 is 289 J/g. What mass of this substance must evaporate to freeze 200 g of water initially at 15 °C? (The heat of fusion of water is 334 J/g; the specific heat of water is 4.18 J/g-K.)

Open Question

Ethanol (C2H5OH) melts at -114 °C and boils at 78 °C. The enthalpy of fusion of ethanol is 5.02 kJ/mol, and its enthalpy of vaporization is 38.56 kJ/mol. The specific heats of solid and liquid ethanol are 0.97 and 2.3 J/g-K, respectively. (a) How much heat is required to convert 42.0 g of ethanol at 35 °C to the vapor phase at 78 °C? (b) How much heat is required to convert the same amount of ethanol at -155 °C to the vapor phase at 78 °C?