Textbook Question
Name the phase transition in each of the following situations and indicate whether it is exothermic or endothermic: (c) Rubbing alcohol in an open container slowly disappears.
Ch.11 - Liquids and Intermolecular Forces
Chapter 11, Problem 42a
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)?
Verified step by step guidance1
Step 1: Understand the concept of specific heat. Specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The specific heat of a substance can vary depending on whether it is in a solid, liquid, or gaseous state.
Step 2: Analyze the given information. Ethyl chloride (C2H5Cl) boils at 12 °C, which means it changes from a liquid to a gas at this temperature. When liquid C2H5Cl under pressure is sprayed on a room-temperature (25 °C) surface in air, the surface is cooled considerably.
Step 3: Understand the process of evaporation. When a liquid evaporates, it absorbs heat from its surroundings, causing a cooling effect. This is because the molecules with the highest kinetic energy are the ones most likely to escape from the liquid and become a gas, leaving the remaining liquid cooler.
Step 4: Compare the specific heats. The observation that the surface is cooled considerably when liquid C2H5Cl is sprayed on it suggests that the specific heat of C2H5Cl(g) is higher than that of C2H5Cl(l). This is because more heat is absorbed from the surroundings during the phase change from liquid to gas, indicating a higher specific heat in the gaseous state.
Step 5: Conclude the comparison. Therefore, the specific heat of gaseous ethyl chloride (C2H5Cl(g)) is higher than that of liquid ethyl chloride (C2H5Cl(l)).
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Phase Change and Boiling Point
The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. Ethyl chloride has a boiling point of 12 °C, meaning it will vaporize at temperatures above this point. When sprayed on a warmer surface, the liquid ethyl chloride evaporates, absorbing heat from the surface, which leads to cooling. Understanding phase changes is crucial for analyzing thermal interactions in this scenario.
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Specific Heat Capacity
Specific heat capacity is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius. The observation that the surface cools when ethyl chloride is sprayed suggests that the gas phase (C2H5Cl(g)) has a lower specific heat capacity compared to the liquid phase (C2H5Cl(l)). This indicates that the gas can absorb heat more efficiently during the phase change, leading to a greater cooling effect.
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Latent Heat of Vaporization
Latent heat of vaporization is the amount of energy required to convert a unit mass of a substance from liquid to gas at constant temperature. When ethyl chloride evaporates on the surface, it absorbs a significant amount of heat from the surroundings, which is related to its latent heat. This energy absorption during the phase change contributes to the cooling effect observed, highlighting the importance of latent heat in thermal processes.
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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.
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. (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?
Textbook Question
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).