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 41
(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?
Verified step by step guidance1
Step 1: Understand the term 'heat of fusion'. The heat of fusion refers to the amount of energy required to change a substance from the solid phase to the liquid phase at its melting point, without changing its temperature.
Step 2: Determine whether the heat of fusion is endothermic or exothermic. Consider that during the melting process, energy is absorbed by the substance to overcome the forces holding the solid together, indicating an endothermic process.
Step 3: Compare the heat of fusion to the heat of vaporization. The heat of vaporization is the energy required to change a substance from the liquid phase to the gas phase at its boiling point. This process generally requires more energy than melting because it involves completely breaking the intermolecular forces.
Step 4: Conclude that the heat of vaporization is generally larger than the heat of fusion. This is because vaporization requires overcoming all intermolecular forces to transition to a gas, whereas fusion only requires enough energy to allow molecules to move past each other in the liquid state.
Step 5: Summarize the concepts: (a) Heat of fusion involves the solid to liquid phase change, (b) it is an endothermic process, and (c) the heat of vaporization is typically larger than the heat of fusion due to the greater energy needed to transition to a gaseous state.
Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Heat of Fusion
The heat of fusion is the amount of energy required to change a substance from a solid to a liquid at its melting point. This phase change involves breaking the intermolecular forces that hold the solid structure together, allowing the molecules to move freely in the liquid state. It is a crucial concept in thermodynamics and is specific to each substance.
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Heat Capacity
Endothermic vs. Exothermic Processes
Endothermic processes absorb heat from the surroundings, resulting in a temperature decrease in the environment, while exothermic processes release heat, causing a temperature increase. The heat of fusion is an endothermic process because energy is absorbed to overcome the forces holding the solid together, allowing it to transition into a liquid.
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Heat of Vaporization
The heat of vaporization is the amount of energy required to convert a liquid into a gas at its boiling point. Generally, the heat of vaporization is larger than the heat of fusion for a given substance because it involves breaking all intermolecular forces to allow the molecules to escape into the gas phase, whereas fusion only requires overcoming some of these forces.
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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. (a) What does this observation tell us about the specific heat of C2H5Cl(g) as compared with that of C2H5Cl(l)?
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).