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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?

Short Answer

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(a) The phase change represented by the "heat of fusion" is the transition of a substance from the solid phase to the liquid phase at its melting point, keeping the temperature constant. (b) The heat of fusion is an endothermic process, as it requires the addition of heat energy for the phase change to occur. (c) Generally, a substance's heat of vaporization is larger than its heat of fusion, as more energy is needed to overcome the intermolecular forces in the transition from liquid to gaseous phase than from solid to liquid phase.

Step by step solution

01

Define heat of fusion

The heat of fusion is the amount of heat energy required to change a substance from the solid phase to the liquid phase at its melting point, while keeping the temperature constant.
02

Determine if the heat of fusion is endothermic or exothermic

The heat of fusion is an endothermic process because it requires the addition of heat energy to the substance in order to change its phase from solid to liquid. As heat energy is absorbed by the substance, its molecules gain energy and break the bonds holding them together in the solid phase, allowing them to move more freely and form the liquid phase.
03

Compare heat of fusion to heat of vaporization

The heat of vaporization is the amount of heat energy required to change a substance from the liquid phase to the gaseous phase at its boiling point, while keeping the temperature constant. In general, the heat of vaporization is larger than the heat of fusion for a substance. This is because it requires more energy to overcome the intermolecular forces between the molecules and allow the molecules to move completely unrestricted in the gaseous phase, compared to the energy needed to overcome the forces holding the molecules together in the solid phase and allow them to move more freely in the liquid phase.

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Key Concepts

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

Phase Change
When substances undergo a phase change, they transition from one state of matter to another. These changes include transitioning between solid, liquid, and gas phases. For example:
  • Melting: solid to liquid
  • Freezing: liquid to solid
  • Vaporization: liquid to gas
  • Condensation: gas to liquid
  • Sublimation: solid to gas
  • Deposition: gas to solid
Each phase change involves a transfer of energy, often as heat. During a phase change, the temperature of the substance remains constant, even though heat energy is being added or removed. This energy is used to change the state, not the temperature. For instance, when ice melts to water, it absorbs heat, yet its temperature stays at 0掳C until the phase change is complete.
Endothermic Process
An endothermic process is one in which a system absorbs energy from its surroundings in the form of heat. This is common in phase changes where energy input is necessary, such as melting and vaporization. During these processes:
  • Molecules gain energy.
  • Intermolecular forces are overcome.
  • Heat is absorbed from the surroundings.
Take melting, for example. Ice absorbs heat energy so that its molecules can move more freely, transitioning into the liquid phase. Similarly, during boiling, heat is absorbed to allow the liquid to vaporize into gas. All these are endothermic processes and are characterized by a positive heat change. This absorbed heat allows substances to change phase without increasing in temperature until the change is complete.
Heat of Vaporization
The heat of vaporization is the energy required to convert a liquid into a gas at its boiling point, while the temperature remains constant. This amount of energy is typically larger than the heat of fusion. The reasons for this include:
  • Breaking all intermolecular forces in a liquid.
  • Allowing molecules to move freely in a gaseous state.
  • Significant energy requirement compared to melting (solid to liquid).
In essence, while melting only needs to partially disrupt molecular bonds, vaporization requires total disruption for molecules to escape into the gas phase. For example, water requires roughly 40.7 kJ/mol for vaporization compared to about 6.01 kJ/mol for melting. These values highlight the substantial energy difference when transitioning from liquid to gas.

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Most popular questions from this chapter

You are high up in the mountains and boil water to make some tea. However, when you drink your tea, it is not as hot as it should be. You try again and again, but the water is just not hot enough to make a hot cup of tea. Which is the best explanation for this result? (a) High in the mountains, it is probably very dry, and so the water is rapidly evaporating from your cup and cooling it. (b) High in the mountains, it is probably very windy, and so the water is rapidly evaporating from your cup and cooling it. (c) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much lower than at sea level. (d) High in the mountains, the air pressure is significantly less than 1 atm, so the boiling point of water is much higher than at sea level.

At room temperature, Si is a solid, \(\mathrm{CCl}_{4}\) is a liquid, and Ar is gas. List these substances in order of (a) increasing intermolecular energy of attraction and (b) increasing boiling point.

Which member in each pair has the stronger intermolecular dispersion forces? (a) Br_ or \(\mathrm{O}_{2},\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH}\) or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{SH},(\mathbf{c}) \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Clor}\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CHCl}\)

(a) What is the relationship between surface tension and temperature? (b) What is the relationship between viscosity and temperature? (c) Why do substances with high surface tension also tend to have high viscosities?

Liquid butane \(\left(\mathrm{C}_{4} \mathrm{H}_{10}\right)\) is stored in cylinders to be used as a fuel. The normal boiling point of butane is listed as \(-0.5^{\circ} \mathrm{C}\) . (a) Suppose the tank is standing in the sun and reaches a temperature of \(35^{\circ} \mathrm{C}\) . Would you expect the pressure in the tank to be greater or less than atmospheric pressure? How does the pressure within the tank depend on how much liquid butane is in it? (b) Suppose the valve to the tank is opened and a few liters of butane are allowed to escape rapidly. What do you expect would happen to the temperature of the remaining liquid butane in the tank? Explain. (c) How much heat must be added to vaporize 250 \(\mathrm{g}\) of butane if its heat of vaporization is 21.3 \(\mathrm{kJ} / \mathrm{mol}\) ? What volume does this much butane occupy at 755 torr and \(35^{\circ} \mathrm{C} ?\)
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