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Chapter 7: Problem 17

What does the kinetic energy of molecules have to do with changing phases?

Short Answer

Expert verified
The kinetic energy of molecules is directly related to phase changes, as it influences the intermolecular forces holding them together. As molecules gain or lose kinetic energy due to heat or other energy sources, they change their behavior, leading to phase transitions like melting, freezing, evaporation, and condensation. In solids, molecules have low kinetic energy and are held tightly together. In liquids, molecules have more kinetic energy, allowing them to move more freely but still remain close. In gases, molecules have the highest kinetic energy and are widely spaced, moving rapidly.

Step by step solution

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1. Definition of Kinetic Energy and Phases

Kinetic energy is the energy of motion, and it plays a crucial role in determining the state or phase of a substance (solid, liquid, or gas). The phases of a substance are primarily determined by the intermolecular forces (forces between molecules) and the amount of kinetic energy the molecules possess.
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2. Kinetic Energy and Phase Transitions

During phase transitions, the kinetic energy of molecules changes as energy is added to or removed from the system. For instance, when a solid is heated, it absorbs energy, and its molecules start to vibrate more vigorously. This increases their kinetic energy, which weakens the intermolecular forces, causing the substance to transition from a solid to a liquid (melting).
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3. Kinetic Energy in Solids

In the solid phase, molecules are closely packed, and their kinetic energy is relatively low. The molecules in the solid phase vibrate in a fixed position within a lattice structure, but they do not have enough energy to overcome the attractive intermolecular forces holding them in place.
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4. Kinetic Energy in Liquids

When a solid melts into a liquid, its molecules acquire more kinetic energy (due to heat or other external energy source). This additional energy allows them to break free from their fixed positions and move more freely but still stay close to each other. The intermolecular forces are weaker in liquids than in solids, resulting in a less structured arrangement of molecules.
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5. Kinetic Energy in Gases

As heat or energy is continually added to a liquid, its molecules gain even more kinetic energy and eventually have enough energy to overcome intermolecular forces altogether. At this point, the substance transitions from the liquid phase to the gas phase. In the gaseous state, molecules are widely spaced and move rapidly, showing the highest kinetic energy among the three phases.
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6. Conclusion

The kinetic energy of molecules is directly related to phase changes. As molecules gain or lose kinetic energy, they change their behavior, which in turn influences the intermolecular forces holding them together. This variation in kinetic energy and intermolecular forces is the primary cause of phase transitions like melting, freezing, evaporation, and condensation.

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Key Concepts

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

Intermolecular Forces
Intermolecular forces are the forces of attraction and repulsion between molecules. They play a critical role in determining the physical properties of substances and their behavior during phase transitions. These forces can vary in strength, affecting the state of matter of a substance.
  • Van der Waals Forces: These are the weakest intermolecular forces, including dispersion forces and dipole-dipole interactions. They influence the state of gases and liquids primarily.

  • Hydrogen Bonds: Stronger than Van der Waals forces, hydrogen bonds occur when a hydrogen atom is attracted to electronegative atoms like oxygen or nitrogen. They are significant in water and alcohols, affecting their boiling and melting points.

  • Ionic and Covalent Bonds: While not typically classified as intermolecular forces, ionic and covalent bonds within structures can influence how molecules interact at a larger scale, especially in solid phases.

These forces are what molecules need to overcome during phase transitions. For example, when a solid melts, its molecules need to overcome the intermolecular forces that hold them in a fixed position.
Phase Transition
A phase transition occurs when a substance changes from one state of matter to another, like from solid to liquid or liquid to gas. These transitions happen due to changes in kinetic energy, leading to variations in intermolecular forces.
During a phase transition:
  • Molecules gain or lose energy, often in the form of heat, which alters their kinetic energy levels.

  • This change in kinetic energy affects the intermolecular forces, making it possible for molecules to change how they interact.

  • For instance, when ice melts, it absorbs heat from the surroundings. This energy increases the kinetic energy of the ice molecules, weakening the intermolecular forces, and allowing the molecules to shift from a solid to a liquid state.

Phase transitions are crucial in nature and technology, influencing everything from weather systems to the creation of new materials. Recognizing how kinetic energy drives these transitions enhances our understanding of phenomena like melting, vaporizing, and sublimating.
States of Matter
The states of matter are primarily characterized as solids, liquids, and gases. Each state has distinct particle arrangements and energy levels.
  • Solids: Molecules are tightly packed in a fixed structure. They have low kinetic energy and strong intermolecular forces, maintaining a definite shape and volume.

  • Liquids: Molecules are less tightly packed than in solids. They have higher kinetic energy, allowing molecules to move more freely, which leads to a definite volume but an indefinite shape.

  • Gases: Molecules have high kinetic energy, enough to completely overcome intermolecular forces. They are widely spaced apart, resulting in no definite shape or volume.

The transition between these states depends on both the kinetic energy and the intermolecular forces present. For example, adding heat to a solid provides energy to the molecules, thereby increasing their kinetic energy and eventually transforming the solid into a liquid.

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Most popular questions from this chapter

Which of the molecules below would not form hydrogen bonds? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}\) (d) \(\mathrm{NH}_{3}\) (e) None of these would form hydrogen bonds.

Which of the following would you expect to have the highest boiling point? Justify your choice. (a) Propane, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (b) Carbon dioxide, \(\mathrm{CO}_{2}\) (c) Ethyl alcohol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (d) Methyl fluoride, \(\mathrm{CH}_{3} \mathrm{~F}\)

For the hydrogen halides, the order of boiling points is \(\mathrm{HF}>\mathrm{HI}>\mathrm{HBr}>\mathrm{HCl}\). (a) Why does HF have the highest boiling point? (b) Why is the boiling point of HI greater than that of \(\mathrm{HBr}\) and \(\mathrm{HCl}\) ?

What do we mean by a "lattice of ions"?

In general, nonmolecular solids have much higher melting points than molecular solids. Why is this so?
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