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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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