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Chapter Five Gases

. Gases are the simplest physical state to consider (rather than solid or liquid) because of the absence of intermolecular forces.Gas molecules move freely, randomly and don't interact significantly with their neighbors.. Physical Characteristics of Gases. See Table 5.1 p. 175 and Figure 5.1 p. 174

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Chapter Five Gases

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    1. Chapter Five –Gases

    2. Gases are the simplest physical state to consider (rather than solid or liquid) because of the absence of intermolecular forces. Gas molecules move freely, randomly and don’t interact significantly with their neighbors.

    3. Physical Characteristics of Gases See Table 5.1 p. 175 and Figure 5.1 p. 174. At room temperature, most non-ionic molecular substances are either gases or easily vaporized liquids (vaporization is the conversion to a gas when a substance isn’t a gas at normal atmospheric conditions). These molecules have weak intermolecular interactions and are therefore gases at room temperature.

    4. Physical Characteristics of Gases Gases occupy entire volume of their container, they take the shape of their container. Gases can be compressed by applying pressure. This can change the shape and volume of their container. The densities of gases are lower than that of liquids and solids and are expressed as g/L rather than g/ml or g/cm3.

    5. Physical Characteristics of Gases If gases don’t react with each other, they will mix evenly and completely in a container.

    6. Kinetic-Molecular Theory Springs from observations of these physical properties and is an attempt to explain what is going on at the microscopic level (in an ideal gas). Kinetic energy is the energy of motion. Molecular applies to molecules or atoms.

    7. Kinetic-Molecular Theory Basic Assumptions Gases are composed of molecules whose size is negligible compared with the average distance between them. We ignore the volume occupied by the molecules and “V” represents the volume of the container. Because they are widely spaced in their container, they lack intermolecular interactions.

    8. Kinetic-Molecular Theory Basic Assumptions Molecules are in constant, random straight-path motion. They collide with each other and the walls of their container. These collisions are “elastic” (no net loss of kinetic energy upon collision). The forces of attraction or repulsion between 2 molecules (intermolecular forces) are very weak or negligible except when molecules collide.

    9. Kinetic-Molecular Theory Basic Assumptions The average kinetic energy of a molecule is proportional to the temperature in kelvin (absolute). Therefore two gases at the same T (K) will have the same average kinetic energy.

    10. Kinetic-Molecular Theory KEY POINTS The higher the temperature (kelvin scale) measured, the faster the average movement of gas molecules. This random, molecular through space motion is called thermal motion. Pressure (P) is measuring the “pushes” of molecules against their container. P is proportional to the frequency of collisions and the average force of these collisions.

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