Atoms and atomic theory
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5.2 Reading: Gases consist of tiny particles widely spaced. Under typical conditions, the average distance between gas particles is about ten times their diameter. Because of these large distances, the volume occupied by the particles themselves is very small compared to the volume of the empty space around them. For a gas at room temperature and pressure, the gas particles themselves occupy about 0.1% of the total volume. The other 99.9% of the total volume is English in chemistry 1 – 2017 TS. Nguyễn Tuyết Phương – TS. Trần Thu Phương 15 empty space (whereas in liquids and solids, about 70% of the volume is occupied by particles). Because of the large distances between gas particles, the attractions or repulsions among them are weak. The particles in a gas are in rapid and continuous motion. For example, the average velocity of nitrogen molecules, N 2 , at 20 °C is about 500 m/s. As the temperature of a gas increases, the particles’ velocity increases. The average velocity of nitrogen molecules at 100 °C is about 575 m/s. The particles in a gas are constantly colliding with the walls of the container and with each other. There are assumed to be no forces between molecules except very briefly during collisions. That is, each molecule acts independently of all the others and is unaffected by their presence, except during collisions. Because of these collisions, the gas particles are constantly changing their direction of motion and their velocity. Individual molecules may gain or lose energy as a result of collisions. In a collection of molecules at constant temperature, however, the total energy remains constant. The model described above applies to real gases, but chemists often simplify the model further by imagining the behaviour of an ideal gas. An ideal gas differs from a real gas in that - The particles are assumed to be point masses, that is, particles that have a mass but occupy no volume. - There are no attractive or repulsive forces at all between the particles. When we add these assumptions to our model for gases, we call it the ideal gas model. As the name implies, the ideal gas model describes an “ideal” of gas behaviour that is only approximated by reality. Nevertheless, the model succeeds in explaining and predicting the behaviour of typical gases under typical conditions. In fact, some actual gases do behave very much in accordance with the model, and scientists may call them ideal gases. The ideal gas assumptions make it easier for chemists to describe the relationships between the properties of gases and allow us to calculate values for these properties. tải về 0.52 Mb. Chia sẻ với bạn bè của bạn:
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