# 146 chapter 8 thermal properties and ideal gases 2

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CHAPTER 8. THERMAL PROPERTIES AND IDEAL GASES - GRADE 11 8.9 2. Forces of attraction do exist between molecules At low temperatures, when the speed of the molecules decreases and they move closer together, the intermolecular forces become more apparent. As the attraction between molecules increases, their movement decreases and there are fewer collisions between them. The pressure of the gas at low temperatures is therefore lower than what would have been expected for an ideal gas (figure 8.7. If the temperature is low enough or the pressure high enough, a real gas will liquify . ideal gas real gas Pressure Temperature Figure 8.7: Gases deviate from ideal gas behaviour at low temperatures 8.9 Summary The kinetic theory of matter helps to explain the behaviour of gases under different conditions. An ideal gas is one that obeys all the assumptions of the kinetic theory. A real gas behaves like an ideal gas, except at high pressures and low temperatures. Under these conditions, the forces between molecules become significant and the gas will liquify. Boyle’s law states that the pressure of a fixed quantity of gas is inversely proportional to its volume, as long as the temperature stays the same. In other words, pV = k or p 1 V 1 = p 2 V 2 . Charles’s law states that the volume of an enclosed sample of gas is directly proportional to its temperature, as long as the pressure stays the same. In other words, V 1 T 1 = V 2 T 2 The temperature of a fixed mass of gas is directly proportional to its pressure, if the volume is constant. In other words, p 1 T 1 = p 2 T 2 In the above equations, temperature must be written in Kelvin . Temperature in degrees Celsius (temperature = t) can be converted to temperature in Kelvin (temperature = T) using the following equation: T = t + 273 147
8.9 CHAPTER 8. THERMAL PROPERTIES AND IDEAL GASES - GRADE 11 Combining Boyle’s law and the relationship between the temperature and pressure of a gas, gives the general gas equation , which applies as long as the amount of gas remains constant. The general gas equation is pV = kT, or p 1 V 1 T 1 = p 2 V 2 T 2 Because the mass of gas is not always constant, another equation is needed for these situations. The ideal gas equation can be written as pV = nRT where n is the number of moles of gas and R is the universal gas constant, which is 8.3 J.K 1 .mol 1 . In this equation, SI units must be used. Volume (m 3 ), pressure (Pa) and temperature (K). The volume of one mole of gas under STP is 22.4 dm 3 . This is called the molar gas volume . s Exercise: Summary exercise 1. For each of the following, say whether the statement is true or false . If the statement is false, rewrite the statement correctly. (a) Real gases behave like ideal gases, except at low pressures and low tem- peratures.

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• Fall '10
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