7. The kinetic theory of gases 7.1. The kinetic theory and the gas laws The kinetic theor y explains the behaviour of gases and all the laws that were discovered in experiments. For the temperature to remain constant, the speed of the gas particles must remain constant. If the volume of the container is reduced , the particles reach the walls sooner. Therefore, the number of collisions per second increases and the pressure increases. When a gas is heated , its particles move faster and its temperature increases. For the pressure of the gas to remain constant, the volume of the container must increase, in order to maintain the rate at which the particles collide with the walls. When the gas cools, its particles move more slowly. For the pressure to remain constant, the volume of the container must decrease, in order to mantain the rate at which the particles collide with the walls. When a gas is heated , its particles move faster and its temperature increases. As the volume of the container remains constant, the particles will hit the walls sooner. The number of collisions per second and the pressure increase. If the gas cools down , the particles move more slowly and the pressure decreases. The par ticles of al l ideal gases behave as explained by the kinetic theor y. In containers of the same volume and t emperature, the pressure depends only on the number of par ticles, regardless of whether they are par ticles of one gas or another ( for example, hydrogen or carbon dioxide). p1 V1 p2 V2 T H E B O Y L E - M A R I O T T E L AW C H A R L E S ' L AW G AY - L U S S A C ' S L AW I D E A L G A S E Q U A T I O N O F S T A T E T = constant p1 × V1 = p2 × V2 p × V = n × R × T T2 p1 T1 p2 T2 p1 T1 p2 V = constant p T p T 1 1 2 2 = p = constant V T V T 1 1 2 2 = T1 T2 V1 V2 22 Explain these facts, taking into account the kinetic theory of gases: a) When we burn incense in a corner of a room, we eventually smell it in the whole room. b) In winter, car tyres must be inflated with air often. 23 We often use density tables of solid and liquid substances, but we never use data for gas density. Use the kinetic theory to explain why the density of gases depends so much on the conditions in which they are found. Starting with a gas that is in one state, think of two different ways to make its density increase. A C T I V I T I E S 22
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