Search for:. The PhET gas simulator may be used to examine the effect of temperature on molecular velocities. Key Concepts and Summary The kinetic molecular theory is a simple but very effective model that effectively explains ideal gas behavior.
Chemistry End of Chapter Exercises Using the postulates of the kinetic molecular theory, explain why a gas uniformly fills a container of any shape. Can the speed of a given molecule in a gas double at constant temperature?
Explain your answer. Describe what happens to the average kinetic energy of ideal gas molecules when the conditions are changed as follows: The pressure of the gas is increased by reducing the volume at constant temperature. The pressure of the gas is increased by increasing the temperature at constant volume. The average velocity of the molecules is increased by a factor of 2. The distribution of molecular velocities in a sample of helium is shown in Figure 9.
If the sample is cooled, will the distribution of velocities look more like that of H 2 or of H 2 O? What is the ratio of the average kinetic energy of a SO 2 molecule to that of an O 2 molecule in a mixture of two gases? What is the ratio of the root mean square speeds, u rms , of the two gases?
What effect do these changes have on the number of collisions of the molecules of the gas per unit area of the container wall? What is the effect on the average kinetic energy of the molecules? What is the effect on the root mean square speed of the molecules?
Answer the following questions: Is the pressure of the gas in the hot air balloon shown at the opening of this chapter greater than, less than, or equal to that of the atmosphere outside the balloon? Is the density of the gas in the hot air balloon shown at the opening of this chapter greater than, less than, or equal to that of the atmosphere outside the balloon?
What is the average molar mass of dry air? The average temperature of the gas in a hot air balloon is 1. Calculate its density, assuming the molar mass equals that of dry air. There is so much empty space in the container that each type of ball bearing hits the walls of the container as often in the mixture as it did when there was only one kind of ball bearing on the glass plate. The total number of collisions with the wall in this mixture is therefore equal to the sum of the collisions that would occur when each size of ball bearing is present by itself.
In other words, the total pressure of a mixture of gases is equal to the sum of the partial pressures of the individual gases. Graham's Laws of Diffusion and Effusion. A few of the physical properties of gases depend on the identity of the gas. One of these physical properties can be seen when the movement of gases is studied.
In Thomas Graham used an apparatus similar to the one shown in the figure below to study the diffusion of gases the rate at which two gases mix.
This apparatus consists of a glass tube sealed at one end with plaster that has holes large enough to allow a gas to enter or leave the tube. When the tube is filled with H 2 gas, the level of water in the tube slowly rises because the H 2 molecules inside the tube escape through the holes in the plaster more rapidly than the molecules in air can enter the tube.
By studying the rate at which the water level in this apparatus changed, Graham was able to obtain data on the rate at which different gases mixed with air. Graham found that the rates at which gases diffuse is inversely proportional to the square root of their densities. To understand the importance of this discovery we have to remember that equal volumes of different gases contain the same number of particles. As a result, the number of moles of gas per liter at a given temperature and pressure is constant, which means that the density of a gas is directly proportional to its molecular weight.
Graham's law of diffusion can therefore also be written as follows. Similar results were obtained when Graham studied the rate of effusion of a gas, which is the rate at which the gas escapes through a pinhole into a vacuum.
The rate of effusion of a gas is also inversely proportional to the square root of either the density or the molecular weight of the gas. Graham's law of effusion can be demonstrated with the apparatus in the figure below.
A thick-walled filter flask is evacuated with a vacuum pump. A syringe is filled with 25 mL of gas and the time required for the gas to escape through the syringe needle into the evacuated filter flask is measured with a stop watch. As we can see when data obtained in this experiment are graphed in the figure below, the time required for mL samples of different gases to escape into a vacuum is proportional to the square root of the molecular weight of the gas.
The rate at which the gases effuse is therefore inversely proportional to the square root of the molecular weight. Graham's observations about the rate at which gases diffuse mix or effuse escape through a pinhole suggest that relatively light gas particles such as H 2 molecules or He atoms move faster than relatively heavy gas particles such as CO 2 or SO 2 molecules.
Evaporation takes place at room temperature which is often well below the boiling point of the liquid. Evaporation happens from the surface of the liquid. As the temperature increases the rate of evaporation increases. Evaporation is also assisted by windy conditions which help to remove the vapour particles from the liquid so that more escape.
Evaporation is a complex idea for children for a number of reasons. The process involves the apparent disappearance of a liquid which makes the process difficult for them to understand. It is not easy to see the water particles in the air. Also, evaporation occurs in a number of quite differing situations - such as from a puddle or bowl of water where the amount of liquid obviously changes, to situations where the liquid is less obvious - such as clothes drying or even those where there is no obvious liquid at all to start with - such as bread drying out.
A further complication is that evaporation may be of a solvent from a solution e. These situations are quite different yet all involve evaporation. Evaporation may also involve liquids other than water e.
The particle model can be used to explain how it is possible to detect smells some distance away from the source. If a liquid is heated the particles are given more energy and move faster and faster expanding the liquid.
The most energetic particles at the surface escape from the surface of the liquid as a vapour as it gets warmer. Bring out the idea that atoms are so much smaller again. Look for other activities that can help reinforce the idea that particles are very, very small. Show students the conventional drawings of particles in solids, liquids and gases and ask them if and how fast they think they are moving.
For more information see: Conservation of mass. With a little encouragement, a class can usually work out by discussion that the particles in gases must be hitting the bottom of the flask harder than the top and hence that they are affected by gravity.
As particles cannot be directly observed, much of the teaching involves looking for apparent problems or inadequacies with the sorts of static pictures of particles given in earlier years. Encourage students to identify these and talk through possible explanations. Some prompts:. If needed, raise issues such as these, which will open up discussion, but it is better if the students themselves come up with some.
Note that many of the issues are to do with gases — it is their properties that we most need a particulate model to explain. To reinforce the notion of elastic collisions, ask what would happen if collisions between gas particles were not elastic.
What practical consequences would there be for people? This can be introduced by dropping different types of balls such as a soccer ball, a table tennis ball and a bouncy ball from toy shops and explaining that a bouncy ball behaves more like gas particles.
Using activities like POE Predict-Observe-Explain can help students think about and then question their existing ideas. The following activity will help students consider their ideas about the movement of particles.
Set up two pairs of flasks each connected by a valve see diagrams below. Both pairs have brown nitrogen dioxide in the left hand side flask. The first pair also has air in the right hand side flask. Students are asked to predict what will happen when the valve between the two flasks is opened. The brown colour will spread very slowly from one flask to the other because the particles have frequent collisions with the air particles.
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