Gases and Their Properties

Gases consist of tiny particles widely spaced (Figute 13.1). Undet typical conditions, the avetage distance between gas particles is about ten times theit diametet. Because of these latge distances, the volume occupied by the particles themselves is vety small Objective 2 compated to the volume of the empty space around them. Fot a gas at room tempetatute 

The particles in a gas are in rapid and continuous motion. For example, the average velocity of nitrogen molecules, N2, 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 rn/s. 

The particles in a gas are constantly colliding with the walls of the container and with each other. Because of these collisions, the gas particles are constantly changing their direction of motion and their velocity. In a typical situation, a gas particle moves Objective 3 a very short distance between collisions. For example, oxygen, O2, molecules at normal temperatures and pressures move an average of 10 m between collisions. 

The model described above applies to real gases, but chemists often simplify the model ftxrther by imagining the behavior of an ideal gas. An ideal gas differs from a real gas in that 

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 behavior that is only approximated by reality. Nevertheless, the model succeeds in explaining and predicting the behavior 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. 

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Domain 3 Kinetic Molecular Theory, Competency 3.1 Gases and Their Properties... 

The experiment involves the decomposition of hydrogen peroxide (H2O2) in the presence of a catalyst (an added molecule that speeds the rate of a given reaction), a surfactant (a dishwashing detergent), and a colorant, carried out in a 2L plastic bottle. The catalyst, potassium iodide (KI), speeds up the decomposition reaction, and the evolved O2 gas is captured by the surfactant, which in turn, foams like toothpaste. The foam shoots out of the bottle, and the heat released by the reaction causes the bottle to shrink. The experiment can be used to demonstrate the energy aspect of reactions, and tell about endothermic and exothermic reactions, about gases and their properties, and about Lavoisier and the other discoverers of the O2 molecule. 

Nowadays, much of our interest in air is actually driven by concerns over the levels and effects of various pollutants. The U.S. Environmental Protection Agency (EPA) defines pollutants as unwanted chemicals or other materials found in the air. Although this definition does include a variety of small solid particles such as dust or soot, most air pollutants are gases. Thus an exploration of some of the issues surrounding the monitoring and control of air pollution will form the backdrop for our study of gases and their properties. 

Andersson, B. Bach, F. (1996). Developing new teaching sequences in science the example of "Gases and their properties". In G. Welford, J. Osborne P. Scott (Eds.), Research in science education in Europe current issues and themes (pp.7-21). London The Falmer Press. 

It follows from this discussion that all of the transport properties can be derived in principle from the simple kinetic dreoty of gases, and their interrelationship tlu ough k and c leads one to expect that they are all characterized by a relatively small temperature coefficient. The simple theory suggests tlrat this should be a dependence on 7 /, but because of intermolecular forces, the experimental results usually indicate a larger temperature dependence even up to for the case of molecular inter-diffusion. The Anhenius equation which would involve an enthalpy of activation does not apply because no activated state is involved in the transport processes. If, however, the temperature dependence of these processes is fitted to such an expression as an algebraic approximation, tlren an activation enthalpy of a few kilojoules is observed. It will thus be found that when tire kinetics of a gas-solid or liquid reaction depends upon the transport properties of the gas phase, the apparent activation entlralpy will be a few kilojoules only (less than 50 kJ). 

TABLE 7-1 Physical Properties of Pollutant Gases and Their Site of Action or Absorption in the Respiratory Tract ... 

AGA The American Gas Association, Arlington. Virginia is an excellent source of information on the properties of natural and other Fuel gases and their measurement, including calorimetry. Publications arc frequently updated. http //WK.aga.oni/... 

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of solids, liquids, and gases, and their mechanical response to external forces can he very complex. 

All the matter around you occurs in one of three physical states—gas, liquid, or solid. Under specific conditions, many pure substances can exist in any of the states. The three states were introduced in Chapter 1 and their properties compared when we examined gases in Chapter 5 now we turn our attention to liquids and solids. A physical state is one type of phase, any physically distinct, homogeneous part of a system. The water in a glass constitutes a single phase. Add some ice and you have two phases or, if there are bubbles in the ice, you have three. 

J. Kestin, et al. Equilibrium and transport properties of the noble gases and their mixtures at low density, J. Phys. Chem. Ref Data, 13, 299, 1984. 

Real gases and their mixtures have the property (see (4.422) and Appendix A.l) that at P 0 they behave as the ideal gases and their mixtures (4.421). This permits to calculate fugacity coefficients and therefore fugacities from state behaviour (equations like (3.204), (4.215)). 

In 4.1 we introduce ideal gases and their mixtures, and we derive equations for computing their thermodynamic properties. Then, we use the rest of the chapter to develop expressions for computing deviations from ideal-gas values the difference measures in 4.2, the ratio measures in 4.3. 

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