Gas Laws
A substance in the gaseous state of matter is one that has no definite shape or volume. The particles in a gas are moving with high energies and speeds. There are many familiar properties of gases. For example, a gas will take on the shape of any container in that it is placed. Likewise, a gas will occupy any volume that is made available to it. A gas can also be easily compressed when pressure is exerted on it. All gases show the same physical properties. They all have mass, they can move through each other, they exert pressure, and the pressure of all gases depends on their temperature.
The physical properties of gases are explained by a model called the kinetic-molecular theory of gases. This theory is composed of several postulates. First of all, a gas is made up of tiny particles. These tiny particles each have a particular mass. Second, the distance between the particles in gases is quite large in comparison with the distance between particles in liquids and solids. Third, the particles of a gas are in a constant motion. They move rapidly and randomly. Fourth, as the gas particles collide with each other or with the wall of a container, their collisions are perfectly elastic. That means no energy is lost by a particle as it makes a collision. Fifth, the only factor that can affect the kinetic energy of the gas particles is the temperature of the gas. As the temperature of the gas increases, so does the kinetic energy of the particles. Finally, a gas particle does not exert a force on another gas particle.
The postulates of the kinetic-molecular model built the foundation for several theories that are now called the gas laws. The gas laws were formed by several scientists and collectively summarize the current understanding of the gaseous state of matter. Each of the gas laws assumes that the gas is under specific conditions, called standard temperature and pressure (STP). The standard temperature is zero degree Celsius or 273 Kelvin (K). The standard pressure is one atmosphere (atm), that is equivalent to 760 mm mercury (Hg) or 101.325 Pascal (Pa). These laws give a mathematical model for the observed properties of pressure, volume, temperature, and quantity of a gas.
The first of the gas laws is known as Boyle's law. Boyle's law describes the relationship between the pressure and the volume of a gas. Gases, as stated above, can be easily compressed. This is because the particles of a gas have a great distance between them. It is this property of gases that make them useful as cushioning devices such as the air bags in an automobile. An English chemist and physicist named Robert Boyle was the first scientist to describe this property of gases. In fact, he wrote about "the spring of air" in more than one publication.
Boyle performed many experiments that explored the "spring of air." In one such experiment, he trapped a certain volume of air and measured the new volume after changing the pressure. To do this, he used a J-shaped tube. The short end of the "J" was closed off, and the long end was left open. Boyle trapped some air in the end of the J-tube by adding mercury in the longer end. By changing the height of the mercury in the long end of the tube, Boyle could alter the pressure on the air in the short end. He measured the volume of the air in the short end of the tube at several different pressures, keeping the temperature constant throughout the experiment. He found that as he increased the pressure on the air, the volume of the air would decrease.
Boyle used these results to formulate his law. Boyle's law maintains that if the temperature of a gas is constant, the product of the pressure times the volume is constant. The mathematical formula that describes this law is PV = k. P is the pressure of the gas, V is the volume, and k is the constant. The value of k depends on the units of pressure and volume used, the amount of gas present, and the temperature of the gas. Using this equation, it can be stated that the volume of a gas and its pressure are inversely proportional to each other. This law can be useful if you measure the pressure and volume of a gas in two trials at constant temperature. If you rearrange the above equation accounting for the two trials, you will come up with PV=PV, where P and V are the pressure and volume of the gas in the first trial, and P and V are the pressure and volume of the gas in the second trial.
The next of the gas laws is Charles's law.Jacques Charles formed this law in order to describe the relationship between the temperature of a gas and its volume. Charles also conducted several experiments in order to develop his theory. In one experiment, he trapped a gas sample in a cylinder with a moveable piston. The temperature of the gas sample was changed by placing the cylinder in water baths at different temperatures. As the gas' temperature changes, so does its volume, that is observed by the piston moving up or down. The volume of the gas can be measured by the position of the piston. Charles found, through these experiments, that the volume of a gas is directly proportional to its temperature. As the temperature rises, so does the volume of a gas. Anyone who has bought a helium balloon during cold weather has observed this phenomenon. When the balloon is removed from the heated store and walked through the cold parking lot, it collapses. As the temperature of the helium decreases, so does its volume. If the temperature is measured using the Kelvin temperature scale and the pressure is kept constant, the relationship can be described mathematically by the equation V = kT. V is the volume of the sample, T is the temperature in Kelvin, and k is called the Charles's law constant of proportionality. Again, this equation can be rearranged for an experiment with two trials to give the relationship VT = VT.
The next of the gas laws is known as Avogadro's law. This law describes the relationship between the amount of a gas and its volume. It was proposed by the Italian chemist Amedeo Avogadro and states that two samples of gases of equal volume at the same temperature and pressure have the same number of particles in each. This relationship can be expressed by the equation V = kn. V is the volume of the sample, n is the number of moles of the gas and k is called Avogadro's law constant.
The fourth gas law is called Dalton's law of partial pressures. John Dalton, an English chemist, explored the nature of gases in mixtures. Dalton determined that each gas present in a mixture exerts the same pressure as it would if it were not in a mixture at the same temperature. In a mixture, each gas exerts a pressure that is called the partial pressure of that gas. The total pressure of the mixture of gases is equal to the sum of all of the partial pressures of the individual gases present in the mixture.
Each of the above laws can be combined to give an equation called the ideal gas equation, PV = nRT. P is the pressure of the gas, V is the volume, n is the number of moles of the gas, T is the temperature, and R is called the gas constant (or, sometimes, the universal gas constant). At STP, R = 0.0821 atm-L/mol-K. This equation summarizes the physical properties of an "ideal" gas. An ideal gas is one that follows each of the postulates given by the kinetic-molecular theory of gases. In reality, no gas completely adheres to the kinetic-molecular theory, so there is no such thing as an ideal gas. Scientists still use the ideal gas equation to describe "real" gases. Real gases generally adhere to the kinetic-molecular postulates, it is only at high pressures and low temperatures that they deviate significantly.
The gas laws described above are a culmination of the works of many scientists. These laws are very useful in describing and predicting the behavior of gases. Even though no ideal gases exist, real gases, for the most part, adhere to the principles outlined above.
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