GS_Properties of Gas Lesson
Properties of Gas
Although you can probably think of many gases that all have different chemical properties, most gases share many of the same physical properties. The way a gas responds to changes in temperature, volume, or pressure is the same, despite the differences in the chemical properties of the gas. These quantities will be used to begin our study of gases.
Pressure
Pressure is defined as force per unit area. To understand pressure, think about a 150 lb woman standing in a high heeled shoe, versus the same 150 lb woman standing in a sneaker. Which one would you allow to stand on your back? The woman in the high heel might puncture your back, while the woman in the sneaker would not. The difference is not in the force of her weight, it is the area of the heel. The high heel exerts much more pressure because the area of the heel is very small compared to the area of the bottom of the sneaker. In this example, the unit used to measure pressure would be pounds per square inch, lb/in2.
Instruments for Measuring Pressure
Pressure can be measured by a variety of instruments and is expressed in many different units. When you begin doing calculations using pressure, pay close attention to these units to make sure you are using the appropriate one.
The simplest device used to measure pressure is a barometer. A barometer is a long tube sealed at one end and filled with a liquid. Once the tube is filled with the liquid, it is inverted, and the liquid is allowed to move out of the open end into a dish of some type. Not all of the liquid will spill out of the tube. The amount that comes out depends upon the pressure exerted by the atmosphere on the liquid in the dish (shown at A in the diagram). As the atmospheric pressure increases, it forces the liquid in the tube to rise (shown at B). If the atmospheric pressure decreases, the fluid level in the tube will decrease. At C, the remaining space in the tube is a vacuum. This is important because there cannot be any pressure exerted downward on the column of liquid in the tube. The lack of pressure in the top of the tube is important because it allows the pressure in the column to be directly proportional with the changes in atmospheric pressure.
By measuring the height of the column of liquid, a pressure reading can be obtained. The idea for measuring pressure in this way suggested by Evangelista Torricelli, an Italian mathematician, in 1643. This barometer described above is often called a Torricelli barometer in his honor.
In addition to mmHg, torr, and lb/in2, there are several more common units for measuring pressure. Pressure can be measured in atmospheres (atm), pascals (Pa), and kilopascals (kPa). The pascal (Pa) is the SI unit for pressure, but kPa are used often. Different industries have histories of using different units of pressure; therefore, all of these are used at different times. As you go through the module, you will see that some equations will require you to use specific units of pressure. For this reason, you will need to be able to convert between any of these units.
The values shown on the list below represent average atmospheric pressure at sea level. This is also known as standard atmospheric pressure. Converting between these units can be done easily using simple dimensional analysis (learned in module 2).
Commonly, the liquid used in a barometer is mercury (Hg). This explains one unit used to measure pressure, millimeters of mercury (mmHg). The unit mmHg is often called a torr, named after Torricelli as well. So, mmHg and torr can be used interchangeably.
Another device for measuring pressure is a manometer. An open-end manometer is composed of a u-shaped tube that is filled with mercury (usually). One end of the u-shaped tube is open to the atmosphere. The other end is open to an enclosed bulb that contains trapped gas. When the pressure of that entrapped gas, Pg, equals the pressure of the atmosphere, Patm, the two columns of mercury are equal in height. If the Patm rises, the mercury on the open end is pushed down, which pushes the mercury on the closed end higher. This difference in the heights of the mercury is measured, PHg.
A closed-end manometer is very similar to an open-end manometer. Instead of one end of the u-shaped tubing being open to the atmosphere, it is closed off. A closed-end manometer is useful for gases that exert a pressure that is less than the atmospheric pressure.
Temperature
Recall that temperature is related to the average kinetic energy of a chemical. While kinetic energy and temperature are not exactly the same thing, they can be used to make interpretations about each other. For example, if a substance has a higher temperature than another, you also know that it has a higher average kinetic energy.
In the United States, we measure temperature in Fahrenheit, while most other countries use Celsius. Most scientific calculations require that temperature be measured in Celsius. However, when doing calculations involving gases, temperature must be in the unit Kelvin.
The development of the Fahrenheit scale was not based on the soundest science. If you are curious, check out the story behind its development HERE Links to an external site..
The Celsius scale was developed so that the freezing point of water was set at 0oC and the boiling point at 100oC (at 1 atm pressure). Eventually it was established that the equivalents on the Fahrenheit scale were 32 oF and 212 oF. This means that the range of 0 to 100 on the Celsius scale is equivalent to the range of 32 to 312 on the Fahrenheit scale. Stated another way, the range of 100 degrees on the Celsius scale equals the range of 180 (212 - 32) on the Fahrenheit scale. This gives us the conversion factor 100oC = 180oF, which reduces to 5oC = 9oF.
In 1848 the British physicist William Thompson, who was later given the title Lord Kelvin, suggested an absolute temperature scale. This was based on the discovery of a relationship between the volume and temperature of a gas. This relationship suggested that the volume of a gas should become zero at a temperature of -273.15oC. On the Kelvin scale, absolute zero (0 K) is the temperature at which the volume of a gas becomes zero. It is therefore the lowest possible temperature, or the absolute zero on any temperature scale. We now know this as the temperature at which all molecular motion ceases to exist. Zero on the Kelvin scale is -273.15oC.
Notice that there is no degree sign when using the Kelvin scale. Since on the Kelvin scale, zero is not arbitrarily defined, the temperatures on the Kelvin scale are not divided into degrees. Temperatures on this scale are reported in units of "kelvin," not in "degrees kelvin."
The formulas for converting temperatures are shown below. You should have learned how to convert between temperature units previous to this class.
Volume
You know that volume is the amount of space something takes up. Gases are unique in that they take up the volume of their container. Whatever size container they are in, they will spread out to consume the entire container. This happens because the intermolecular attractions (discussed in the previous module) as so weak that they are virtually zero. With no attractions between the gas molecules, they spread out as much as possible.
The SI unit for volume is L. Often mL are used to measure volume in the lab. Pay close attention to when you have a choice of which unit of volume to use and when you must use L.
Remember to work on the module practice problems as you complete each section of content.
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