C.9AC: Gas Laws Gases and Thermochemistry So Much Pressure (Lexile 1130L) 1 Gases have long been studied and understood by early scientists. One of the reasons for this is that gases behave in predictable ways: they have low density; they can be compressed; they expand to fill their container; they can be mixed; and they exert pressure on the walls of their containers. Initial experiments using variations of temperature and volume supported these properties, and scientists learned much regarding the behavior of gases. 2 Volume and temperature are easy factors to manipulate. Scientists knew that gases, unlike liquids or solids, always filled their container. This meant that the volume of the gas would always be the same as the volume of the container. The temperature of the gas could be manipulated by adding or removing thermal energy. But, how could scientists manipulate pressure, and how did they come up with the pressure units that we use today? 3 Some of the earliest studies of pressure examined atmospheric pressures. In the early 1600s, a student of Galileo named Evangelista Torricelli was looking for an explanation for the difficulty of pumping water out of deep mines. He set up an experiment and made the first mercury barometer in order to observe the behavior of liquids as they moved up a tube. In doing so, he noticed that the height of the mercury in the column changed from one day to the next. Torricelli determined that the changing height of the mercury in the tube must be due to the changing “weight” of the atmosphere from day to day, but he could not prove it. 4 There needed to be better proof of this than Torricelli's hypothesis. In the mid-1600s, another scientist named Blaise Pascal took Torricelli's barometer to the top of a mountain and found that the height of the mercury column decreased at the higher elevation. He postulated that this was due to the atmosphere exerting less pressure at the top of the mountain than it did in the valley. Therefore, he realized that the atmosphere was exerting a force on the column of mercury. In other words, the atmosphere had “weight.” These two men and their early studies set the standards for modern day pressure measurements. 1 C.9AC: Gas Laws Gases and Thermochemistry 5 What exactly is pressure? Pressure equals the force exerted over a unit of area. We use several units to measure pressure. The first is the millimeter of mercury (mm Hg), as the early barometers used mercury to measure the pressure exerted by the atmosphere. As Torricelli was the first to use this device, we also use the torr as a unit of pressure measurement. The pressure exerted by a millimeter of mercury or a torr are identical and can be written as 1 mm Hg or 1 torr. On average, normal atmospheric pressures range between 760 mm Hg at sea level and 270 mm Hg on the top of very tall mountains. 6 However, a more accurate way to measure pressure is to use the Newton to measure the force exerted. In honor of Pascal and his work, the current SI unit of pressure measurement is known as the pascal (Pa), which equals a pressure of one Newton per square meter. This unit is very small, however, so for many calculations it is easier to use the kilopascal (kPa). An English unit of measure called pounds per square inch, or psi, also exists. 7 One of the most common units of measure in current use is the atmosphere, or atm. This unit can be used to convert between any of the common pressure units, and it is important in your study of chemistry to know how to use these conversions. The table below shows the conversions between pressure units, using the unit atm as the starting point. 1 atm = 760 mm Hg = 760 torr 1 atm = 101.3 kPa 1 atm = 14.69 psi You can use the information contained on this table to convert between any pressure units when calculating gas laws equations. 2 C.9AC: Gas Laws Gases and Thermochemistry 1 Paragraph 3 discusses Evangelista Torricelli and his work with the mercury barometer. What did he determine was exerting pressure on the mercury in his barometer? A The mercury B The water C The atmosphere D The height of the column 2 There are several units of pressure discussed in this reading. Which of the following is not a unit of pressure? A psi B mm Hg C torr D mL Hg 3 What was the best evidence that showed scientists that the volume of a gas would always equal the volume of its container? A Gases expand to fill their containers. B Gases have low densities. C Gases can be mixed. D Gases exert pressure on the container walls. 3 C.9AC: Gas Laws Gases and Thermochemistry 4 Paragraph 7 provides some basic pressure unit conversions. If a gas has a pressure of 0.5 atm, what will be the pressure in torr? A 760 torr B 380 torr C 150 torr D 480 torr 5 You are measuring the pressure of a gas and need to convert from psi to kPa. If the gas has a pressure of 12.02 psi, what will be the pressure in kPa? A 82.8 kPa B 0.818 kPa C 123.8 kPa D 6.90 kPa 4.