Gases & Atmospheric Chemistry Atmospheric Pressure

A history in Atmospheric Pressure Quantitative measurements on gases were first made by the English chemist Robert Boyle (1627-1691). Boyle used two instruments to measure pressure: The manometer, which measures differences in pressure The barometer, (any device that) measures the total pressure of the atmosphere

Early Studies in Atmospheric Pressure A manometer is simply a bent piece of tubing (preferably glass) with one end closed or open (depends on type). CLOSED-TUBE MANOMETER Used to measure pressure below atmospheric pressures Pressure is the difference in the heights of the mercury levels in the two arms OPEN-TUBE MANOMETER Used to measure pressure that are near atmospheric pressures The difference in the heights of the mercury levels in the two arms relates the gas pressure to the atmospheric pressure

Manometer The difference in pressure can be measured in units of length of the vertical column of liquid. mmHg, or its modern version the torr, originated with the use of the manometer. When the fluid level in both arms is the same, the pressure of the sample of gas inside the closed end must equal the external atmospheric pressure. Pgas = h

Manometer When the fluid level differs, then the pressures must be different. If the pressure of the gas in bulb is the greater than the atmospheric pressure, then the column exposed to atmosphere will be higher Pgas = P (atm + h) If the pressure of the gas in the bulb is less than the atmospheric pressure, the mercury column exposed to the mercury will be higher Pgas = P(atm) - h

Importance of Mercury Mercury is particularly convenient for use in manometers (and barometers) because at room temperature it has a low vapor pressure, does not wet glass and has a high density (13.6 times > water). Why would this be important?

Early Studies in Atmospheric Pressure Galileo Galilei (1564-1642) – designed a suction pump to pump water from under ground. There was a limit of 10 m. He concluded that this was because the water had reached the “limit of vacuum”. Evangelista Torricelli (1608-1647) – continued Galileo’s work and concluded that the water was able to move up the column because “the weight of the air was pushing down on the rest of water. He improved his experiments by using mercury, which has a density that is 13.6 times greater than water. He designed what we now call a barometer. The mercury ran up the tube to a height of 760 mm (this is about 13.6 times less the height of water that Galilei could pump).

Early Studies in Atmospheric Pressure Torricelli repeated this with tubes of various diameters; each time the column rose to a height of 760 mmHg. Thus, he concluded that it is the air pushing on the mercury in the dish that caused the column of mercury to rise to a height of 760 mm. The height of the mercury column will be the height which the atmospheric pressure will support. The standard atmospheric pressure (STP), one atmosphere (atm) is 760 mmHg, but actual atmospheric pressure varies with altitude and weather conditions. Falling barometer- arrival of low pressure system – stormy (rain/showers) Rising barometer – arrival of high pressure system – clear

Four Common Units of Pressure UNIT NAME UNIT SYMBOL DEFINITION/CONVERSION Pascal Pa 1 Pa = 1N/m2 Atmosphere atm 1 atm = 101.325 kPa mm of mercury mm Hg 760mm of Hg=1 atm=101.325kPa Torr torr 1 torr = 1mm of Hg Thus, there are FOUR (4) common units of pressure used in chemistry: *** Know how to convert these units***

Practice Work Complete the worksheet “Manometer Worksheet” Continue working on “Kinetic Molecular theory Review” Questions from Yesterday