10.1 Characteristics of gases Gas is one of the three states of matter. It is characterized by it's low density. The other states of matter are liquid and solid. Only a few elements exist as gases under ordinary conditions: - The noble gases: He, Ne, Ar, Kr and Xe (monoatomic) - H2, N2, O2, F2 and Cl2 (diatomic gases) Gas differ significantly from solids and liquids: A gas expands spontaneously to fill its container The volume of a gas equals the volume of the container in which it is held Gases also are highly compressible
10.2 Pressure Pressure is a force exerted by the substance per unit area on another substance. Gases exert a pressure on any surface with which they are in contact Force Area Pressure = F A P =
Atmospheric pressure and the barometer Atmospheric pressure is the pressure exerted by earth’s atmosphere Value of atmospheric pressure depends on location, temperature and weather condition A barometer measures atmospheric pressure by balance the weight of mercury in a glass tube against the weight of air in the atmosphere. 76 cm Standard atmospheric pressure (1 atm) is equal to the pressure that supports a column of mercury exactly 760 mm (or 76 cm) height at 0C at sea level. 1 atm = 760 mmHg = 760 torr = 76 cmHg = 101325 Pa = 101.325 kPa
Example: The atmospheric pressure at a certain location is 732 mmHg Example: The atmospheric pressure at a certain location is 732 mmHg. Convert the pressure in atm and Pa? Pressure (in atm)= 732 mmHg 1 atm 760 mmHg = 0.963 atm Pressure (in Pa)= 732 mmHg 101325 Pa 760 mmHg = 97591.9 Pa = 97.5919 kPa
A manometer is used to measure the pressure of an enclosed gas. Their operation is similar to the barometer, and they usually contain mercury A mercury manometer, used to measure gas pressures near atmospheric pressure.
Example: The level of mercury in an open-end arm of the manometer has a height of 136.4 mm, and the mercury in the arm that is in contact with the gas has a height of 103.8 mm. What is the pressure of the gas (a) in atmospheres, (b) in kPa [atmospheric pressure is 764.7 torr] Pressure of gas, Pgas = Patm + h (a) = 764.7 torr + (136.4 torr – 103.8 torr) = 797.3 torr (797.3 torr) 1 atm 760 torr Pgas = = 1.049 atm (b) P in kPa =(1.049 atm) 101.325 kPa 1 atm = 106.3 kPa