What is PRESSURE? Chapter 5: Properties of Gases. force pressure = area

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1 Chapter 5: Properties of Gases Gases have a number of properties that are very different from liquids and solids: ) compressible ) exerts pressure 3) Gas pressure depends on the amount of confined gas 4) Gases fill their container 5) Gases mix freely with each other 6) Gas pressure increases with temperature a gas is comprised of widely spaced molecules in rapid motion collisions of molecules with the walls are responsible for the gas pressure What is PRESSURE? force pressure = area What about atmospheric pressure?

2 Atmospheric pressure is measured with a barometer. A Torricelli barometer consists of a glass tube sealed at one end, about 80 cm in length. The tube is filled with mercury, capped, inverted, and the capped end immersed in a pool of mercury. When the cap is removed the atmosphere supports a column of mercury about 760 mm high. Torricelli observed that the height of the column varied a little with temperature. He found, too, that the height varied from day to day, even when there was no change in temperature. Thus, he discovered that atmospheric pressure is a variable. The weather tends to be fine when pressure is high unsettled with rain, thick cloud and strong winds when pressure is low However the markings that are found on the dials of domestic barometers - 'Stormy', 'Rain', 'Change', 'Fair', 'Very dry' - cannot be considered reliable indicators of the weather to be expected. barometer.php

3 The height of the mercury column varies with altitude The average pressure at sea level or the standard atmosphere (atm) was defined as the pressure needed to support a column of mercury 760 mm high measures at 0 o C The SI unit of pressure is the pascal (Pa) N Pa = = N m m atm = 0,35 Pa (exactly) The height of the mercury column varies with altitude Specifically, with increasing altitude, pressure decreases You may encounter a number of pressure units The standard atmosphere is o 760 mm Hg (measured at 0 C) 760 torr 0,35 Pa = 0.35 kpa.03 bar = 03 mb 4.7 lb in Chemical reactions often involve gases 3

4 The barometric pressure in a cavern was measured using a mercury barometer. A value of 74. cm of Hg was obtained. How many atmospheres is this? A manometer is used to measure the pressure inside closed containers Open-end manometer. (a) The pressure of the trapped gas, P gas equals the atmospheric pressure, P atm. Trapped gas pressure (b) higher and (c) lower than atmospheric pressure. Mercury is so dense (3.6 g ml - ) that small pressure changes are difficult to measure Other liquids can be used to make manometers Columns of mercury and water that exert the same pressure. Mercury is 3.6 times more dense than water. Both columns have the same weight and diameter, so they exert the same pressure. 4

5 Thus for a given difference in pressure, the difference in heights between the two levels is inversely proportional to the density of the liquid used in the manometer There are four variables that affect the properties of a gas: pressure, volume, temperature, and the amount of the gas Simple experiments can be conducted that relate how these variables change The gas laws summarize these experiments Compressing a gas increases its pressure. A molecular view of what happens when a gas is squeezed into a smaller volume. The number of collisions with a given area of the walls increases which causes the pressure to rise. Robert Boyle studied how the volume of a fixed amount of gas varies with pressure at constant temperature. (a) Air trapped in a J-tube by mercury. (b) As more mercury is added, the pressure of the trapped gas increases and the volume decreases. 5

6 (a) A typical graph of volume versus pressure showing volume decreasing as pressure increases. (b) A straight line is obtained when volume is plotted against (/P), which shows that V P The relationship between volume and pressure is called Boyle s law or the pressure-volume law The volume of a given amount of gas held at constant temperature varies inversely with the applied pressure The proportionality can be removed by introducing a proportionally constant, C V = C or PV = C (constant amount and temperature) P Boyle s law is remarkably successful, especially for common laboratory conditions However, no real gas obeys Boyle s law exactly over a wide range of temperatures and pressures The hypothetical gas that does exactly obey Boyle s law is called an ideal gas Real gases act more like ideal gases as their pressures decrease and temperatures increase 6

7 A sample of a gas in a cylindrical chamber with a movable piston occupied a volume of 4.66 liters when the pressure was atm and the temperature was 7. o C. The pressure was readjusted to.388 atm by moving the piston. What was the volume occupied by the sample under the later conditions if the temperature remained constant throughout? Jacques Alexander Charles studied how the volume of a gas sample varied with temperature Charles law plots. Each line shows how the gas volume changes with temperature for different sized samples of the same gas. When extrapolated to zero volume all the samples have the same temperature: o 73.5 C This temperature is called absolute zero and is the basis of the Kelvin temperature scale: TK = To C Charles law or the temperature-volume law can be expressed mathematically 7

8 V T or V = C' T (constant amount and pressure) A sample of a gas in a cylindrical chamber with a movable piston occupied a volume of 6.44 liters when the pressure was 850 torr and the temperature was 7. o C. The temperature was re-adjusted to 65.5 o C while the load on the piston was kept constant to keep the pressure constant in the system. What was the volume occupied by the sample at the new temperature? Joseph Louis Gay-Lussac studied how the pressure and temperature of a fixed amount of gas at constant volume are related Gay-Lussacs law or the pressuretemperature law states: The pressure of a fixed amount of gas held at constant volume is directly proportional to the Kelvin temperature Mathematically this is 8

9 P T or P = C" T (constant amount and volume) The three gas laws are often used in a single equation called the combined gas law PV P V PV = constant or = (fixed amount) T T T When using this equation the temperature must always be in kelvins Alternate forms of the previous gas laws result when certain variables cancel Boyle's Law : PV = PV Charles' Law : V / T = V / T Gay - Lussac's Law : P / T = P / T (when T = T ) (when P = P ) (when V = V ) Problems involving the gas laws are important Example: What will be the the final pressure of a sample of oxygen with a volume of 850 m 3 at 655 torr and 5.0 o C if it is heated to 80.0 o C and given a final volume of 066 m 3? 9