Seattle Central CHEM 161 - Chapter 6 - Properties of Gases - The Air We Breathe

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CHEM 161: Chapter 6 v1013 Page 1 of 16 Chapter 6: Properties of Gases: The Air We Breathe Problems: 6.1-6.5, 6.13-6.18, 6.21-6.22, 6.6.25, 6.27-6.32, 6.35-6.38, 6.41-6.50, 6.55-6.122, 6.133-6.138,6.145-6.158, 6.161-6.167, 6.169-6.170, 6.184-6.186 Activity: Cap the end of the syringe with your finger to make it airtight then pull the plunger out at least half way. Release the plunger. Record your observations. Explain why this occurs. Example: Explain what would happen if the hole in an airtight can of coconut juice was just barely big enough to fit a straw and you tried to use a straw to drink the coconut juice. 6.1 THE GAS PHASE gas pressure: force per unit area exerted by gas molecules colliding against the inside walls of their container – a measure of how often gas particles hit the container walls In the 1600’s, Evangelista Torricelli published the first explanation for a vacuum. – Ancient Greeks observed that a wine barrel empties slowly if only one hole is drilled. – Torricelli explained that a “sea of air” surrounding the Earth slows the flow of wine out of the barrel. – If you create a second hole on the top of the barrel, air molecules can rush in and push the liquid out.  The wine flows out faster. vacuum: empty space with no gas molecules present  gas pressure equals zero – Don’t think of a vacuum cleaner because there’s no suction, just empty space!CHEM 161: Chapter 6 Page 2 of 16 Know a few substances are gases at "normal atmospheric conditions" (25°C and 1 atm) – Elements that are gases: H2, N2, O2, F2, Cl2, ozone (O3), all Noble Gases – Some molecules are gases (CO, CO2, HCl, NH3, CH4); most are solids or liquids. – No ionic compounds exist as gases. Physical characteristics of gases - Gases assume the volume and shape of their containers. - Gases are the much more compressible than liquids or solids. – Because of the empty space between particles in a gas, the particles can be pushed closer together at higher pressures or move further apart at lower pressures. - Gases will mix evenly and completely when confined to the same container - Gases have much lower densities than liquids and solids. – Gas density is generally reported in units of g/L Example: On the moon’s surface daytime temperatures average 107°C and nighttime temperatures average -153°C. Explain the more extreme temperatures on the moon given the Earth and moon are similar distances from the Sun. 6.2 ATMOSPHERIC PRESSURE The Earth’s atmosphere is a layer of gases about 50 km (~31 miles) thick, consisting primarily of nitrogen (78%), oxygen (21%), and trace gases (Ar, CO2, water vapor, etc.). Gravity pulls the gases in the Earth’s atmosphere towards the planet’s surface.  The weight of these gases exerts pressure on surfaces in the environment.  Atmospheric pressure is exerted by gas molecules in the Earth’s atmosphere.  Anything entering the Earth's atmosphere from space comes into contact with all of these gas molecules  friction and heat.  Objects burn up in the atmosphere before reaching the Earth’s surface! CHEM 161: Chapter 6 v1013 Page 3 of 16 Atmospheric Pressure – At sea level the pressure exerted by the gases in the atmosphere is strong enough to hold up a column of mercury about 760 mm (or 29.92 inches) high.  Thus, atmospheric pressure is often expressed in millimeters of mercury or inches of mercury. – However, atmospheric pressure varies with location, temperature, and weather conditions. – It depends on the overlying mass of the atmosphere  It decreases as altitude increases  Thus, air becomes thinner at higher altitudes.  Atmospheric pressure is ~760 mmHg at sea level but 15-20% lower in Denver (~1 mile above sea level) and 65% lower at the top of Mt. Everest (~5.5 mi. above sea level) Example: Altitude Training is a practice by endurance athletes to train at intermediate altitude (1500 m or ~5000’ high) or high altitude (2400 m or ~8000’ high) for several weeks. The body adapts to the high altitude conditions via oxygen transport, and these changes persist for about two weeks even after the athletes return to sea level. Explain how the body adapts at these altitudes and why this helps endurance athletes. barometer: instrument that measures atmospheric pressure Units of Pressure – Since most instruments used to measure pressure often contain mercury, pressure is often reported in heights of mercury (e.g. inHg, mmHg, etc.) or units of atmosphere (atm). Standard Atmospheric Pressure (1 atm): 760 mmHg at 0°C at sea level 1 atm  760 mmHg  760 torr = 101.325 kPa (Note: 1 inch2.54 cm) Ex. 1: The atmospheric pressure in the lab was measured to be 29.22 inHg. Express this pressure in units of mmHg, torr, atm, and kPa.CHEM 161: Chapter 6 Page 4 of 16 manometer: instrument that measures gas pressure Consider these two manometers: Ex. 1: Gas pressure is higher than atmospheric pressure for which one? (Circle one) left right Ex. 2: Explain why. Ex. 3 If the height difference for the manometer on the left is 95 mmHg and atmospheric pressure is 725 mmHg, calculate the gas pressure. Ex. 4 If the height difference for the manometer on the right is 85 torr and atmospheric pressure is 0.975 atm, calculate the gas pressure. Variables Affecting Gas Pressure Example: Consider pressure to be a measure of the frequency of gas particles colliding against the surfaces of a container. What happens to the pressure of a gas given the following changes? (Circle one for each below.) a. If Volume (of the container) , pressure_____.   stays same Thus, pressure and volume are _____ related. not directly inversely b. If Temperature , pressure _____.   stays same Thus, pressure and temperature are _____ related. not directly inversely c. If the # of gas molecules , pressure _____.   stays same Thus, pressure and # of molecules are _____ related. not directly inversely CHEM 161: Chapter 6 v1013 Page 5 of 16 6.3 THE GAS LAWS Combined Gas Law – Consider when there are changes in P,V, and T for a gas, and T must be in Kelvins! Exercises:


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