10.1 Characteristics of Gases. Air behaves physically as one gaseous material N 2 (78%), O 2 (21%) and Ar (0.9%) Only a few elements exist as gases under standard conditions H 2 , N 2 , O 2 , F 2 , and Cl 2 , the noble gases (He, Ne, Ar , Kr, Xe )
Figure 10.2 A mercury barometer invented by Torricelli
mercury manometer or related device
closed, air-filled cuff
The volume of a fixed amount of gas at constant pressure is directly proportional to its absolute temperature.
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules
The volume of a gas at constant temperature and pressure is directly proportional to the number of moles of the gas
At the same volume, pressure and temperature, samples of different gases have the same number of molecules but different masses
The term R is called the gas constant
R = 0.08206 L-atm/mol-K = 8.314 J/mol-K
Molar volume: the volume occupied by one mole of ideal gas at STP (273.15K and 1 atm), 22.41 L
One mole of an ideal gas at STP occupies a volume of 22.41 L. One mole of various real gases at STP occupies close to this ideal volume
▲ Figure 10.11 Comparison of molar volumes at STP
Calcium carbonate, CaCO3(s), decomposes upon heating to give CaO(s) and CO2(g). A sample of CaCO3 is decomposed, and the carbon dioxide is collected in a 250-mL flask. After the decomposition is complete, the gas has a pressure of 1.3 atm at a temperature of 31 °C. How many moles of CO2 gas were generated?
The gas pressure in an aerosol can is 1.5 atm at 25 °C. Assuming that the gas inside obeys the ideal-gas equation, what would the pressure be if the can were heated to 450 °C?
▲ Figure 10.12 Carbon dioxide gas flows downhill because it is denser than air.
The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone.
- Dalton’s law of partial pressure
The pressure exerted by a particular component of a mixture of gases
Applying Dalton’s Law to Partial Pressures
A gaseous mixture made from 6.00 g O2 and 9.00 g CH4 is placed in a 15.0-L vessel at 0 °C. What is the partial pressure of each gas, and what is the total pressure in the vessel?
Each gas in a mixture behaves independently
We can relate the amount of a given gas in a mixture to its partial pressure
Relating Mole Fractions to Partial Pressures
A study of the effects of certain gases on plant growth requires a synthetic atmosphere composed of 1.5 mol% CO2, 18.0 mol% O2, and 80.5 mol% Ar.
(a) Calculate the partial pressure of O2 in the mixture if the total pressure of the atmosphere is to be 745 torr.
(b) If this atmosphere is to be held in a 121-L space at 295 K, how many moles of O2 are needed?
How to measure the amount of gases generated from a chemical reaction
▲ Figure 10.15 Collecting water-insoluble gas over water.
mp: most probable speed
av: average speed
rms: root-mean-square speed
→ pressure decreases
→ pressure increases
(a) Constant (b) Constant (c) Increase (d) Increase
▲ Figure 10.19 The effect of molecular mass on molecular speeds.
▲ Figure 10.19 Effusion. Gas molecules in top half effuse through pinhole only when they happen to hit pinhole
▲ Figure 10.24 Gases behave more ideally at low pressure than at high pressure. The volume of gas molecules is not negligible at high pressure.
▲ Figure 10.25 In any real gas, attractive intermolecular forces reduce pressure to values lower than in an ideal gas.
At high P, gas volumes are not negligible
Attractive forces between molecules reduce the pressure
▲ Figure 10.22 The effect of pressure on the behavior of several real gases at constant T. The deviations increases with increasing P.
▲ Figure 10.23 The effect of temperature and pressure on the behavior of nitrogen gas. The deviations increase with decreasing T.
p397, 399, 402, and 412
Due on 06-13 (Thur)