Chemistry and Gases Help (page 3)
Introduction to Gases
Gases have always been a mystery. Ancient humans saw bubbles of gas form when they brewed ales and spiced ciders from grain and fermented fruit. Some tribes believed intestinal gases were somehow connected to the spirit. When the spirit was unhappy, ill humors (sickness) would plague a person with an excess of gas. When naturally occurring pockets of natural gas from the earth were discovered, it was thought the earth’s spirit was releasing the errors of the people upon the land.
Until the 1700s, spontaneous combustion, the explosive occurrence of fire, was thought to be caused by mice, since the sudden combustion of stored grain in barns always seemed to happen when there were lots of mice around.
Modern chemists have learned a lot since then. With the development of precise equipment that can measure minute amounts of elements, accurate information about gases can now be gathered and studied.
The gas club is a lot more active than the liquid or solid clubs. Gases have livelier characters and no set boundaries. When allowed to escape from a container, they spread out into whatever space there is. If the space is the size of a room, they expand to fill the space. If the space is the outside atmosphere, they spread out infinitely, contained only by temperature changes and directed by wind currents.
Gases are the least compacted form of matter.
Gases are known for preferring to be as far as possible from each other, with no special shape or volume. Unlike solids and liquids, they are independent of one another.
Some common gases found in this free form club include nitrogen, oxygen, air, steam, carbon dioxide, helium, and argon. The air we breathe is a gas, except on very humid summer days when the air seems so loaded with water as to be nearly a liquid.
The table below gives some of the general characteristics of gases.
No set shape or volume
Expand to fill shape of container
Can be compressed by increasing pressure
Mix completely and spontaneously
Move constantly, quickly, and randomly
Smaller mass gases move more quickly than gases with larger masses
No strong molecular forces between particles
When particles collide, no energy is lost
All collisions are elastic
Although it may seem like it, the air we breathe is not limitless. It reaches out about 30 kilometers from the surface of the Earth, but is only breathable to humans to about 14,000 feet or 4.3 kilometers. The air we breathe, in fact, is not just one gas, but several.
Outside and inside air is made up of roughly 78% nitrogen, 21% oxygen, and 1% argon with a smattering of 3–4% water vapor, carbon dioxide, sulfur dioxide, and the list goes on depending on where in the world you live.
Something important to consider is that the amount of polluting (chemicals not found in high levels in nature) gases in the atmosphere is rising. Levels at the times of the pyramids with fires and local industry, were around 80–100 parts per million (ppm), in 1900, the levels of CO 2 were less than 300 ppm. Today carbon dioxide levels approach 400 ppm. Since the human body does not do well breathing low levels of oxygen or pollutants, this is a significant problem. Not only is the air no longer pure in many parts of the world, but increasing levels of carbon dioxide add to the problem of rising global temperatures.
When gases expand and mix with other gases to fill available space, it is called diffusion . This is how environmentalists measure the levels of industrial gases in the air. When there is a gas release, they measure the amount of gas in parts per million or parts per billion. From these measurements, they can figure out the released amount and whether or not the diffused concentration is harmful to humans.
Kinetic Gas Theory
Gas molecules are always on the move. They are always bouncing off each other and other things like the walls of a container, or people, places, and things. They are super charged with energy. When scientists talk about this crazy motion of gases, they call it kinetic energy .
The kinetic energy of gases can be calculated. It is equal to one-half the mass (m) of the sample multiplied by the velocity squared ( v 2 ):
Kinetic energy = ½ m v 2
Then if a scientist has a sample of a known mass in a container with the molecules bouncing all over the place, the kinetic energy can be calculated using the equation above.
Chemists calculate the kinetic energy of gases based on their temperature, which in turn affects their velocities. The average kinetic energy of a gas molecule depends on the absolute temperature of the gas.
The atmosphere contains different gases as we learned earlier. These gas molecules collide with everything in our world, all the time. The Earth’s gravity affects the force with which gas molecules hit people, objects, and each other. Gravity’s pull on gas molecules decreases when molecules get farther and farther away from the Earth. Their weight is decreased without the constant tug of gravity although their mass remains the same. It is the same reason why astronauts are weightless in space and weigh only about a third of their weight on the moon (with a lot less gravity) as they would on Earth.
Atmospheric pressure is caused by the weight of the air per unit of area.
Since the first experiments to find this difference in pressure were performed with mercury and tall glass tubes by Italian scientist Evangelista Torricelli, the standard unit for pressure was called the torr . Experiments found that at sea level, the lowest land location to measure, that atmospheric pressure is equal to 1 atmosphere (1 atm).
1 atm = 760 mm Hg = 760 torr
Note: When engineers and mechanics calculate pressure, they sometimes use the units pounds per square inch (psi).
Standard Temperature And Pressure
When studying the gas laws, you will sometimes see a problem talk about standard temperature and pressure (STP). Table 17.2 lists the values for temperature and pressure that are called gas standards .
Since some gases are non-polar and don’t mix with water, they can be collected using water as a filter. Water is displaced by the gas that is bubbled through it and collected. The vapor pressure of a gas is directly affected by temperature. As temperature increases, vapor pressure increases.
Vapor pressure of a sample is equal to the partial pressure of the gas molecules above the liquid phase of the sample added to the pressure of the water vapor.
P total = P gas collected + P water pressure
P gas collected = P total − P water pressure
To figure out the volume of a mole of gas, use Avogadro’s number of 6.022 × 10 23 molecules in a mole. To figure out the molar volume of a gas, multiply the density times its molar mass.
Molar volume describes the density of a gas times its molar mass. At STP all gases have the same molar volume.
Molar volume = d × m
The density of oxygen at STP is 1.43 grams/liter and its molar mass is 32.0 grams/mol. To figure out the molar volume of oxygen at STP, use the following formula:
32.0 grams/mol × 1 liter/1.43 grams = 22.4 liters/mol
Since Avogadro’s number is so huge, every gas works out to be very close to 22.4 liters/mol, so it is commonly said that the molar volume of every gas at STP is 22.4 liters/mol.
The activity of gases can be calculated using a lot of different ratios. If you know the temperature, pressure, and volume of gases, then lots of different ideas can be tested. The gas club is definitely the most changeable of the solid, liquid, and gas forms of matter.
Practice problems for these concepts can be found at – Chemistry and Gases Practice Test
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