The Earth’s Atmosphere

Composition The air that we breathe is composed of: – 78% N 2 (nitrogen gas). N 2 is very non-reactive (chemically inert) This is good – why don’t we want a highly reactive atmosphere? – 21% O 2 (oxygen gas). Oxygen is very reactive and we need to breathe it because it is an important reactant in our metabolic chemistry. – 1% “other” Includes: H 2 O vapor, CO 2, argon, trace amounts of other gases.

Layers The atmosphere has distinct layers as you go up from the ground. We live in the “troposphere” which goes up to about 60,000 feet. Above the troposphere is the “stratosphere”, which contains a lot of a chemical called ozone (O 3 ). This ozone forms the “ozone layer”

Temperature and Pressure in Gases

Review of Kinetic Molecular Theory Gases are made of moving particles Particles are very far apart relative to their size. Movement is in straight lines and bounce off of each other and off of the walls of their container. The higher the temperature, the faster the particles are moving.

Particle Collisions = Pressure! Moving gas particles are constantly crashing into the walls of the gas’ container Each collision exerts a little bit of force on the wall All of these collisions add up to a fair amount of force. – This force is called “pressure”. If there are no particles at all, there is no pressure = this is called a “vacuum” (i.e. outer space)!

Measuring Pressure Device for measuring air pressure = barometer. The higher the air pressure, the higher the fluid (usually mercury) in the barometer rises. Common units of pressure – millimeters of mercury (mmHg), aka Torr, and inches of mercury. In this class you don’t need to use these units.

Fun Fact A barometer can tell you if a storm is approaching. Good weather generally correlates to high atmospheric pressure. In a storm, the air pressure drops rapidly (you will hear storms called “low pressure systems” sometimes). In the old days people would watch the barometer – high pressure meant good weather, a dropping barometer reading meant the weather was about to get worse!

Pressure Units Lots of different units to measure pressure If you have ever pumped up a tire or a ball you might be familiar with the unit “pounds per square inch (psi).” 1 psi = one pound of force on every square inch of surface. Some real-world psi pressure levels: – A car tire is typically at about 32 psi – A full scuba tank holds air at around 3,000 psi (don’t drop it!) – Dog bite – 400 psi – Great White Shark bite = 4,500 psi (ouch!)

Pressure Units (continued) “psi” is not a metric or Si unit so we don’t use it in science. In Chemistry, we use “atmospheres” of pressure (atm), Torr, and Pascals (Pa or kPa) 1 atm = the average air pressure at sea level. 1 atm = 14.7 psi. *Right now you have almost 15 pounds of force pressing on every inch of your body! Note: as we are surrounded by air on all sides  this force squeezes us, it does not push us down (it is gravity that holds us on the ground, not air pressure!)

Air Pressure in Action: Demo!

Wait – 15 pounds per square inch? Why don’t I get squished? Cells are mostly water =liquids is virtually incompressible. The squeezing doesn’t actually stress our body at all. Our eardrums can be affected however! It has air on both sides of it. Too much pressure on one side or the other causes the eardrum to bend and can even break it. To prevent this, our ears can be “equalized” so that air pressure inside and outside the ear drum are the same. Air from the outside travels into your mouth and nose, up through the Eustachian tube, and into your inner ear – this means that the pressure inside your ear is always the same as the pressure outside the ear. Eustachian tube - leads to nasal passages! Air pressure equal on both sides

Ears “popping” Sometimes the Eustachian tube closes up and you start to feel pain from changing air pressure (i.e. when ascending or descending in a plane) Open the tube by: yawning, chewing gum, wiggling your jaw – anything that moves that part of your face around can help. The “pop” you hear = air flowing in to equalize pressure (also why you feel a big relief) If you have a cold, mucus can clog the Eustachian tube, making it difficult or impossible to equalize your ears – very painful and can even cause ruptured eardrums in young children!

Under Water When you dive down deep in the pool, the water pressure on the outside of your eardrum is much greater than the air pressure inside the ear – that’s why it hurts! To fix – pinch your nose shut and blow gently. This blows air up from your lungs into the inner ear and equalizes the pressure. This allows scuba divers to dive to very deep depths without pain – they just keep equalizing their ears! Why would this be a very bad idea to try if you aren’t under water?

Why am I out of breath while skiing? Lower air pressure in the high mountains means that you have to work harder to get air into your lungs! Also less oxygen in each breath because there are less particles in the air That means you work harder to get the air in – and it does you less good once it is in! The lowered oxygen supply can lead to altitude sickness within 12 hours – headache, nausea, and vomiting. Extreme cases = you can get a buildup of fluid in the lungs and brain which causes death (only at Mt. Everest-style altitudes). Your body can adapt to this – after about 2 weeks at high altitude you will be producing a LOT more red blood cells that carry oxygen.

Elevation and Air Pressure The higher you are above sea level, the lower the air pressure Think of the atmosphere as an ocean of air – when you are at the bottom of the ocean, the pressure is really high because of all of the water on top of you! Similarly, when you are at sea level, the air pressure is highest and the pressure drops as you go up - this is because there is less air on top of you the higher up you are! Lots of air on top = higher pressure Less air on top = lower pressure!

Movie: Everest

Gas Pressure, Temperature, and Volume: Gases present a special challenge in the lab because unlike solids and liquids, their volumes, temperatures, and pressures are linked and can change easily! For example, if you heat a gas, the gas will expand a LOT If that expanding gas is trapped in a container it will create a lot more pressure on the container – possibly becoming dangerous.dangerous This means that we have to take extra care and do some special calculations when dealing with gases.

STP Since the volume, temperature, and pressure of a gas are dependent on their environment, scientists use a “standard environment” when describing a gas. STP = “standard temperature and pressure” STP = O°C (273K) and 1atm pressure

Temperature and Pressure Temperature and pressure are directly proportional. – For a gas, higher temperature means higher pressure (assuming the gas isn’t allowed to expand) and vice versa! – This happens because the when you heat a gas, the gas particles are moving faster and hitting the walls of the container with more force. This is why you should never throw an aerosol can into a fire!

Temperature and Volume Temperature and Volume are directly proportional. If you heat a gas (that is not in a fixed-size container,) the gas will expand. If you cool it, it will contract. This is why hot air rises – as the air expands it becomes less dense and rises because it is lighter than the surrounding air.

Volume and Pressure Volume and pressure are inversely proportional If you take a container of gas and squish it down (make its volume smaller) the pressure in the container will get larger! This is because the particles are hitting the walls of the container more often! The opposite is also true. This is why a balloon pops when you sit on it – you are squeezing the balloon into a smaller volume, which increases the pressure until it breaks the balloon!

The Combined Gas Law We can take all of these factors into account and relate them to each other using a simple formula – the “Combined Gas Law” formula. It is this: P 1 V 1 /T 1 = P 2 V 2 /T 2 (note: it’s on your cheat sheet) 1 = initial p, v, and t of the gas 2 = final p, v, and t of the gas. Note: If one of the variables does not change, you just leave it out entirely. Note: if a problem says “at STP”, that gives you two of your variables – O°C (273 K) and 1 ATM pressure! Note: always work in Kelvin in this unit!