Properties of Gases

• Gases expand to fill their containers.

• They spread out easily and mix with one another.

• They have low densities and are compressible.

• Gases are mostly empty space.

• Gases exert pressure on their containers.

Kinetic Theory

• 1. Gas molecules are in constant, rapid motion, moving in straight lines until they collide with each other or other substances (like walls)

• 2. Collisions between gas particles and between particles and container walls are elastic collisions (no energy is lost)

• 3. Gas molecules are very far apart and are negligible in volume compared to the volume of the gas.

• 4. There are no forces of attraction between gas particles.

• 5. The kinetic energy of gas molecules is directly related to temperature.

Changing Pressure

Have your ears ever popped? If your ears have "popped", you have experienced a change in air pressure. Ears "pop" because the air pressure is different on the inside and the outside of your ears.

Air Pressure

Because air is a gas, its molecules have a lot of energy. Air molecules move a lot and bump into things. For this reason, they exert pressure. Air pressure is defined as the weight of the air pressing against a given area.

At sea level, the atmosphere presses down with a force of about 1 kilogram per square centimeter (14.76 pounds per square inch). If you are standing at sea level you have more than a ton of air pressing against you. Why doesn't the pressure crush you?

Altitude and Air Pressure

Like density, the pressure of the air decreases with altitude. There is less air pressing down from above the higher up you go. Look at the bottle in the picture. It was drained by a hiker at the top of the mountain. Then the hiker screwed the cap on the bottle and carried it down to sea level. At the lower altitude, air pressure crushed it.

Summary

• Air density and pressure decrease with increasing altitude

• Gravity pulls more air molecules towards the center of the planet

• Ears pop as air pressures inside and outside the ear equalize

Standards in Chemistry

Torricelli tried an experiment where he filled a very thin (straw like) test tube with mercury.

​He inverted it into a pool of mercury.

He expected all the mercury to empty, just as would happen if you replace it with water.

Air pressure on the larger surface = Pressure on the smaller​ surface by Hg

New unit of measure: ___ mm of Hg​

​

Standard atmospheric pressure = 760 mm of Hg

​Barometer

​How? If one nail can tear through your skin and cause tones of damage why doesn't adding more cause more pain?

More Nails = Less Pain?

​There are 2 ways to increase pressure, increase force or decrease surface area.

Math Note: the bigger the number you divide by the smaller the fraction, and vice versa. ​

P= F/A

If you have a dull knife you must add more force, but tomatoes don't like that. If you make the surface area smaller, you use less force and get a better cut.

Dull Knife

Which gas is under more pressure?

Pressure and Area

There are many ways we can calculation pressure.

We can use the formula for pressure P=F/A (force is in Newtons (N) and area in m²)​

​1 Pascal (Pa) = 1 N/m²

​In this case atmospheric pressure is measured in kPa.

​

​NOTE: Standard Atmospheric Pressure is measured at sea level.

Standards In Chemistry

Torricelli tried an experiment where he filled a very thin (straw like) test tube with mercury.

​He inverted it into a pool of mercury.

He expected all the mercury to empty, just as would happen if you replace it with water.

Air pressure on the larger surface = Pressure on the smaller​ surface by Hg

New unit of measure: ___ mm of Hg​

​

Standard atmospheric pressure = 760 mm of Hg

​Barometer

Standards in Chemistry

Different Units of Measure

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Sample Problem

Even gas particles have a weight

Gravity will pull more particles closer to Earth's surface.

​At the surface air is more dense.

The 2 pink boxes are the same size but the bottom has more particles. ​

Atmospheric Pressure and Altitude

​As the plane takes off surrounding pressure drops and the inner ear pressure remains the same.

Pressure on the ear drum makes your ears "plugged"

​

Air Pressure Difference

​Oops!!!

After heating to expand the air (to remove all the gas and create a vacuum) the tanker was sealed.

They did such a great job of sealing it, the atmospheric pressure was so much greater, it crushed it. ​

​

Air Pressure Difference

​Video

Click Here

Measuring Air Pressure cont.

A manometer is an instrument used to measure gas pressure in a closed container.

WHAT IS MANOMETER

an instrument for measuring the pressure acting on a column of fluid, consisting of a U-shaped tube of liquid in which a difference in the pressures acting in the two arms of the tube causes the liquid to reach different heights in the two arms

TYPES OF MANOMETER

• U-Tube Manometer.

• Differential U-Tube Manometer.

• Inverted U-Tube Manometer.

• small Manometer.

• Inclined Manometer.

• 
  

U-TUBE MANOMETER

U-tube manometer contains water or mercury in a U-shaped tube, and is usually used to measure gas pressure. One end of the U tube is exposed to the unknown pressure field (P) and the other end is connected to a reference pressure source (usually atmospheric pressure) (P_ref)

Manometer Calculations

Size matters:

• Small gas particles travel faster than large gas particles

• Small molecules effuse faster thsn larger molecules

Equation:

The speed at which a gas effuses is dependent upon the size of the molecule.

Solving the equation:

We usual make gas B the BIGGER gas and gas A the smaller gas for calculations.

Example:

How much faster does hydrogen gas travel than neon gas?

Than answer is 3 x faster....

Try this one on paper....

If an unknown gas travels 3 times faster than neon gas , what is the molar mass of the gas?

How to solve the last problem