Warmup 11/20/15

Describe a gas. What's it actually made of? How does it work?

Objective Tonight’s HomeworkTo learn how we model a gas physically

pp 382: 5, 6, 7

Notes on Kinetic TheoryMolecules in Motion

Imagine we could look at a sample of a gas up close. On the molecular level.

If solids are just molecules bound together, then what’s a gas?

Notes on Kinetic TheoryMolecules in Motion

Imagine we could look at a sample of a gas up close. On the molecular level.

If solids are just molecules bound together, then what’s a gas?

A gas is just molecules bouncing around a container at high speeds. These molecules bounce off the walls and off each other. How fast are we talking here? Try 1,000 mph with 4 billion collisions every molecule every second.

Notes on Kinetic TheoryPressure

As each molecule hits the edge of the container, it pushes on it a little. This push isn’t much, but with quadrillions of molecules it adds up.

Notes on Kinetic TheoryPressure

As each molecule hits the edge of the container, it pushes on it a little. This push isn’t much, but with quadrillions of molecules it adds up.

We measure this pressure in terms of force per square area. One pascal is a pressure of 1 Newton spread out over 1 square meter.

1 Newton is a small force and 1 square meter quite big, so 1 pascal is quite small.

Notes on Kinetic TheoryAir exerts quite a bit of pressure. Standard atmospheric pressure at sea level is 101.325 kilopascals.

This is equivalent to 14.7 pounds per square inch.

That means every inch of your body is feeling almost 15 pounds of pressure pushing on it just from the air!

Notes on Kinetic TheoryMeasuring Pressure

So how did we get that number? Measuring pressure can be quite difficult. Essentially, all the devices work like this:

Notes on Kinetic TheoryIn an open arm manometer, the pressure of the gas we want is balanced against the pressure of the air outside.

We have liquid mercury in the middle. Whichever gas has more pressure will push the mercury toward the other side.

The pressure of the gas wewant is the pressure of theoutside air plus the height difference in the tube.

Notes on Kinetic TheoryIn a closed arm manometer, the end that was open to the air is now sealed and emptied. If both sides are empty, the mercury will be level. When pressure is added to the chamber, it pushes the mercury up the other end.

The pressure can becalculated as simply theheight difference in one end of the mercury from the other.

Notes on Kinetic TheorySo what’s standard in terms of height difference?

We said standard air pressure is 101.325 kPa. Using a closed arm manometer, this same measurement comes out to 760 mm of mercury.

As for a conversion factor…

1kPa = 7.501 mm of Hg

Notes on Kinetic TheoryExample: “An open end manometer shows a difference of 245 mm of mercury favoring the air end. If the air pressure nearby is standard, what is the pressure of the gas in the chamber?”

Notes on Kinetic TheoryExample: “An open end manometer shows a difference of 245 mm of mercury favoring the air end. If the air pressure nearby is standard, what is the pressure of the gas in the chamber?”

We know that the pressure we want is given by:P0 = Pair + hPair = 101.325 kPa

H = 245 mm • = 32.6 kPa

P0 = 101.325 kPa + 32.6 kPa

P0 = 133.925 kPa

245 mm 1 kPa _ 1 7.501mm

Notes on Kinetic TheoryKinetic Energy and Temperature

If we look at the motion of molecules, there’s something else we can conclude. Motion is related to temperature. In fact, temperature can be defined as motion of molecules.

Notes on Kinetic TheoryKinetic Energy and Temperature

If we look at the motion of molecules, there’s something else we can conclude. Motion is related to temperature. In fact, temperature can be defined as motion of molecules.

If a group of molecules aren’t moving at all, we say it has a temperature of absolute zero. Since you can’t have anything move slower than not moving at all, this temperature represents the coldest that something can ever get.

Nothing in the universe can ever be as cold as absolute zero. (Because of quantum stuff.)

Notes on Kinetic TheoryNeither Fahrenheit or Celsius have clean numbers for absolute zero.

In Fahrenheit, absolute zero is -459.6°In Celsius, absolute zero is -273.15°

Notes on Kinetic TheoryNeither Fahrenheit or Celsius have clean numbers for absolute zero.

In Fahrenheit, absolute zero is -459.6°In Celsius, absolute zero is -273.15°

Given this, in the 1800s, a scientist named Lord Kelvin created a scale based on absolute zero.

This scale is the same as Celsius but shifted so that absolute zero is 0 K.

Notes on Kinetic TheoryNeither Fahrenheit or Celsius have clean numbers for absolute zero.

In Fahrenheit, absolute zero is -459.6°In Celsius, absolute zero is -273.15°

Given this, in the 1800s, a scientist named Lord Kelvin created a scale based on absolute zero.

This scale is the same as Celsius but shifted so that absolute zero is 0 K.K = °C + 273.15

Exit Question

What does the pressure of a gas physically represent?

a) How tightly packed the gas molecules areb) How “solid” the gas molecules arec) The temperature and density of the gasd) How fast gas molecules hit against each othere) How hard and how fast molecules are bouncing against

a surfacef) None of the above