13.1 Describing the Properties of Gases

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13.1 Describing the Properties of Gases

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Today…

We are going to look at the characteristics of gases:

-Pressure

-Volume

-Temperature

-Amount

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What are some things gases can do?

Think about hurricanes and the damage they cause.

Even though we move through gases easily, they still can cause significant damage.

They can exert a huge amount of pressure.

http://www.secondawareness.com/wp-content/uploads/2007/01/stormglobalwarmingawareness.jpg



PressurePressure is the amount of force on a given

area.

Gases exert pressure on its surroundings.

You can see this when you blow up a balloon; the air pushes against the sides of the balloon.

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Pressure

We are surrounded by air, and the air above us is pushing down on us.

The air around us is exerting a lot of pressure on us.

We use this pressure to our advantage.Without this pressure, our lungs would not

function. (We’ll discuss this a little later.)

http://www.asc-csa.gc.ca/images/spacesuit_pressure.jpg

Pressure

Before we get into the way lungs work, we are quickly going to talk about:

-how we measure pressure

-units of pressure

-what affects atmospheric pressure



Measuring Pressure

To measure atmospheric pressure (the pressure due to the air around us), we use a device called a “barometer.”

This is tube filled with mercury and inverted in a dish a mercury.

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Measuring PressureWhy does the mercury stay

in the tube?The pressure from the

atmosphere pushes down on the surface of the mercury in the dish, which keeps the mercury in the tube.

The more pressure pushing down, the higher the mercury in the tube goes.

http://www.hko.gov.hk/education/edu01met/wxobs/pressure/pres-fig2e.jpg

QuickTime™ and a decompressorare needed to see this picture.

Units of PressureUsing a barometer, we saw that the

more pressure there is, the higher the mercury climbs.

We use the height of mercury (in millimeters) to describe the amount of pressure.

The units of pressure are “mm Hg” (millimeters of mercury).

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Units of Pressure

There are other units of pressure…-a torr - named after Torricelli, the man who invented the barometer, same thing as “mm Hg.”-a standard atmosphere (atm) - the pressure at sea level.- a pascal (Pa) = 1 N / m2

-pounds per square inch (psi)

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Units of Pressure

1.000 atm = 760.0 mm Hg = 760.0 torr

=101,325 Pa = 14.69 psi

28 psi =



What Affects Atmospheric Pressure?

1. Weather - can cause more or less air to be packed into a certain space.

- Low pressure = stormy weather

- High pressure = nice weather

2. Altitude - the higher you go, the less air there is pushing down on you.

http://csc.gallaudet.edu/soarhigh/density.gif





Pressure and Volume

What happens to the volume when you apply pressure to a balloon?

The volume gets smaller.

What happens when you open a shaken pop can?

The pressure is lowered and the volume of the gases gets larger.

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Pressure and VolumeSome things to notice are:

When pressure increases, volume decreases.

When pressure decreases, volume increases.

This shows an inverse relationship between the two.

http://www.antonine-education.co.uk/Physics_AS/Module_2/Topic_8/Boyle_1.gif



Boyle’s Law

Pressure X Volume = constant

P x V = k

(This happens at a specific temperature and a given amount of gas)

http://www.asc-csa.gc.ca/images/neemo_graph_boyles_law.jpg



Boyle’s LawAnother way to look at things:

We can see how a change in pressure will change the volume (and vice versa).

P1V1 = P2V2

http://www.colorado.edu/intphys/Class/IPHY3430-200/image/17-5.jpg

Our Lungs use Boyle’s Law!

The diaphragm’s main function is to help us breath.

How does it do that?When the diaphragm expands and contracts, it is

changing the volume that your lungs can hold.

http://www.emptynosesyndrome.org/pics/diaphragm%20animation.gif


As you breath in, your lungs expand. This lowers the pressure in your lungs.

Air flows from high pressure to low pressure.

So if the pressure in your lungs is lower than the pressure around you, the high pressure will force air into your lungs.




This also explains why it is harder to breath at higher altitudes.

At higher altitudes, there is enough oxygen to support you. However, there is less pressure.

This means your lungs have to expand more to lower the pressure in your lungs enough for air to flow in.

Using Boyle’s LawConsider a 1.5 liter sample of gas at a pressure

of 56 torr. If the pressure is changed to 150 torr, what will the new volume of the gas be?



Volume and Temperature

What happens to the volume of gas when it is heated up?

It expands (takes up more volume).

When it is cooled down, it condenses (takes up less volume).

http://bt.kieding.org/projects/watercooling/images/condensation1.jpg

Volume and TemperatureSo more heat = more volume Less heat = less volume

Charles’s Law: Volume and temperature are directly proportional.

Charles’s Law

V = bT or V / T = constant

V1 = V2

T1 T2

(At constant pressure and amount of gas)

Using Charles’s Law:A 2.0-L sample of air is collected at 298 K and

then cooled to 278 K. The pressure is held constant at 1.0 atm. What is the new volume?



Volume and Moles

Remember, moles is just another word for “the number of particles”

Think about blowing up a balloon…

When you blow into it, you are putting more particles into the balloon.

Then the volume increases.

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Volume and Moles

The more particles, the more volume.

Avogadro’s Law: The volume is directly proportional to the number of moles of gas.

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Avogadro’s Law

Volume = (some constant) x (# moles)

V = a x n or V / n = constant

V1 = V2

n1 n2

(At constant T and P)

Using Avogadro’s LawSuppose we have a 12.2-L sample of 0.50 mol of

oxygen gas, O2, at 1 atm and 25oC. If all of this is converted to ozone, O3, what will the new volume be?

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13.1 Describing the Properties of Gases