GasesGases

I. Real Gases (we will not study these much)

A. Do NOT apply the Kinetic Molecular Theory

1. The particles in a real gas can NOT be thought of as small, well spread points in space.

2. The particles in a real gas have significant volume and tend to be closer together.

3. Interactions between the particles in a real gas are NOT negligible.

Kinetic Theory of MatterKinetic Theory of Matter

All particles are in constant motion:– The faster the particles move, the more

heat the substance will have.– The slower the particles move, the less

heat the substance will have.

Add Heat

The opposite is true too:

– When heat is removed particles get slower and move closer together

Remove Heat

How can you identify a real gas?

• Gases at low temperature tend to act as real gases.

• Gases at high pressure tend to act as real gases.

• Gases of larger or more polar particle units tend to act as real gases.

Fundamentally, a real gas acts as though it is approaching the liquid phase. The particles are slowing down (low temperature) and moving closer together (high pressure).

II. Ideal Gases (we WILL study these a lot!)

5 Assumptions of an ideal gas:

1. Gas consists of large #’s of particles that are far apart relative to their size.

- most volume occupied by gas is empty space

2. Collisions between ideal gas particles are elastic (the particles don’t lose energy).

3. Ideal gas particles are considered to have kinetic energy directly related to their temperature & mass.

II. Ideal Gases (we WILL study these a lot!)

5 Assumptions continued:

4. Ideal gas particles are considered to be in continuous random motion.

5. No force of attraction between particles

How can you identify an ideal gas?

• Gases at high temperature tend to act as ideal gases.

• Gases at low pressure tend to act as ideal gases.

• Gases of smaller, non-polar particle units tend to act as ideal gases.

Fundamentally, an ideal gas acts as a bunch of small balls randomly bouncing around with no

attractions between them.

Real or Ideal Gas Behavior?

Nitrogen Gas at room temperature and 1 atm

Real or Ideal Gas Behavior?

Water vapor at 25oC and 1 atm

Real or Ideal Gas Behavior?

Helium Gas at room temperature and 1 atm

III. Four Important Characteristics (Ideal Gases)

A. Amount (moles)B. Volume (liters)C. Temperature (K)D. Pressure (atm)

STP means Standard Temperature & Pressure

O oC 1 atm 273 K

One mole of an ideal gas at STP = 22.4 L

At STP, 1 mole any gas takes At STP, 1 mole any gas takes up 22.4 Liters of space.up 22.4 Liters of space.

IV. Characteristic Details

A. Amount 1. Indicated in moles (mol)

2. One mole of an ideal gas at STP = 22.4 L

B. Volume – the space occupied1. Indicated in liters (L)

2. Most of the space is empty – particles are far apart

3. One mole of an ideal gas at STP = 22.4 L

4. Easily Changed – Expand or Contract

IV. Characteristic Details (cont.)

C. Temperature 1. Indicated in Kelvin (K)

2. A measure of particle speed- independent of mass

3. Related to heat, but NOT heat- heat depends on mass

4. Absolute Zero = O K or -273oC

To Convert oC to K:

K = oC + 273

The container on the right experiences a greater number of particle collisions per unit area – the pressure is greater!

Greater Pressure

D. Pressure (Cont.)

3. Dalton’s Law

Total Pressure = Partial Pressures Added Ptotal = P1 + P2 + P3…

4. Air Pressure

Pair = Poxygen + Pnitrogen + Pother

14.7 psi (lbs per in2) or 1 atm

Measured with a barometer – air pushes mercury (Hg) up a tube.

Air Pressure – Related to the weight of a column of air on top of us!

D. Pressure (Cont.)

5. Conversions

1 atm = 760 mm Hg = 14.7 psi

DON’T FORGET

One mole of an ideal gas at 1 atm and 273 K = 22.4 L

But, it must be…at ST….P

If the barometer shows 765 If the barometer shows 765 mm Hg, how many mm Hg, how many

Atmospheres is this?Atmospheres is this?

atmmmHgatm

mmHg

atmmmHg

01.1760

1765

?765

V. Gases in the Laboratory

To get the pressure of To get the pressure of just the hydrogen in the just the hydrogen in the

flask:flask:• You subtract the partial pressure of the water

vapor from the total pressure in the flask.  (Table on page 859)

• PH2 gas = Ptotal - PH2O vapor • IE: If the pressure in the room is 762 mmHg and

the temp. in the room is 21 C, then the partial pressure of just H2 would be:

• 762-19 mm Hg = 743 mm Hg• (If you look up Water Vapor Pressure on Pg. 859,

it’s 19 mm Hg at 21 C.)