Mr. Matthew Totaro Legacy High School Regular Chemistry Properties of Gases Mr. Matthew Totaro Legacy High School Regular Chemistry

Gases

Properties of Gases Expand to completely fill their container. Take the shape of their container. Low density. Much less than solid or liquid state. Compressible. Mixtures of gases are always homogeneous. Fluid.

Properties—Indefinite Shape and Indefinite Volume Because the gas molecules have enough kinetic energy to overcome attractions, they keep moving around and spreading out until they fill the container. As a result, gases take the shape and the volume of the container they are in.

Properties—Compressibility Because there is a lot of unoccupied space in the structure of a gas, the gas molecules can be squeezed closer together.

Gas Properties Explained— Low Density Because there is a lot of unoccupied space in the structure of a gas, gases do not have a lot of mass in a given volume, the result is that they have low density.

The Structure of a Gas Gases are composed of particles that are flying around very fast in their container(s). They move in straight lines until they encounter either the container wall or another particle, then they bounce off. If you were able to take a snapshot of the particles in a gas, you would find that there is a lot of empty space in there.

Kinetic Molecular Theory The particles of the gas (either atoms or molecules) are constantly moving. The attraction between particles is negligible. When the moving particles hit another particle or the container, they do not stick, but they bounce off and continue moving in another direction. Like billiard balls.

Kinetic Molecular Theory of Gases There is a lot of empty space between the particles in a gas. Compared to the size of the particles. The average kinetic energy of the particles is directly proportional to the Kelvin temperature. As you raise the temperature of the gas, the average speed of the particles increases. But don’t be fooled into thinking all the particles are moving at the same speed!!

Kinetic Molecular Theory

Pressure of a Gas

Gas Particles Pushing Gas molecules are constantly in motion. As they move and strike a surface, they push on that surface. Push = force. If we could measure the total amount of force exerted by gas molecules hitting the entire surface at any one instant, we would know the pressure the gas is exerting. Pressure = force per unit area.

The Effect of Gas Pressure The pressure exerted by a gas can cause some amazing and startling effects. Whenever there is a pressure difference, a gas will flow from area of high pressure to low pressure. The bigger the difference in pressure, the stronger the flow of the gas. If there is something in the gas’ path, the gas will try to push it along as the gas flows.

Which Way Would Air Flow? Two filled balloons are connected with a long pipe. One of the balloons is plunged down into the water. Which way will the air flow? Will air flow from the lower balloon toward the top balloon; or will it flow from the top balloon to the bottom one? 15

Soda Straws and Gas Pressure The pressure of the air inside the straw is the same as the pressure of the air outside the straw—so liquid levels are the same on both sides. The pressure of the air inside the straw is lower than the pressure of the air outside the straw—so liquid is pushed up the straw by the outside air.

The Pressure of a Gas Pressure is the result of the constant movement of the gas molecules and their collisions with the surfaces around them. The pressure of a gas depends on several factors: Number of gas particles in a given volume. Volume of the container. Average speed of the gas particles.

Density and Pressure When more molecules are added, more molecules hit the container at any one instant, resulting in higher pressure. Also higher density.

Gas Pressure

Air Pressure The atmosphere exerts a pressure on everything it contacts. On average 14.7 psi. The atmosphere goes up about 370 miles, but 80% is in the first 10 miles from Earth’s surface. This is the same pressure that a column of water would exert if it were about 10.3 m high.

Atmospheric Pressure

Measuring Air Pressure Use a barometer. Column of mercury supported by air pressure. Force of the air on the surface of the mercury balanced by the pull of gravity on the column of mercury. gravity

Atmospheric Pressure and Altitude The higher up in the atmosphere you go, the lower the atmospheric pressure is around you. At the surface, the atmospheric pressure is 14.7 psi, but at 10,000 ft it is only 10.0 psi. Rapid changes in atmospheric pressure may cause your ears to “pop” due to an imbalance in pressure on either side of your ear drum.

Effect of Air Pressure 14,000 Feet 9,000 Feet 1,000 Feet

Pressure Imbalance in the Ear If there is a difference in pressure across the eardrum membrane, the membrane will be pushed out—what we commonly call a “popped eardrum.”

Common Units of Pressure Average air pressure at sea level Pascal (Pa) 101,325 Kilopascal (kPa) 101.325 Atmosphere (atm) 1 (exactly) Millimeters of mercury (mmHg) 760 (exactly) Inches of mercury (inHg) 29.92 Torr (torr) Pounds per square inch (psi, lbs./in2) 14.7

Practice—Convert 45.5 psi into kPa, Continued Given: Find: 45.5 psi kPa Concept Plan: Relationships: 1 atm = 14.7 psi, 1 atm = 101.325 kPa psi atm kPa Solution: Check: Since kPa are smaller than psi, the answer makes sense.