It’s a Gas

Air is a gas

• It is a mixture of several gases.It is a mixture of several gases.

• It surrounds you all of the time.It surrounds you all of the time.

• It inflates tiresIt inflates tires

• It provides cushioning in an air mattressIt provides cushioning in an air mattress

• It transmits sound waves so you can hearIt transmits sound waves so you can hear

• So, how do you know it exists???So, how do you know it exists???

Gases

What gases are important for each of the What gases are important for each of the following: Ofollowing: O22, CO, CO22 and/or He? and/or He?

A. B. C. D.A. B. C. D.

Gases

• What gases are important for each of What gases are important for each of the following: Othe following: O22, CO, CO22 and/or He? and/or He?

• A. COA. CO22 B. O B. O22/CO/CO22 C. O C. O22 D. He D. He

The Nature of Gases

The systematic study of gases began 300 The systematic study of gases began 300 years ago. What did they learn?years ago. What did they learn?

What did my high school chemistry

teacher say?

1. Gases have mass

• Proof: Weigh a basketball deflated and Proof: Weigh a basketball deflated and inflated. The mass increases. The increase inflated. The mass increases. The increase is due to air molecules.is due to air molecules.

Mass: 2567 g

+Mass: 2571 g

What was the mass of gas added?

2. It is easy to compress a gas

• Why can you put more air in a tire, but Why can you put more air in a tire, but can’t add more water to a glass full of can’t add more water to a glass full of water?water?

• If you squeeze a gas, its volume can be If you squeeze a gas, its volume can be reduced considerably.reduced considerably.

• This is why gases are used as shock This is why gases are used as shock absorbers and in air bags.absorbers and in air bags.

3. Gases completely fill their containers

• This property explains why nowhere This property explains why nowhere around you is there an absence of air.around you is there an absence of air.

• Air in a balloon is distributed evenly Air in a balloon is distributed evenly throughout the balloon not just on the throughout the balloon not just on the bottombottom

4. Different gases can move through each other quite rapidly

• The movement of one substance through The movement of one substance through another is called DIFFUSION.another is called DIFFUSION.

• Therefore, gases diffuse easily through each Therefore, gases diffuse easily through each other.other.

• You observe diffusion when smell popcorn You observe diffusion when smell popcorn at the theater or when a skunk is nearby.at the theater or when a skunk is nearby.

5. Gases exert pressure• You have experienced the effects of You have experienced the effects of

changing air pressure when your ears “pop”changing air pressure when your ears “pop”

• You have observed air pressure when you You have observed air pressure when you inflate a balloon.inflate a balloon.

Units of Pressure

1 atm = 760 mm Hg

1 atm = 760 torr

1 atm = 101.325 kPaBarometer

Pressure = ForceArea

Sea level 1 atm

4 miles 0.5 atm

10 miles 0.2 atm

6. The pressure of a gas depends on its temperature.

• The higher the temperature of a gas, the The higher the temperature of a gas, the higher the pressure.higher the pressure.

• Your tire pressure can become dangerously Your tire pressure can become dangerously high on hot southwest summer days.high on hot southwest summer days.

Summary of Gas Properties

• Gases have mass.Gases have mass.

• It is easy to compress a gas.It is easy to compress a gas.

• Gases completely fill their containers.Gases completely fill their containers.

• Different gases can move rapidly through Different gases can move rapidly through each other.each other.

• Gases exert pressure.Gases exert pressure.

• The pressure of a gas depends on temperature.The pressure of a gas depends on temperature.

Kinetic-Molecular Theory

• All of the gas properties covered are All of the gas properties covered are explained by the kinetic-molecular theory.explained by the kinetic-molecular theory.

• Kinetic means motionKinetic means motion

• Molecular means moleculesMolecular means molecules

• Therefore, Kinetic-Molecular means motion Therefore, Kinetic-Molecular means motion of molecules.of molecules.

K-M Theory

• Gases consist of discrete molecules that have mass.

• Every molecules is independent of other gas molecules.

K-M Theory

• Individual molecules are small and far apart compared to their size.

• This assumption explains why gases can be so easily compressed.

K-M Theory

• Gas molecules are in continuous, random, straight line motion with varying velocities.

• This explains why gases immediately fill their containers.

K-M Theory

• Gases exert pressure because their particles frequently collide with the wall of the container in which they are held.

K-M Theory

• Collisions are elastic.

• Collisions occur without any loss of energy (speed)

K-M Theory

• Gas molecules exert no attraction

• or repulsion force on one another.

Attraction

Repulsion

K-M Theory: Temperature

Temperature is a Temperature is a measure of the amount of the average kinetic energy of measure of the amount of the average kinetic energy of the particles in matter. The more kinetic energy the the particles in matter. The more kinetic energy the

particles have, the higher the temperature.particles have, the higher the temperature. The The temperature of particles are typically recorded in one of temperature of particles are typically recorded in one of three ways:three ways:

1. Fahrenheit (1. Fahrenheit (ºF)ºF)

2. Celsius (2. Celsius (ºC)ºC)

3. Kelvin (K)3. Kelvin (K)Do you remember whichDo you remember which

is the standard unit????is the standard unit????

•In this analogy each billiard ball represent different gas molecules moving in random motion. Pressure result from collision of each between ball and the boundary. Each collision is perfectly elastic with each ball exhibiting no attractive or repulsive force between each other.•Motion (energy) Temperature (K)•Collision (impact) Pressure (atm)•Boundary (container size) Volume (L)

KMT: Billiard analogyKMT: Billiard analogy

Summary of KMT Postulates

• Gas particles are in constant random motion.Gas particles are in constant random motion.

• Gas particles occupy no volume.Gas particles occupy no volume.

• Collisions between gas particles are perfectly Collisions between gas particles are perfectly elastic: there is no loss of kinetic energy.elastic: there is no loss of kinetic energy.

• There are neither attractive nor repulsive forces There are neither attractive nor repulsive forces between gas particles.between gas particles.

• The higher the absolute temperature, the higher The higher the absolute temperature, the higher the the averageaverage kinetic energy of the gas. kinetic energy of the gas.

Boyle’s Law• At constant temperature, the volume of a At constant temperature, the volume of a

gas varies inversely with its pressure.gas varies inversely with its pressure.

• In other words, as the pressure increases, In other words, as the pressure increases, the volume of the gas decreases.the volume of the gas decreases.

• As the pressure decreases, the volume of As the pressure decreases, the volume of the gas increases.the gas increases.

• UP → DOWN and DOWN → UPUP → DOWN and DOWN → UP

The Math of Boyle’s Law

• PP11VV11 = P = P22VV22 • 10 liters of air at 1 atm is compressed to a 10 liters of air at 1 atm is compressed to a

pressure of 4 atm. What is the volume of pressure of 4 atm. What is the volume of the compressed air. the compressed air.

• (1 atm)(10 L) = (4 atm)V(1 atm)(10 L) = (4 atm)V22

• (1 atm)(10 L)(1 atm)(10 L) = = (4 atm)V(4 atm)V22

(4 atm) (4 atm) (4 atm) (4 atm)

The Math of Boyle’s Law (continued)

• PP11VV11 = P = P22VV22 • (1 atm)(10 L)(1 atm)(10 L) = = (4 atm)V(4 atm)V22

(4 atm) (4 atm) (4 atm) (4 atm)

• VV22 = = (1 atm)(10 l)(1 atm)(10 l)

(4 atm) (4 atm)

• VV22 = = 2.5 L2.5 L

Kinetic Molecular Theory and Boyle’s Law

• In a smaller volume, the number of collisions In a smaller volume, the number of collisions between the particles and the walls of the between the particles and the walls of the container is concentrated on a smaller area container is concentrated on a smaller area (think high heels), so the pressure is greater.(think high heels), so the pressure is greater.

• In a larger volume, the number of collisions In a larger volume, the number of collisions between the particles and the walls of the between the particles and the walls of the container is spread out over a larger area container is spread out over a larger area (think snowshoes), so the pressure is less.(think snowshoes), so the pressure is less.

Avogadro’s Hypothesis

• At constant temperature and pressure, At constant temperature and pressure, equal volumes of gases contain equal equal volumes of gases contain equal numbers of particles. numbers of particles.

• Or restated in mole-speak, at constant Or restated in mole-speak, at constant temperature and pressure, equal temperature and pressure, equal volumes of gases contain equal volumes of gases contain equal numbers of moles. numbers of moles.

The Math of Avogadro’s Hypothesis

• VV11 = = VV22

n n1 1 nn22 Where V is Volume and n Where V is Volume and n

is moles. is moles.

• One mole of ozone gas (OOne mole of ozone gas (O33) occupies 22.4 L. ) occupies 22.4 L.

The ozone decomposes to 1.5 moles of The ozone decomposes to 1.5 moles of molecular oxygen (Omolecular oxygen (O22). What is the volume ). What is the volume

of the resulting molecular oxygen?of the resulting molecular oxygen?

• 22.4 L22.4 L == V V22___ ___

1 mol1 mol 1.5 mol 1.5 mol

The Math of Avogadro’s Hypothesis (continued)

• 22.4 L22.4 L == V V22___ ___

1 mol1 mol 1.5 mol 1.5 mol

• 22.4 L ( 1.5 mol )22.4 L ( 1.5 mol ) == VV22

1.0 mol 1.0 mol

• VV22 = 33.6 L = 33.6 L

Kinetic Molecular Theory and Avogadro’s Hypothesis

• As the number of gas particles increases, the As the number of gas particles increases, the frequency of collisions with the walls of the frequency of collisions with the walls of the container must increase. This, in turn, leads to container must increase. This, in turn, leads to an increase in the pressure of the gas. Flexible an increase in the pressure of the gas. Flexible containers, such as a balloon, will expand until containers, such as a balloon, will expand until the pressure of the gas inside the balloon once the pressure of the gas inside the balloon once again balances the pressure of the gas outside. again balances the pressure of the gas outside. Thus, the volume of the gas is proportional to Thus, the volume of the gas is proportional to the number of gas particles. the number of gas particles.

Summary• Boyle’s Law: PBoyle’s Law: P11VVII = P = P22VV22

• Avogadro’s Hypothesis:Avogadro’s Hypothesis:VV11 = = VV22

n n1 1 nn22

• Put the right numbers in the right Put the right numbers in the right places.places.

• Isolate the unknown variable and Isolate the unknown variable and solve.solve.