CHAPTER 14 Gases 14.1 Pressure and Kinetic Energy

Chapter 9 Water and Solutions Chapter 10 Chemical Reactions Chapter 11 Stoichiometry Chapter 12 Reaction Rates and Equilibrium Chapter 13 Acids and Bases So far we have primarily focused on reactions that happen in the liquid phase

We will now discuss the gas phase Chapter 9 Water and Solutions Chapter 10 Chemical Reactions Chapter 11 Stoichiometry Chapter 12 Reaction Rates and Equilibrium Chapter 13 Acids and Bases Chapter 14 Gases We will now discuss the gas phase

Properties of gases No interaction between atoms or molecules, except during collisions Straight trajectory until a collision occurs Mostly empty space Gases consist of widely separated atoms or molecules in constant, random motion

Properties of gases Gases have a unique set of physical properties: Gases are translucent or transparent. Gases have very low densities when compared to liquids or solids. Gases are highly compressible compared to liquids and solids. Gases can expand or contract to fill any container. Mostly empty space

Properties of gases Gases have a unique set of physical properties: Gases are translucent or transparent. Gases have very low densities when compared to liquids or solids. Gases are highly compressible compared to liquids and solids. Gases can expand or contract to fill any container. These can be explained by the kinetic molecular theory

Gases consist of atoms or molecules with a lot of space in between, that are in constant, random motion kinetic molecular theory: the theory that explains the observed thermal and physical properties of matter in terms of the average behavior of a collection of atoms and molecules.

Properties of gases Evidence for the atomic/molecular nature of matter: As a liquid, water molecules are packed very close together, so water appears to be a continuous substance, not made of individual particles (molecules).

Properties of gases Evidence for the atomic/molecular nature of matter: When heated, the water seems to disappear, but it is still there as a gas.

Properties of gases Evidence for the atomic / molecular nature of matter: If the liquid and gas are both made from the same molecules (H2O), you can explain the “disappearance” by assuming that the molecules are much more spread out in the gas phase.

Brownian motion Brownian motion can be seen by magnifying diluted milk and observing tiny fat globules getting knocked around by the surrounding water molecules

Brownian motion What Brownian motion tells us: This was discussed in Chapter 3: Temperature, Energy, and Heat What Brownian motion tells us: Matter consists of discrete particles (molecules or atoms) Molecules (or atoms) are in constant, vigorous motion as a result of temperature

Brownian motion Brownian motion provides a peek into the microscopic world of atoms to see details that are normally hidden by the law of averages, and the enormous number of incredibly small atoms.

Pressure Gases have pressure A gas can easily change shape. When you push down on a balloon, sideways forces push the sides of the balloon outward.

Pressure Two jars contain air at a pressure of 35 psi. One jar is 3 inches in diameter and the other is 1 inch in diameter. The larger gar blows its lid off, but the smaller one does not. Calculate the force on each lid resulting from pressure. (Hint: Area of a disc = πr2)

Pressure Two jars contain air at a pressure of 35 psi. One jar is 3 inches in diameter and the other is 1 inch in diameter. The larger gar blows its lid off, but the smaller one does not. Calculate the force on each lid resulting from pressure. (Hint: Area of a disc = πr2) Asked: Force on the lid from pressure Given: P = 35 psi Relationships:

Pressure Two jars contain air at a pressure of 35 psi. One jar is 3 inches in diameter and the other is 1 inch in diameter. The larger gar blows its lid off, but the smaller one does not. Calculate the force on each lid resulting from pressure. (Hint: Area of a disc = πr2) Asked: Force on the lid from pressure Given: P = 35 psi Relationships: Solve: For the 3 inch diameter jar:

Pressure Two jars contain air at a pressure of 35 psi. One jar is 3 inches in diameter and the other is 1 inch in diameter. The larger gar blows its lid off, but the smaller one does not. Calculate the force on each lid resulting from pressure. (Hint: Area of a disc = πr2) Asked: Force on the lid from pressure Given: P = 35 psi Relationships: Solve: For the 3 inch diameter jar: For the 1 inch diameter jar:

Pressure Two jars contain air at a pressure of 35 psi. One jar is 3 inches in diameter and the other is 1 inch in diameter. The larger gar blows its lid off, but the smaller one does not. Calculate the force on each lid resulting from pressure. (Hint: Area of a disc = πr2) Asked: Force on the lid from pressure Given: P = 35 psi Relationships: Solve: For the 3 inch diameter jar: For the 1 inch diameter jar: Answer: The lid of the 3 inch jar feels a force of 247 lb; the lid of the 1 inch jar only feels a force of 27.5 lb.

Earth is covered with a thin layer of air Image credit: NASA (photo from Apollo 7 spacecraft)

Atmospheric pressure How much pressure does the surrounding air apply on the palm of your hand?

Atmospheric pressure How much pressure does the surrounding air apply on the palm of your hand? The pressure you would feel if a 260 lb person were standing on your hand!

Atmospheric pressure How much pressure does the surrounding air apply on the palm of your hand? The pressure from the air around us is significant! Your hand is not forced to the floor because the same pressure acts on the other side of your hand.

Units of pressure Standard pressure = 14.69 = 14.69 psi = 1.000 atm = 760.0 mmHg = 101,305 Pa lb in2 Convert 60,000 lb/in2 of pressure to atmospheres (atm).

Units of pressure Standard pressure = 14.69 = 14.69 psi = 1.000 atm = 760.0 mmHg = 101,305 Pa lb in2 Convert 60,000 lb/in2 of pressure to atmospheres (atm). Relationships: Standard pressure = 14.69 lb/in2 = 1 atm

Units of pressure Standard pressure = 14.69 = 14.69 psi = 1.000 atm = 760.0 mmHg = 101,305 Pa lb in2 Convert 60,000 lb/in2 of pressure to atmospheres (atm). Relationships: Standard pressure = 14.69 lb/in2 = 1 atm Solve:

Atmospheric pressure The air pressure at the top of Mt. Everest is only one third the air pressure at sea level (0.33 atm compared to 1.00 atm)

Atmospheric pressure A barometer Empty space Gravity pulls mercury down the tube Air pressure pushes mercury up the tube A barometer As air pressure changes, the height of the mercury column changes. barometer: an instrument that measures atmospheric pressure.

Variations in atmospheric pressure affect the weather

Kinetic theory of pressure Pressure comes from the collisions of the many, many atoms inside and outside the balloon.

A gas will expand to fill any size Kinetic theory of pressure Remember: Gases consist of atoms or molecules with a lot of space in between, that are in constant, random motion A gas will expand to fill any size (or shaped) container

Kinetic theory of pressure The average force from molecular impacts depends on two things: 1. Faster (hotter) molecules mean more force per impact and higher pressure. Higher temperature means harder collisions and higher pressure

Kinetic theory of pressure The average force from molecular impacts depends on two things: 1. Faster (hotter) molecules mean more force per impact and higher pressure. 2. More molecules per cubic centimeter (higher density) mean more impacts and therefore higher pressure. Higher density means more collisions and higher pressure

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph!

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph! Random motion A typical air molecule has 140,000 collisions in a centimeter

Diffusion Gas molecules move around quite fast. The average speed of a nitrogen molecule (N2) in air is 417 m/s, or 933 mph! The average speed of an oxygen molecule (O2) in air is 390 m/s, or 873 mph Random motion Heavier molecules move slower

Diffusion diffusion: the spreading of molecules through their surroundings through constant collisions with neighboring molecules.

Boltzmann’s constant The average molecular speed is derived by assuming all the energy is kinetic energy of motion

Boltzmann’s constant

Boltzmann’s constant As temperature increases, the average speed of molecules increases. As temperature increases, the spread of molecular speeds increases.

Boltzmann’s constant As temperature increases, the average speed of molecules increases. As temperature increases, the spread of molecular speeds increases.

The graph is known as a Maximilian distribution Boltzmann’s constant As temperature increases, the average speed of molecules increases. As temperature increases, the spread of molecular speeds increases. The graph is known as a Maximilian distribution

No interaction between atoms or molecules, except during collisions The basis of kinetic molecular theory, which explains gas behavior Straight trajectory until a collision occurs Mostly empty space Gases consist of widely separated atoms or molecules in constant, random motion

Pressure increases when: Standard pressure = 14.69 = 14.69 psi = 1.000 atm = 760.0 mmHg = 101,305 Pa lb in2 Pressure increases when: the temperature (speed of molecules) increases. the density (number of molecules) increases. The energy of molecules only depends on temperature therefore, heavier molecules move slower. Diffusion is the slow spreading of one type of molecules within another type.