Chapter 8

Gases

What is pressure?

the amount of force applied over a certain area. In chemistry, pressure usually comes from particles colliding with a surface.

P= F/A

• P = pressure

• F = force

• A = area

If the same force is applied over a smaller area, pressure increases. If more gas particles hit the walls of a container, pressure also increases.

Why do gases create pressure?

Gas particles are always moving. As they move, they constantly collide with the walls of their container. Those collisions create force, and that force spread over the wall area is pressure. More collisions, or stronger collisions, mean greater pressure.

What happens to pressure when there are more particles inside a container?

increases because more particles means more collisions with the walls of the container. More collisions = more force on the walls = greater pressure.

What is atmospheric pressure?

the pressure caused by Earth’s atmosphere pushing down on everything. Air has mass, and gravity pulls that air downward, so the atmosphere exerts pressure on us and on every object at Earth’s surface.

Why don’t we constantly feel atmospheric pressure?

We usually do not notice itbecause our bodies are already adjusted to it and our internal body pressure balances the outside pressure. We mostly notice pressure changes, not the pressure itself.

What are examples of situations where we notice pressure changes?

• ears popping on an airplane

• diving underwater

• weather changes
  These situations change the pressure around us enough for us to feel it.

What are the common units of pressure?

• atm = atmosphere

• mmHg = millimeters of mercury

• torr = equal to mmHg

• kPa = kilopascal

• Pa = pascal, the SI unit

• psi = pounds per square inch

• bar = another pressure unit often used in weather and science contexts

What are the standard pressure conversions?

1 atm=760 mmHg=760 torr=101.3 kPa=101325 Pa1 atm=760 mmHg=760 torr=101.3 kPa=101325 Pa

Also:

1 torr=1 mmHg1 torr=1 mmHg

These are used all the time in gas law and pressure problems.

What is a barometer?

an instrument used to measure atmospheric pressure. a glass tube closed at one end, filled with a nonvolatile liquid like mercury, then inverted into a container of the same liquid. Atmospheric pressure pushes on the liquid in the container and supports a column of liquid in the tube.

Why is a nonvolatile liquid used in a barometer?

this is used because it does not evaporate easily. This makes the liquid stable and allows the barometer to measure pressure accurately without the liquid disappearing into vapor.

How does a barometer measure atmospheric pressure?

the atmosphere pushes down on the liquid in the reservoir. That pressure supports a column of liquid in the tube. The height of the liquid column reflects the atmospheric pressure. A higher column means greater atmospheric pressure.

What is hydrostatic pressure?

the pressure exerted by a liquid due to gravity.

P=hρg

Where:

• h = height of the liquid column

• p = density of the liquid

• g = acceleration due to gravity

This idea explains how barometers and manometers work.

What must be true for a liquid column in a barometer to stay at a certain height?

The atmospheric pressure pushing on the liquid must balance the hydrostatic pressure of the liquid column. When those are equal, the liquid remains at that height.

What is a manometer?

a device used to measure the pressure of a trapped gas. It uses the height difference between two liquid columns to compare gas pressure to either atmospheric pressure or another gas pressure.

What does the height difference in a manometer represent?

The difference in liquid height between the two arms represents a pressure difference. If one side pushes harder, it forces the liquid lower on that side and higher on the other side.

What is the difference between an open-end manometer and a closed-end manometer?

• In an open-end manometer, one side is open to the atmosphere, so the gas pressure is compared to atmospheric pressure.

• In a closed-end manometer, one side is sealed, often with a vacuum, so the liquid height directly represents the gas pressure.

What unit must temperature always be in for gas law problems?

Temperature must always be in Kelvin for gas law problems. Kelvin is an absolute temperature scale and is required for formulas like the ideal gas law, Charles’s law, and Gay-Lussac’s law.

K=∘C+273.15K=∘C+273.15

Why must temperature be in Kelvin for gas laws?

Gas laws require absolute temperature. Celsius can be negative, which would break the proportional relationships in gas laws. Kelvin starts at absolute zero, so it works correctly in the equations.

What is Boyle’s law?

This law describes the relationship between pressure and volume when temperature and amount of gas stay constant.

P1V1=P2V2

P1​V1​=P2​V2​

Pressure and volume are inversely related:

• if pressure increases, volume decreases

• if pressure decreases, volume increases

When do you use Boyle’s law?

Use this law when:

• pressure changes

• volume changes

• temperature stays constant

• amount of gas stays constant

What is Charles’s law?

]This law describes the relationship between volume and temperature when pressure and amount of gas stay constant.

V1/T1=V2/T2

Volume and temperature are directly related:

• if temperature increases, volume increases

• if temperature decreases, volume decreases

When do you use Charles’s law?

Use this law when:

• volume changes

• temperature changes

• pressure stays constant

• amount of gas stays constant

What is Gay-Lussac’s law?

This law describes the relationship between pressure and temperature when volume and amount of gas stay constant.

P1​​/T1=T2​/P2​​

Pressure and temperature are directly related:

• if temperature increases, pressure increases

• if temperature decreases, pressure decreases

When do you use Gay-Lussac’s law?

Use law when:

• pressure changes

• temperature changes

• volume stays constant

• amount of gas stays constant

What are the five layers of Earth’s atmosphere?

The five layers are:

1. Troposphere

2. Stratosphere

3. Mesosphere

4. Thermosphere

5. Exosphere

What is the troposphere?

The is the lowest layer of the atmosphere, closest to Earth. It contains the air humans breathe and it is where weather happens. In this layer, temperature generally decreases as altitude increases.

What is the stratosphere?

This is above the troposphere and contains the ozone layer. The ozone layer absorbs much of the sun’s radiation. Airplanes often fly in the lower stratosphere to avoid turbulence found in the troposphere.

What is the mesosphere?

The layer where meteors burn up. The air is thinner here, meaning molecules are farther apart than in the lower atmosphere.

What is the thermosphere?

This layer is strongly affected by the sun and can reach very high temperatures. Even though it is very hot, the air is extremely thin.

What is the exosphere?

The outermost layer of the atmosphere. It is where the atmosphere gradually fades into space.

What does it mean that the Ideal Gas Law “relates” pressure, volume, moles, and temperature?

It means these four variables are connected mathematically. If one changes, at least one of the others must also change to keep the equation true.

Examples:

• If temperature increases and moles stay the same, pressure or volume must change.

• If more gas particles (n) are added to a container, pressure may increase if volume stays the same.

• If volume decreases, pressure usually increases when temperature and moles stay constant.

• If pressure decreases, volume may increase under constant temperature.

So “relates” means the formula shows how changing one variable affects the others.

What is the Kinetic Molecular Theory of gases?

Theory explains gas behavior by describing gas particles as tiny particles in constant random motion.

Main ideas:

1. Gas particles are very small compared to the space between them.

2. Most of a gas container is empty space.

3. Particles move constantly in straight lines until collisions occur.

4. Collisions are elastic (no net energy lost).

5. Gas particles do not attract or repel each other significantly.

6. Average kinetic energy depends on temperature.

What causes gas pressure according to KMT?

is caused when moving gas particles collide with the walls of the container.

More collisions = higher pressure.
Harder collisions = higher pressure.

What does temperature measure in a gas?

this measures the average kinetic energy of gas particles.

Higher temperature means particles move faster on average.

Use Kelvin in gas calculations.

Why can gases be compressed easily?

Gases compress easily because there is a large amount of empty space between particles.

Particles can be pushed closer together.

Why do gases expand to fill any container?

Gas particles move randomly and freely in all directions, so they spread out until they occupy all available space.

Gas has no loyalty to one corner of the container.

What is diffusion of a gas?

the spreading and mixing of gas particles from an area of higher concentration to an area of lower concentration due to random motion.

Example: perfume smell spreading across a room.

What is effusion of a gas?

the escape of gas particles through a tiny hole into a vacuum or lower-pressure area.

Example: helium leaking slowly from a balloon.

What is the difference between diffusion and effusion?

• Diffusion: gases mix and spread through space.

• Effusion: gas escapes through a tiny opening.

Diffusion = spreading
Effusion = sneaking out

What is Graham’s Law?

This law states that lighter gases move faster and therefore diffuse or effuse faster than heavier gases.

• Rate = speed of diffusion/effusion

• M = molar mass

Which gas effuses faster: He or O₂?

Helium effuses faster because it has a much lower molar mass.

• He = 4 g/mol

• O₂ = 32 g/mol

Lighter gas wins the race.

Why do lighter gases diffuse faster?

At the same temperature, lighter particles move faster than heavier particles because they require less mass to move at higher speeds.

What is an ideal gas?

A gas that is a theoretical gas

It assumes:

• No particle volume

• No intermolecular attractions

Why are real gases called non-ideal gases?

This is because actual gas particles:

1. Occupy space (have volume)

2. Attract each other somewhat

Because of this, they do not always follow PV = nRT exactly.

When do gases behave most ideally?

• High temperature

• Low pressure

Why:

• Particles move fast

• Far apart

• Attractions matter less

When do gases behave least ideally?

• Low temperature

• High pressure

Why:

• Particles move slower

• Closer together

• Attractions and particle volume become important

What is the van der Waals equation used for?

It corrects the Ideal Gas Law for real gases by accounting for:

• Particle volume

• Attractive forces

How are gases used in stoichiometry?

Balanced equations can relate gas amounts using:

• Moles

• Volumes

• Masses

• Pressure/temperature (Ideal Gas Law)

What volume does 1 mole of gas occupy at STP?

• 1 atm pressure

• 273.15 K (0°C)

1 mole of ideal gas occupies:

22.4 L

What is STP?

Standard Temperature and Pressure.

• Temperature = 273.15 K

• Pressure = 1 atm

Used often in gas problems.

How do you solve gas stoichiometry problems?

Steps:

1. Balance equation

2. Convert given amount to moles

3. Use mole ratio

4. Convert to liters (or other unit)

Always let moles be the middleman.

In gaseous reactions at same T and P, what do coefficients represent?

Coefficients represent volume ratios as well as mole ratios.

Example:

2H₂ + O₂ → 2H₂O

Means:

• 2 L H₂ reacts with 1 L O₂ to form 2 L H₂O vapor

What is Avogadro’s Law?

Equal volumes of gases at the same temperature and pressure contain equal numbers of particles.

More volume = more moles (if T and P constant).

What is a gas mixture?

this contains two or more gases physically mixed together, not chemically bonded.

Example: Air contains N₂, O₂, Ar, CO₂, etc.

What is partial pressure?

the pressure exerted by one gas in a mixture.

Each gas contributes its own share of total pressure.

What is Dalton’s Law of Partial Pressures?

Total pressure of a gas mixture equals the sum of individual gas pressures.

If O₂ = 0.30 atm and N₂ = 0.70 atm, what is total pressure?

Add them:

0.30 + 0.70 = 1.00 atm

What is mole fraction?

the fraction of total moles contributed by one gas.

Used to find partial pressure.

How do you calculate partial pressure using mole fraction?

Multiply mole fraction by total pressure.

Why do gases in a mixture act independently?

Because gas particles are far apart and move randomly, each gas behaves almost as if the others are not present.

How can you tell if a gas is produced in a chemical reaction?

Common clues:

• Bubbling/fizzing

• Pressure increase in closed container

• Product listed as (g)

Examples:

• CO₂

• H₂

• O₂

• NH₃

If gas moles increase in a sealed rigid container, what happens to pressure?

Pressure increases.

More particles = more collisions with container walls.

Why are gas reactions often sensitive to temperature?

Temperature changes particle speed, collision frequency, and pressure.

Hotter gases are more active and dramatic.

What is the most common mistake in gas stoichiometry?

Forgetting to convert Celsius to Kelvin.

What is the easiest way to think about gas chapter formulas?

• Pressure = collisions

• Temperature = particle speed

• Volume = available space

• Moles = number of particles

• Molar mass = particle heaviness