What is Gas?

Gas Definition in Chemistry: Matter forms into a gas when molecules are moving very fast

• No definite shape/volume

• Scents are gasses; eventually diffuse

• Hot air balloons use gas

Characteristics of Gas

• Air is one of the biggest gasses

  • Mixture composed of many things

  • Composed of Nitrogen, Oxygen, CO2, NE, HE, CH4, etc.

• Noble Gasses

  • Unreactive

• Any liquid/solid can become a gas with enough energy/heat

• Gasses fill a container

• Can be compressed

  • Ex: Oxygen in a scuba tank; air in tire

• To understand gasses, you need to gain an understand of pressure

Pressure

Definition: The amount of force exerted by molecules against a container

• Formula:

  • Pressure = Force/Area (PSI = Pounds per Square Inch)

    • Force = Weight/Mass of molecule (lb. or kg)

    • Area = length * width (in² or cm²)

• Not the same pressure as in physics

• Area is inversely proportional to pressure

  • Generally more important than force when determining pressure because force is constant

• Barometer: Device measuring atmospheric pressure

  • A mercury barometer measures atmospheric pressure

    • Mercury rises or falls based on atmospheric pressure by showing how much air pressure pushes down on mercury (Hg), which rises or falls in a tube.

  • In warm weather, air moves faster and expands, increasing atmospheric pressure.

    • This increased pressure pushes down more on the mercury in the open dish, causing it to rise in the tube.

  • The standard atmospheric pressure at sea level is 760 mm Hg

    • But in warm conditions, the mercury can rise above this as faster-moving air exerts more force on the mercury, leading to a higher column.

Boyles Law

4 Variables needed to determine the state of a gas

• Temperature (Constant in Boyles law)

• Pressure (Boyles Law)

• Volume (Boyles Law)

• Number of Molecules (Constant in Boyles law)

Boyles Law: P₁V₁₌P₂V₂

Robert Boyles Experiment

• Manometer:

  • One end sealed

• One end open to atmosphere

  • 1 atm is the standard atmospheric pressure on the mercury

• Robert Boyle’s Experiment

  • Tested different volumes effect on pressure using a manometer

  • Created graph based on his results

    • High Pressure = Low Volume

    • High Volume = Low Pressure

• Volume and Pressure are inversely proportional

  • If one increases, the other decreases

Creates a Constant:

• Constant: PV = k

• Another way of stating Boyles Law is:

  • P₁V₁₌P₂V₂

• (For a constant temperature and amount of gas)

• Pressure Units must be the same

Practice Problem: A sample of oxygen gas has a volume of 150.0 mL when its pressure is 0.947 atm. What will the volume of the gas be at a pressure of 745 mm Hg if the temperature remains constant?

Boyles Law Demonstration Notes

Use ACE (Answer, Cite, Explain)

Charles Law

Demo Experiment:

• We put an Erlenmeyer flask with water on a hot plate and a balloon on top sealing it

• What happened?

  • The water started boiling

    • Temperature increase

  • The balloon expanding

    • Volume increase

• What variables are constant?

  • Amount of gas

    • Constant because it’s a closed system; nothing enters or leaves)

  • Pressure

    • Constant because volume is changing

Direct Relationship: Temperature Increase = Volume Increase

• Holding Pressure and number of molecules constant

• Volume varies with temperature

• They are directly proportional

Graphing Data for Several Gasses

• Observe starting point of temperature in graph

  • -273 degrees Celsius

  • All gasses start at -273 Celsius

  • Point of absolute zero temperature

Volume and Temperature: Charles Law

• V₁/T₁ = k

• Volume and Temperature are directly proportional

  • If one increases the other increases

• Another way of stating Charles Law is

  • V₁/T₁ = V₂/T₂

  • (constant pressure and amount of gas)

Charles Law Origin

• Discovered by Jacques Charles in 1787

• Found volume changes by 1/273 of each original volume for each Celsius degree, at a constant P and initial T of 0 degrees C

• This temp. = -273.15 is called absolute zero and given a value of 0 in the Kelvin scale

• K = 273.15 + degrees in Celsius

• Charles Law applies to Kelvin temperature also

  

Practice Problems

Problem 1: A sample of neon gas occupies a volume of 752mL at 25 0 C. What volume will the gas occupy at 50 degrees C if the pressure remains constant?

Gay-Lussac Law

P1/T1=P2/T2

• Temperature and Pressure are variables

• Volume and number of moles/molecules constant

• Direct Relationship (if pressure increases, temperature increases)

  • Similar to Charles Law

• P1/T1 = k

Direct Relationship: Division in equation

Inverse Relationship: Multiplication in equation

Check your Understanding: Explain how you can breath air into your lungs.

• Use ACE method

• Answer: When the pressure in your lungs is higher than atmospheric pressure, you exhale, and when it is lower than atmospheric pressure, you inhale.

• Cite: P1*V1=P2*V2

• Explain: When you inhale, your diaphragm contracts, increasing the volume of your lungs, decreasing the pressure inside them. Air rushes in to equalize the pressure, filling your lungs with air. When you exhale, the diaphragm expands, decreasing the volume in your lungs, therefore increasing the pressure and pushing the air out. The atmospheric pressure is 1 atm so when the pressure inside your lungs is higher or lower than 1 atm, it causes you to exhale or inhale.

Example Problem:

Combined Gas Law

• 4 Variables needed to know to determine state of a gas

  • Temperature

  • Volume

  • Pressure

  • Number of moles/molecules

• To explain gas interactions, 2 must be constant, while 2 are changing

P1*V1 = P2*V2 (Inverse relationship)

V1/T1 = V2/T2 (Direct relationship)

P1/T1 = P2/T2 (Direct relationship)

• Temperature is always on bottom to make the combined gas law equation

• Useful for any problem

• Temperature musk be in Kelvin (K)

• Units must be the same

Practice Problem:

Kinetic Molecular Theory

1. Gases consist of molecules in continuous random motion.

2. Volume of all the molecules is negligible compared to total volume of container they are in.

3. Attractive and repulsive forces between gas molecules are negligible.

4. Collisions are elastic. Energy can be transferred, but average kinetic energy stays same, as long as temperature is constant.

5. The average kinetic energy of the molecules is proportional to the absolute temperature.

Application on Gas Laws

• What happens when we increase V at constant T and number of molecules?

  • The increased volume means decreased pressure because the gas particles hit the container less frequently if they are moving at the same speed (temp.

Temperature is the measure of kinetic movement of particles

• If two substances have the same temperature, then their average kinetic movement is the same

Units of Pressure

• 1 standard atmosphere =

  • 1.000 atm

  • 760.0 mm Hg

  • 760.0 torr (named after Torricelli)

  • 101,325 Pa

  • 101.3 kPa

  • 14.7 psi

• 1000 mL = 1 L

Graphing in Chemistry

• Create X, Y, scatter plot

• Label the X and Y axis and put the units in

  parenthesis, create a title for the graph

  • Title should be “X vs. Y” or “Boyles Law Graph” in this lab

• X axis always longest edge of paper

• X is the independent variable – what you

  knew when you started the lab

• Y is dependent variable – what changed

• SCALE – most important part of graph

Scale

• Must be easy to figure out

• Each line must represent a value so that

  you can estimate between lines

• For this lab (Boyle’s law lab) each axis, X and Y, start at zero

  • But not all labs must start at zero

• Each scale, X and Y, can be different –

  they do not have to be the same