GASESGASES

Importance of GasesImportance of Gases

• Airbags fill with NAirbags fill with N22 gas in an gas in an accident. accident.

• Gas is generated by the Gas is generated by the decomposition of sodium decomposition of sodium azide, NaNazide, NaN33..

• 2 NaN2 NaN33 ---> 2 Na + 3 N ---> 2 Na + 3 N22

Atmosphere•The atmosphere is composed of nitrogen, oxygen, argon, water vapor, and a number of trace gases.

•Chemical reactions maintain the ratios of major constituents of the atmosphere to each other.

•For example, oxygen is released into the atmosphere by photosynthesis and consumed by respiration.

Altitude vs. Temperature

Altitude vs. Pressure

General Properties of General Properties of GasesGases

• There is a lot of “free” space There is a lot of “free” space in a gas.in a gas.

• Gases can be expanded Gases can be expanded infinitely.infinitely.

• Gases fill containers uniformly Gases fill containers uniformly and completely.and completely.

• Gases diffuse and mix rapidly.Gases diffuse and mix rapidly.

PressurePressurePressure of air is measured with a Pressure of air is measured with a

BAROMETERBAROMETER (developed by (developed by Torricelli in 1643)Torricelli in 1643)

Hg rises in tube until force of Hg (downHg rises in tube until force of Hg (downbalances the force of atmosphere(pushing balances the force of atmosphere(pushing

up). (Just like a strawup). (Just like a strawin a soft drink)in a soft drink)

P of Hg pushing down related to P of Hg pushing down related to • Hg densityHg density• column heightcolumn height

PressurePressureColumn heightColumn heightmeasures Pressure ofmeasures Pressure ofatmosphereatmosphere• 1 standard 1 standard

atmosphere (atm) *atmosphere (atm) *= 760 mm Hg (or torr) = 760 mm Hg (or torr)

**= 29.92 inches Hg *= 29.92 inches Hg *= 14.7 pounds/in= 14.7 pounds/in2 2 (psi)(psi)= 101.3 kPa= 101.3 kPa

* Memorize these!* Memorize these!

Pressure Conversions

A. What is 475 mm Hg expressed in atm?

1 atm 760 mm Hg

B. The pressure of a tire is measured as 29.4 psi. What is this pressure in mm Hg?

760 mm Hg 14.7 psi

= 1.52 x 103 mm Hg

= 0.625 atm475 mm Hg x

29.4 psi x

Pressure Conversions

A. What is 2 atm expressed in torr?

B. The pressure of a tire is measured as 32.0 psi. What is this pressure in kPa?

Kinetic Molecular Theory• To fully understand the world around us requires

that we have a good understanding of the behavior of gases.

• The description of gases and their behavior can be approached from several perspectives.

• The Gas Laws are a mathematical interpretation of the behavior of gases.

• However, before understanding the mathematics of gases, a chemist must have an understanding of the conceptual description of gases.

• That is the purpose of the Kinetic Molecular Theory.

Kinetic Molecular Theory• The Kinetic Molecular Theory is a single set of

descriptive characteristics of a substance known as the Ideal Gas.

• All real gases require their own unique sets of descriptive characteristics. Considering the large number of known gases in the World, the task of trying to describe each one of them individually would be an awesome task.

• In order to simplify this task, the scientific community has decided to create an imaginary gas that approximates the behavior of all real gases. In other words, the Ideal Gas is a substance that does not exist.

• The Kinetic Molecular Theory describes that gas.

KINETIC MOLECULAR THEORY

• Gases are composed of particles- usually molecules or atoms: Small, hard spheres, relatively far apart from each other

• Particles in a gas move rapidly in constant random motion. Move in straight paths, changing direction only when colliding with one another or other objects

• Collisions are perfectly elastic- meaning kinetic energy is transferred without loss from one particle to another- the total kinetic energy remains constant

ALL TEMPERATURE HAS TO BE IN

KELVIN

And now, we pause for this commercial message from STP

OK, so it’s really not THIS kind of STP…

STP in chemistry stands for Standard Temperature and Pressure

Standard Pressure = 1 atm (or an equivalent)

Standard Temperature = 0 deg C (273 K)

STP allows us to compare amounts of gases between different pressures and temperatures

STP allows us to compare amounts of gases between different pressures and temperatures

Boyle’s LawBoyle’s Law

Robert Boyle was among the first to note the relationship between pressure and volume of a gas.He measured the volume of air at different pressures, and observed a pattern of behavior which led to his mathematical law.During his experiments Temperature and amount of gas weren’t allowed to change

Robert Boyle was among the first to note the relationship between pressure and volume of a gas.He measured the volume of air at different pressures, and observed a pattern of behavior which led to his mathematical law.During his experiments Temperature and amount of gas weren’t allowed to change

As the pressure increases

As the pressure increases

VolumedecreasesVolume

decreases

How does Pressure and Volume of gases relate graphically?

How does Pressure and Volume of gases relate graphically?

Volu

me

Volu

me

PressurePressure

PV = kPV = k

Temperature, # of particlesremain constant

Temperature, # of particlesremain constant

Boyle’s Mathematical Law:Boyle’s Mathematical Law:

since PV = ksince PV = k

P1V1 = P2V2P1V1 = P2V2

Eg: A gas has a volume of 3.0 L at 2 atm. What is its volume at 4

atm?

Eg: A gas has a volume of 3.0 L at 2 atm. What is its volume at 4

atm?

What if we had a change in conditions?What if we had a change in conditions?

1)determine which variables you have:

1)determine which variables you have:

P and V = Boyle’s LawP and V = Boyle’s Law

2)determine which law is being represented:

2)determine which law is being represented:

P1 = 2 atm V1 = 3.0 L P2 = 4 atm V2 = ?

P1 = 2 atm V1 = 3.0 L P2 = 4 atm V2 = ?

3) Rearrange the equation for the variable you don’t know

3) Rearrange the equation for the variable you don’t know

4) Plug in the variables and chug it on a calculator:

4) Plug in the variables and chug it on a calculator:

P1V1 = V2P1V1 = V2

P2P2

(2.0 atm)(3.0L) = V2(2.0 atm)(3.0L) = V2

(4atm)(4atm)V2 = 1.5LV2 = 1.5L

Charles’s LawCharles’s Law

Jacques Charles determined the relationship between temperature and volume of a gas.He measured the volume of air at different temperatures, and observed a pattern of behavior which led to his mathematical law.During his experiments pressure of the system and amount of gas were held constant.

Jacques Charles determined the relationship between temperature and volume of a gas.He measured the volume of air at different temperatures, and observed a pattern of behavior which led to his mathematical law.During his experiments pressure of the system and amount of gas were held constant.

Volume of balloon at room temperature

Volume of balloon at room temperature

Volume of balloon at 5°C

Volume of balloon at 5°C

Temp

How does Temperature and Volume of gases relate

graphically?

How does Temperature and Volume of gases relate

graphically?V

olu

me V/T = k

Pressure, # of particlesremain constant

Pressure, # of particlesremain constant

Charles’s Mathematical Law:Charles’s Mathematical Law:

since V/T = ksince V/T = k

Eg: A gas has a volume of 3.0 L at 127°C. What is its volume at 227 °C? Eg: A gas has a volume of 3.0 L at

127°C. What is its volume at 227 °C?

V1 V2

T1 T2

=

What if we had a change in conditions?What if we had a change in conditions?

1)determine which variables you have:

1)determine which variables you have:

T and V = Charles’s LawT and V = Charles’s Law

2)determine which law is being represented:

2)determine which law is being represented:

T1 = 127°C + 273 = 400K

V1 = 3.0 L T2 = 227°C + 273 =

5ooK V2 = ?

T1 = 127°C + 273 = 400K

V1 = 3.0 L T2 = 227°C + 273 =

5ooK V2 = ?

4) Plug in the variables:4) Plug in the variables:

(500K)(3.0L) = V2 (400K)(500K)(3.0L) = V2 (400K)

V2 = 3.8LV2 = 3.8L

3.0L V23.0L V2

400K 500K400K 500K = =

5) Cross multiply and chug5) Cross multiply and chug

Gay Lussac’s LawGay Lussac’s Law

Old man Lussac determined the relationship between temperature and pressure of a gas.He measured the temperature of air at different pressures, and observed a pattern of behavior which led to his mathematical law.During his experiments volume of the system and amount of gas were held constant.

Old man Lussac determined the relationship between temperature and pressure of a gas.He measured the temperature of air at different pressures, and observed a pattern of behavior which led to his mathematical law.During his experiments volume of the system and amount of gas were held constant.

Pressure Gauge

Pressure Gauge

Car before a tripCar before a trip

Think of a tire...Think of a tire...

Let’s get onthe road

Dude!

Car after a long tripCar after a long trip

Think of a tire...Think of a tire...

WHEW!

Pressure Gauge

Pressure Gauge

Temp

Pre

ssu

re

How does Pressure and Temperature of gases relate

graphically?

How does Pressure and Temperature of gases relate

graphically?

P/T = k

Volume, # of particlesremain constant

Volume, # of particlesremain constant

Lussac’s Mathematical Law:Lussac’s Mathematical Law:

What if we had a change in conditions?What if we had a change in conditions?

since P/T = ksince P/T = k

P1 P2

T1 T2

=

Eg: A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C?

Eg: A gas has a pressure of 3.0 atm at 127º C. What is its pressure at 227º C?

T and P = Gay-Lussac’s LawT and P = Gay-Lussac’s Law

T1 = 127°C + 273 =

400K P1 = 3.0 atm T2 = 227°C + 273 =

500K P2 = ?

T1 = 127°C + 273 =

400K P1 = 3.0 atm T2 = 227°C + 273 =

500K P2 = ?

1)determine which variables you have:

1)determine which variables you have:

2)determine which law is being represented:

2)determine which law is being represented:

4) Plug in the variables:4) Plug in the variables:

(500K)(3.0atm) = P2 (400K)(500K)(3.0atm) = P2 (400K)

P2 = 3.8atmP2 = 3.8atm

3.0atm P23.0atm P2

400K 500K400K 500K = =

5) Cross multiply and chug5) Cross multiply and chug

LAWLAW RELAT-RELAT-IONSHIPIONSHIP LAWLAW CON-CON-

STANTSTANT

BoyleBoyle’’ss PP V V PP11VV1 1 = P= P22VV22 T, nT, n

CharleCharless ’’ VV T T

VV11/T/T11 = = VV22/T/T22

P, nP, n

Gay-Gay-LussacLussac’’

ssPP T T

PP11/T/T11 = = PP22/T/T22

V, nV, n