GASES%20Chemistry%20I%20Honors%20

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GASESChemistry I Honors Chapter 11
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Importance of Gases

• Airbags fill with N2 gas in an accident.
• Gas is generated by the decomposition of sodium
  azide, NaN3.
• 2 NaN3---gt2 Na 3 N2

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General Properties of Gases

• There is a lot of free space in a gas.
• Gases can be expanded infinitely. (they will fill
  whatever container they are in.)
• Gases fill containers uniformly and completely.
• Gases diffuse and mix rapidly.

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Pressure

• Pressure is the amount of force per unit area.
• Gas Pressure is the pressure caused by particles
  of gas striking an object.
• Atmospheric (Air) pressure is the pressure of
  the column of atmosphere above you ,pressing down
  on you.

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Atmospheric Pressure

• (COPY)
• Measured with a BAROMETER (developed by
  Torricelli in 1643)
• Hg rises in tube until force of Hg (UP) balances
  the force of atmosphere (DOWN). (Just like a
  straw in a soft drink)
• (DONT COPY)
• Q Why is Hg so good for use in barometer?
• A If you tried to use water, it would rise about
  34 high!

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Atmospheric. Pressure

• The column height measures atmospheric pressure
• 1 standard atmosphere (atm)
• 760 mm Hg
• 14.7 pounds/in2 (psi)
• 101.3 kPa (SI unit is PASCAL)

These are the 2 units we will use
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Pressure Conversions

• A. What is 2.71 atm expressed in kPa?
• B. The pressure in a tire reads 262 kPa.
• What is this pressure in atm?

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Properties of Gases

• Gas properties can be modeled using math.
• The factors that affect gases are
• V volume of the gas (L)
• T temperature (K)
• n amount (moles)
• P pressure (atm or kPa)

All temperatures must be in Kelvins! No
exceptions!
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How of Moles(n) affects gas pressure

• Pressure is caused by the particles striking the
  walls of the container.
• If the gas is in a rigid container, the volume is
  constant
• If you triple the number of gas particles (n),
  you triple the amount of pressure. (see p 415 of
  text)

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Properties of Gases, cont.

• We can study the relationship between 2 variables
  if we keep the others the same.

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Boyles Law

• 2 variables we will study P V
• All other variables kept the same (T, n, etc.)
• This relationship is seen in a flexible/adjustable
  container
• Ex a rising balloon or a cylinder with a piston

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Boyles Law

• P a 1/V
• This means Pressure and Volume are INVERSELY
  PROPORTIONAL
• For example, P goes up as V goes down.
• P1V1 P2 V2

Robert Boyle (1627-1691). Son of Earl of Cork,
Ireland.
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Boyles Law

• A bicycle pump is a good example of Boyles law.
• As the volume of the air trapped in the pump is
  reduced, its pressure goes up, and air is forced
  into the tire.

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Sample Problem 14.1 (p 419)

• A balloon contains 30.0 L of helium gas at
  103kPa. What is the volume of the helium when the
  balloon rises to an altitude where the pressure
  is only 25.0 kPa?

Unknown V2
Givens P1 103 kPa P2 25.0kPa V1 30.0L
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Solution

• 1. Identify label all your variables (given
  unknown)
• 2. Find the formula that has all the variables
  youve listed.
• 3. Isolate the variable for which you are
  solving.
• 4. Substitute your values
• 5. Solve for the unknown

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Charless Law

• 2 variables we will study V T
• All other variables kept the same (P, n, etc.)

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Charless Law

• If n and P are constant, then V a T
• V and T are directly proportional.
• V1 V2
• T1 T2
• If one temperature goes up, the volume goes up!

Jacques Charles (1746-1823). Isolated boron and
studied gases. Balloonist.
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Sample Problem 14.2 Using Charles Law (p 421)

• A balloon inflated in a room at 24C has a volume
  of 4.00L. The balloon is then heated to a
  temperature of 48C. What is the new volume if
  the pressure remains constant?

Givens T1 24C 273 297K T2 48 273
321K V1 4.00 L
Unknown V2
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Solution

• V1 V2 ? V2 V1T2
• T1 T2 T1
• V2 4.00L321K
• 297K
• V2
• CW/HWPractice Problems, pp 419-423 7-12

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Charless Law
What will happen if the syringe is put in a hot
water bath? A cold water bath?
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Gay-Lussacs Law

• If n and V are constant, then P a T
• P and T are directly proportional.
• P1 P2
• T1 T2
• If one temperature goes up, the pressure goes up!

Joseph Louis Gay-Lussac (1778-1850)
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Section 14.1 Assessmentp 417

• 1. Why is a gas easy to compress?
• 2. List 3 factors that can affect gas pressure.
• 3. Why does a collision with an air bag cause
  less damage than a collision with a steering
  wheel?
• 4. How does a decrease in temp affect the
  pressure of a contained gas?
• 5. If the temp is constant, what change in volume
  would cause the pressure of an enclosed gas to be
  reduced to ¼ of its original value?
• 6. Assuming the gas in a container remains at a
  constant temp, how could you increase the gas
  pressure in a container a hundredfold?

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Combined Gas Law

• Since all 3 gas laws are related to each other,
  we can combine them into a single equation. BE
  SURE YOU KNOW THIS EQUATION!
• P1 V1 P2 V2
• T1 T2

No, its not related to R2D2
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Combined Gas Law

• If you should only need one of the other gas
  laws, you can cover up the item that is constant
  and you will get that gas law!

P1
V1
P2
Boyles Law Charles Law Gay-Lussacs Law
V2
T1
T2
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Combined Gas Law Problem

A sample of helium gas has a volume of 180 mL, a
pressure of 0.800 atm and a temperature of 29C.
What is the new temperature(C) of the gas at a
volume of 90.0 mL and a pressure of 3.20
atm? Set up Data Table P1 0.800 atm V1
180 mL T1 302 K P2 3.20 atm V2
90 mL T2 ??
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Solution

• P1 0.800 atm V1 180 mL T1 302K
• P2 3.20 atm V2 90 mL T2 ??
• P1 V1 P2 V2
• P1 V1 T2 P2 V2 T1
• T1 T2
• T2 P2 V2 T1
• P1 V1
• T2 3.20 atm x 90.0 mL x 302 K
  0.800 atm x 180.0 mL
• T2 604 K - 273 331 C

604 K
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Learning Check

• A gas has a volume of 675 mL at 35C and 0.850
  atm pressure. What is the temperature in C when
  the gas has a volume of 0.315 L and a pressure of
  802 mm Hg?

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One More Practice Problem

• A balloon has a volume of 785 mL on a fall day
  when the temperature is 21C. In the winter,
  the gas cools to 0C. What is the new volume of
  the balloon?

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And now, we pause for this commercial message
from STP
OK, so its 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
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Try This One

• A sample of neon gas used in a neon sign has a
  volume of 15 L at STP. What is the volume (L) of
  the neon gas at 2.0 atm and 25C?

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Avogadros Hypothesis

• Equal volumes of gases at the same T and P have
  the same number of molecules.
• V and n are directly related.

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IDEAL GAS LAW
P V n R T

• Brings together gas properties.
• BE SURE YOU KNOW THIS EQUATION!

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Using PV nRT

• P Pressure n number of moles
• V Volume T Temperature
• R is a constant, called the Ideal Gas Constant
• 
  8.31 LkPa
• Kmol
• NOTE We must convert the units to match R.

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Using PV nRT

• Ex p 439, Q55
• 1.24 moles of gas at 35 C and 96.2 kPa pressure.
  What is the volume the gas occupies?
• V?
• n 1.24 mol
• T 35 273 308K
• P 96.2kPa
• R8.31LkPa/Kmol
• (96.2kPa)V (1.24 mol ) (8.21LkPa/Kmol) (308K)
• V33L

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Learning Check

• Dinitrogen monoxide (N2O), laughing gas, is used
  by dentists as an anesthetic. If 2.86 mol of gas
  occupies a 20.0 L tank at 23C, what is the
  pressure (mm Hg) in the tank in the dentist
  office?

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Learning Check

• A 5.0 L cylinder contains oxygen gas at 20.0C
  and 735 mm Hg. How many grams of oxygen are in
  the cylinder?

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Gases in the Air

• The of gases in air Partial pressure (STP)
• 78.08 N2 593.4 mm Hg
• 20.95 O2 159.2 mm Hg
• 0.94 Ar 7.1 mm Hg
• 0.03 CO2 0.2 mm Hg
• PAIR PN2 PO2 PAr PCO2 760 mm
  Hg
• (Total Pressure 760mm Hg)

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Daltons Law of Partial Pressures
2 H2O2 (l) ---gt 2 H2O (g) O2 (g)
0.32 atm 0.16 atm

• What is the total pressure in the flask?
• Ptotal in gas mixture PA PB ...
• Therefore,
• Ptotal PH2O PO2 0.48 atm
• Daltons Law total P is sum of PARTIAL
  pressures.

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Daltons Law
John Dalton 1766-1844
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Health Note

• When a scuba diver is several hundred feet
  under water, the high pressures cause N2 from the
  tank air to dissolve in the blood. If the diver
  rises too fast, the dissolved N2 will form
  bubbles in the blood, a dangerous and painful
  condition called "the bends". Helium, which is
  inert, less dense, and does not dissolve in the
  blood, is mixed with O2 in scuba tanks used for
  deep descents.

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Gases and Stoichiometry

• 2 H2O2 (l) ---gt 2 H2O (g) O2 (g)
• Decompose 1.1 g of H2O2 in a flask with a volume
  of 2.50 L. What is the volume of O2 at STP?

Bombardier beetle uses decomposition of hydrogen
peroxide to defend itself.
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Gases and Stoichiometry

• 2 H2O2 (l) ---gt 2 H2O (g) O2 (g)
• Decompose 1.1 g of H2O2 in a flask with a volume
  of 2.50 L. What is the volume of O2 at STP?
• Solution
• 1.1 g H2O2 1 mol H2O2 1 mol O2
  22.4 L O2
• 34 g H2O2 2 mol H2O2 1
  mol O2

0.36 L O2 at STP
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Gas Stoichiometry Practice!

• A. What is the volume at STP of 4.00 g of CH4?
• B. How many grams of He are present in 8.0 L of
  gas at STP?

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What if its NOT at STP?

• 1. Do the problem like it was at STP. (V1)
• 2. Convert from STP (V1, P1, T1) to the stated
  conditions (P2, T2)

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Try this one!

• How many L of O2 are needed to react 28.0 g NH3
  at 24C and 0.950 atm?
• 4 NH3(g) 5 O2(g) 4 NO(g) 6
  H2O(g)

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GAS DIFFUSION AND EFFUSION
HONORS only

• diffusion is the gradual mixing of molecules of
  different gases.

• effusion is the movement of molecules through a
  small hole into an empty container.

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GAS DIFFUSION AND EFFUSION
HONORS only

• Grahams law governs effusion and diffusion of
  gas molecules.

Rate of effusion is inversely proportional to its
molar mass.
Thomas Graham, 1805-1869. Professor in Glasgow
and London.