Honors Chemistry

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Honors Chemistry

• Chapter 5
• Gases

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5.1 Gases

• Temperature vs. Intermolecular attraction
• Atomic Gases
• Noble Gases, H2, N2, O2, F2, Cl2
• Molecular Gases
• Usually light molecules with weak attraction
  forces
• Eg HCl, CO2, NH3, H2S, NO2
• Ionic Compounds
• Strong forces not normally gases

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5.2 Pressure

• Force per unit area
• P F/A
• N/m2 unit defined as Pascal (Pa)
• Standard air pressure 101.325 kPa
• Also called 1 atmosphere (atm)
• Measured by unequal mercury levels
• Manometers and barometers
• Common unit called mmHg (or Torr)
• Standard air pressure 760 mmHg

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5.2 Dimensional Analysis

• Convert 75.0 kPa to mmHg
• 75.0 kPa 760 mmHg----------- x
  --------------- 563 mmHg 1
  101.325 kPa
• Try this one
• Convert 1.25 atm to kPa

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5.3 Boyles Law

• Pressure is inversely proportional to volume
• Hold temperature and amount of gas constant
• V a 1/P
• V k x (1/P)
• PV k
• Best used with changing conditions
• P1V1 P2V2

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5.3 Boyles Law Problems

• A 175 mL sample of methane is stored at 125 kPa.
  What pressure is needed to compress the gas to a
  volume of 50.0 mL?
• P1V1 P2V2
• (125 kPa) (175 mL) P2 (50.0 mL)
• P2 438 kPa
• Try this one
• A sample of argon occupies 476 mL at 650 Torr.
  Find the volume at 975 Torr.

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5.3 Charles Law

• Also credited to Gay-Lussac
• Volume is directly proportional to temperature
• Hold pressure and amount of gas constant
• V a T
• V kT
• Linear relationship
• Must use Kelvins!
• V1 V2--- ---T1 T2

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5.3 Charles Law Problems

• A 5.00 L helium balloon is heated from 20oC to
  75oC. Find its new volume.
• V1/T1 V2/T2
• 5.00 L V2--------- --------293 K
  348 K
• V2 5.94 L
• Try this one
• A 670 mL sample of chlorine is stored at 50oC.
  At what temperature will its volume be 450 mL?

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5.3 More Gas Laws

• Another form of Charles Law
• Pressure is directly proportional to temperature
• P kT
• P1/T1 P2/T2
• Avogadros Law
• Volume is directly proportional to the amount of
  gas present
• V a n
• Volume relationships in chemical reactions

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5.3 Avogadros Law Problems

• How many liters of hydrogen are needed to
  completely react with 1 liter of oxygen?
• 2 H2 O2 ? 2 H2O
• 2 mol hydrogen react with 1 mol oxygen
• V a n, so.
• 2 L hydrogen react with 1 L oxygen
• Try this one
• How many liters of ammonia are formed when 1 L of
  hydrogen reacts with excess nitrogen?

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5.4 The Ideal Gas Equation

• Ideal Gas
• No intermolecular attraction forces
• Particles have no volume
• Combine Boyles, Charles, and Avogadros Laws
• PV nRT
• STP 1 atm, 273 K
• Molar volume of a gas 22.414 L at STP
• R 0.0821 atm L / mol K

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5.4 Ideal Gas Equation Problems

• A sample of fluorine occupies 3.65 L at 45oC and
  2.50 atm. How many moles of fluorine are
  present?
• PV nRT
• (2.50 atm)(3.65 L) n (0.0821)(318 K)
• n 0.350 mol
• Try this one
• A 0.500 mol sample of propane occupies 2.15 L.
  If the temperature is 28oC, find the pressure.

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5.4 Gas Density

• Since n m/M.
• PV (m/M) RT
• MPV mRT
• Divide by V to get density (m/V)
• MP rRT
• Gas density expressed in g/L

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5.4 Gas Density Problems

• Find the density of nitrous oxide at STP.
• First, find molecular mass of N2O
• MP rRT
• (44.0 g/mol)(1.00 atm) r (0.0821)(273 K)
• r 1.96 g/L
• Try this one.
• A gas is found to have a density of 2.54 g/L at
  15oC and 1.50 atm. Find its molecular mass.

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5.5 Gas Stoichiometry

• Mass-Mass problems (review)
• Volume-Volume problems
• Volume is proportional to moles, so.
• Mol relationship from reaction can be used
  directly
• No conversions needed!

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5.5 Volume-Volume Problem

• 2 H2 O2 ? 2 H2O
• If 3.25 L of oxygen react, how many liters of
  water vapor are formed?
• 3.25 L O2 2 L H2O------------- x
  ------------ 6.50 L H2O 1 1 L
  O2
• Volume-Volume is just Avogadros Law!

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5.5 Mass-Volume Problems

• Key step get to moles!
• Mass conversion use molecular mass
• Volume conversion use gas equation
• Need to know temperature and pressure conditions

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5.5 Mass-Volume Problems

• 25.0 g of sodium react with excess water at STP.
  How many liters of hydrogen are produced?
• 2 Na 2 H2O ? 2 NaOH H2
• 25.0 g Na 1 mol Na 1 mol H2------------ x
  ----------- x ----------- 0.543 mol 1
  23.0 g Na 2 mol Na
• Now use ideal gas equation to get volume

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5.5 Mass-Volume Problems

• PV nRT
• (1.00 atm) V (0.543 mol)(0.0821)(273 K)
• V 12.2 L
• Try this one
• Potassium chlorate decomposes into potassium
  chloride and oxygen gas. How many grams of KClO3
  are needed to produce 5.00 L of oxygen at 0.750
  atm and 18oC?
• Hint This one is backwards!

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5.6 Daltons Law

• Partial pressure the pressure of an individual
  gas in a mixture of gases
• Total pressure of a mixture equals the sum of the
  partial pressures of each gas
• Pt P1 P2 P3 ...
• Partial pressure is proportional to the mol
  fraction (X1 n1 / nt)
• P1 X1 Pt

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5.6 Daltons Law

• 2.00 mol He is mixed with 1.00 mol Ar. Find the
  partial pressure of each at 1.75 atm pressure.
• XHe 2.00 mol / 3.00 mol 0.667
• XAr 1.00 mol / 3.00 mol 0.333
• PHe (0.667) (1.75 atm) 1.17 atm
• PAr (0.333) (1.75 atm) 0.583 atm
• Try this...
• Find the partial pressure of oxygen in air if it
  makes up 21 of the Earths atmosphere by volume.
  (Note The volume gives you the mole ratio
  because of Avogadros law.)

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5.7 Kinetic Molecular Theory

• Explains gas behavior in terms of molecular
  motion
• Energy
• Work done by a moving object
• Measured in SI unit Joule (J)
• Kinetic energy
• Energy due to motion
• K ½ mv2
• KMT is a simplification of reality (ideal gas)

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5.7 Kinetic Molecular Theory

• Gas molecules are separated by great distances
• They can be treated as point masses
• Gas molecules are in constant random motion
• Frequent elastic collisions (no energy lost)
• No attractive or repulsive forces
• Average K is proportional to Temperature

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5.7 Distribution of Molecular Speeds

• Maxwell-Boltzmann Distribution
• Molecular speeds distributed around average
• Peak velocity depends on temperature and on
  molec. mass
• Root Mean Square Speed
• _____vrms v3RT/M
• Rate of diffusion

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5.8 Deviations from Ideal Behavior

• We made approximations!
• Point masses
• No intermolecular forces
• These approximations become bad at...
• High pressure
• Low temperature
• Liquefaction
• van der Waals Equation
• (P an2/V2) (V nb) nRT