Gases

1
Chapter 10

• Gases

2
GASES
John Dalton
3
Characteristics, Pressure, Laws, and Ideal-Gas
Equation

• Sections 10.1-10.4

4
Objectives

• Compare distinguishing characteristics of gases
  with those of liquids and solids
• Study gas pressure and the units that express it
• Examine volume, pressure, and temperature and
  their relationship to gases.
• Use the ideal-gas equation to make calculations.

5
Characteristics of Gases

• Earths Atmosphere
• N2 (78)
• O2 (21)
• Other gases, ex Argon (0.9)
• Diatomic gases- halogens
• Monatomic gases- nobles
• Vapors

6
Gases vs. Solids Liquids

• Gases, unlike solids and liquids,
• Expand to fill their containers volumes
• Are highly compressible
• Form homogeneous mixtures with each other

7
Pressure

• P F
• A
• Gases exert pressure on any surface they contact

8
Atmospheric Pressure

• Gases experience gravitational acceleration BUT
  they have tiny masses
• So gravity operates on atmosphere as a whole to
  press down on Earth
• Atmospheric pressure

9
Magnitude of Atmospheric Pressure

• F ma
• Force mass x acceleration
• a 9.8 m/s2

10
SI Units

• Force kg-m/s2, the newton (N)
• Pressure N/m2, the pascal (Pa)

11
Barometer

• Early 17th century
• Evangelista Torricelli, student of Galileo
• Proved that atmosphere had weight

Height of Hg, h, measures atmospheric pressure
12
Standard Atmospheric Pressure

• Pressure at sea level
• Supports a column of Hg 760mm high
• 1.01325 x 105 Pa
• Defines non-SI units of pressure
• Atmospheres (atm)
• Millimeters of mercury (mm Hg)
• a.k.a. torr (for Torricelli)

13
Pressure Conversions

• 1 atm 760 mm Hg 760 torr 1.01325 x 105 Pa

14
Gas Laws

• Pressure and Volume
• (Boyles Law)
• Temperature and Volume
• (Charles Law)
• Quantity and Volume
• (Avogadros Law)

15
Boyle's Law

• British chemist Robert Boyle (1627-1691)
• The pressure of a gas is inversely related to the
  volume when T does not change
• PV product remains constant
• P x V k (constant) when T remains constant

16
Boyles Law and the Breathing
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P and V Changes

P1 8atm
P2 4atm
V1 2L
V2 4L
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Pressure/Volume Changes

• P1V1 P2V2
• P1V1 8.0 atm x 2.0 L 16 atm L
• P2V2 4.0 atm x 4.0 L 16 atm L

19
Charles Law

• V 125 mL V 250 mL
• T 273 K T 546 K
  
• Observe the V and T of the balloons. How does
  volume change with temperature?

20
Charles Law

• French scientist, Jacques Charles (1746-1823)
• At constant pressure, the volume of a gas is
• directly related to its absolute (K) temperature
• V constant x T
• V1 V2
• T1 T2

21
Learning Check

• Use Charles Law to complete the statements
• 1. If final T is higher than initial T, final V
• is (greater, or less) than the initial V.
• 2. If final V is less than initial V, final T is
• (higher, or lower) than the initial T.

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Solution

• V1 V2
• T1 T2
• 1. If final T is higher than initial T, final V
• is (greater) than the initial V.
• 2. If final V is less than initial V, final T is
  (lower) than the initial T.

23
Avogadros Law

• Joseph Louis Gay-Lussac (1778-1823)
• At a given P and T, the V of gases react with one
  another in a ratio of small whole numbers
• Amedeo Avogadro (1776-1856)
• Hypothesis Equal volumes of gases at the same T
  and P contain equal numbers of moecules
• Law V of a gas maintained at constant T and P is
  directly proportional to the number of moles of
  gas.
• V constant x n

24
Ideal Gas Law

• The four variables in the gas laws all deal with
  proportionality
• Boyles V ? 1/P (constant n, T)
• Charles V ? T (constant n, P)
• Avogadros V ? n (constant P, T)
• They combine into a general gas law
• V ? nT
• P

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Ideal Gas Equation

• If the proportionality constant is R, than
• PV nRT
• R ideal gas constant

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Units for Ideal-Gas Equation

• T must be in K
• n is expressed in moles
• P is usually atm
• V is typically L

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STP

• Standard Temperature and Pressure
• 0 ºC
• 1 atm
• V of 1 mole of ideal gas at STP 22.41 L

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Temperature Conversions
ºF
-459
32
212
ºC
-273
0
100
K
0
273
373