Chapters 13

1
Chapters 13 14

• Gases

2
13.1 A model to explain gas behavior.

• Gases can exist as
• Single atoms He, Ne, Ar, Kr, Xe, Rn
• Diatomic molecules H2, N2, O2, F2, Cl2, Br2,
  I2
• Polyatomic molecules CO2, CH4, NO
• Physical properties common to all gases
• Gases have mass.
• -mass per unit volume is much less than a solid
  or liquid (low density)
• Gases are compressible.
• -can be forced into smaller containers.

3

• Gases fill their container completely
• -the particles distribute themselves evenly
  throughout
• Different gases can move through each other quite
  rapidly
• -diffusion
• Gases exert pressure
• The pressure of a gas depends on the temperature.
• -pressure and temperature have a direct
  relationship

4

• Postulates of the Kinetic-Molecular Theory
• Gases consist of tiny particles that have mass.
• -most gases are molecules, except the noble
  gases
• The distance between gas particles is large
  relative to the size of the particles.
• -explains why a gas is compressible
• Particles of a gas are in constant, rapid, random
  motion
• -explains why gases fill their containers and
  diffuse easily

5

• Particles of a gas frequently collide with each
  other and the walls of their container (causing
  pressure) with perfectly elastic collisions.
• Perfectly elastic collision-no energy of motion
  is lost
• The average kinetic energy of the particles
  depends on the temperature.
• Gas particles exert no force on one another.

6

• Four Variables used to describe gases
• Amount of Gas (n)- expressed in moles
• Volume (V)- the volume of the container,
• expressed in liters
• Temperature (T)- expressed in Kelvin
• K C 273
• Pressure (P)- expressed in
• - atmospheres(atm)
• - millimeters of mercury (mmHg)
• - torr
• - pounds per square inch (lb/in2)
• - Pascal's (Pa)

7

• Units of Pressure Equality Equations
• 1 atm 101,325 Pa
• 101,325 Pa 760. mm Hg
• 1 atm 760. mm Hg 760. Torr
• 1 atm 14.70 lb/in2

8
14.1 The Gas Laws

• Boyles Law The Pressure-Volume Relationship
• Inverse relationship at constant temperature.
• V ? P ?
• V ? P ?
• P1V1 P2V2
• P1 - initial pressure V1- initial volume
• P2 - final pressure V2 - final volume

9

• 1. A gas occupies a volume of 458 mL at a
  pressure of 1.01 kPa. When the pressure is
  changed, the volume becomes 477 mL. If there has
  been not change in temperature, what is the new
  pressure?
• P1 1.01 kPa
• V1  458 mL
• P2   ?
• V2 477 mL
• P2 (P1 x V1) ? V2
• P2 (1.01 kPa x 458 mL) ? 477 mL
• P2 0.97 kPa

10

• 2. A gas occupies a volume of 2.45 L at a a
  pressure of 1.03 atm. What volume will the gas
  occupy if the pressure changes to 0.980 atm and
  the temperature does not change?
• P1 1.03 atm
• V1  2.45 L
• P2  0.980 atm
• V2 ?
• V2 (P1 x V1) ? P2
• V2 (1.03 atm x 2.45 L) ? 0.980atm
• V2 2.58 L

11
.

• Charless Law The Temperature-Volume
  Relationship
• Direct relationship at constant pressure.
• T ? V ?
• T ? V ?
• V1T2V2T1
• V1 initial volume T2 final temperature
• V2 - final volume T1 initial temperature
• Absolute zero-lowest possible temp., no movement
  of particles (0 Kelvin, -273 C)

12

• 1. What will be the volume of a gas sample
• at 309 K if its volume at 215 K is 3.42 L?
• Assume that pressure is constant.
• V1 3.42 L
• T2 309 K
• V2 ? 
• T1 215 K
• V2 (T2 x V1) ? T1
• V2 (309 K x 3.42 L ) ? 215 K
• V2 4.92 L

13

• 2. A gas sample at 83 ?C occupies a volume
• of 1400 m3. At what temperature will it
• occupy 1200 m3?
• V1 1400 m3
• T2   ?
• V2  1200 m3
• T1 83 ?C 273 356 K ?
• T2 (T1 x V2) ? V1
• T2 (356 K x 1200m3) ? 1400 m3
• T2 305.14 K