Kinetic Molecular Theory and Gas Stoichiometry

1
Kinetic Molecular Theory and Gas Stoichiometry

• Chapter 13 Sections 13.7-13.11

2
Review

• What are the Four Gas Law Variables
• Temperature
• Pressure
• Volume
• and Moles (or amount of the gas)

3
Laws vs. Models

• Since the ideal gas law is about an IDEAL gas,
  under normal conditions real gases do not obey
  the law exactly
• Only under very high temperatures and low
  pressure do most gases get close to the ideal gas
  law.
• The ideal gas LAW only applies to ideal gases,
  however it is a good MODEL for other gases
• In other words a model predicts an approximate
  future behavior of most gases

4
Kinetic Molecular Theory

• Remember a LAW is a summary of observations, a
  THEORY is a scientifically tested and accepted
  reason why the law occurs
• The kinetic molecular theory is a MODEL that
  explains the ideal gas LAW
• The theory is made only for what is called an
  ideal gas. They cannot all be rigorously applied
  (i.e. mathematically) to real gases, but can be
  used to explain their observed behavior
  qualitatively.

5
Kinetic Molecular Theory cont

• So in order for kinetic theory to be applied
  these are the postulates (things that hold true)
• Gas consists of tiny particles (atoms or
  molecules)
• These particles are so small that the distances
  between the particles are WAY bigger than the
  particles themselves (particle volume is
  negligible)
• Particles are in constant motion, when the
  particles hit the container, this is pressure
• Particles are neutral
• Average Kinetic energy is equal to the
  temperature in degrees Kelvin
• KE ½ mv2
• KE Kinetic Energy
• m mass of the particle
• v velocity of the particle

6
Kinetic Molecular Theory cont

• So Kinetic Molecular Theory really does explain
  the laws we have seen qualitatively!
• Temperature This is how fast molecules or atoms
  are moving
• So the temperature in Kelvin is directly
  proportional to the movement (Kinetic energy) of
  the molecules!

7
Kinetic Molecular Theory cont

• Pressure and Temperature
• Pressure is how often and how fast the gas
  particles hit the walls of a container (or
  anything else)
• and Temperature is how fast they are moving

8
Kinetic Molecular Theory cont

• Volume and Temperature
• Volume How much space the gas is taking up
• Temperature How fast the particles are moving
• So if they move faster they spread out more

9
Gas Stoichiometry

• Since the Ideal Gas Law is useful for predicting
  the behavior of real gases
• We can use it to calculate the number of moles
  present in a gas!!!
• PV nRT or
• n PV/RT

10
Gas Stoichiometry cont

• And once you get the moles you can use
  stoichiometry to do all sorts of stuff!!!
• Example Calculate the volume of hydrogen
  produced at 1.50 atm and 19.0oC by the reaction
  of 26.5 g of zinc with excess hydrochloric acid
• Zn 2HCl ? ZnCl2 H2
• do a MOLE TRAIN to figure out how many moles H2
  you have
• First convert grams of zinc to moles of zinc
• 26.5 moles / 65.38 .405 moles
• Since it is a 1 to 1 ratio, we have .405 moles of
  H2
• Then calculate the volume of H2 using the
  pervnert
• PV nRT
• (1.50) V (.405)(.08206)(292)
• V 6.47 L

11
Gas Stoichiometry cont

• Here is another VERY cool thing
• At standard temperature and pressure (1.00 atm
  and 273 K) 1 mole of an ideal gas has a volume of
  22.4 L
• (or 22.4 liters of a gas contains 1 mole of that
  gas)

12
Gas Stoichiometry cont

• You have 2.00 L of dry H2 at STP. How many moles
  is this?
• 2.00 L 22.4 L
• x mol 1 mol
• X .0823 mol

13
Gas Stoichiometry cont

• 0.250 moles of HCl will occupy how many liters at
  STP?
• 0.250 mol x 22.4 L
• 5.60 L

14
Homework

• Read pages 425-433. Problems 2 and 3 on page
  433, Problems 43, 47 and 53 on pages 437-438