Chemistry 1011

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Chemistry 1011

• TOPIC
• Gaseous Chemical Equilibrium
• TEXT REFERENCE
• Masterton and Hurley Chapter 12

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12.1 The N2O4 - NO2 Equilibrium

• YOU ARE EXPECTED TO BE ABLE TO
• Distinguish equilibrium from steady state
  situations.
• Recognize chemical equilibrium as a dynamic
  process taking place in a closed system.
• Identify the changes taking place at the
  molecular level in a chemical equilibrium
  process.
• Identify the equilibrium constant as a
  temperature dependent constant related to the
  equilibrium partial pressures of reactants and
  products.

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Review - Partial Pressure

• Total gas pressure is proportional to the number
  of moles present it is independent of the
  identity of the material
• The partial pressure of a gas in a gas mixture is
  the fraction of the total pressure that is due to
  that gas
• The partial pressure of gas X2 in a mixture is
  equal to the
• mole fraction of X2 x Total Pressure

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Partial Pressure Example

• Air is approximately 80 N2 and 20O2
• Total air pressure is approximately 100kPa
• PN2 PO2 PTOTAL
• PN2 80 x 100kPa 80kPa
• PO2 20 x 100kPa 20kPa
• Partial pressures can be used to express the
  concentrations of reactants and products in
  chemical equilibria

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The N2O4 - NO2 Equilibrium

• At any given temperature, a sample of NO2 will
  exist as an equilibrium mixture of NO2 and N2O4
• N2O4(g) 2NO2(g)
• The forward and reverse reactions are taking
  place at the same rate
• The concentrations of the species remain constant

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Getting to Equilibrium

• Equilibrium can be approached starting from
  either reactants or products
• In the N2O4 - NO2 example, we can theoretically
  start from either 100 reactants or 100 products
• The equilibrium position will be the same

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The Equilibrium Constant

• Equilibrium Measurements for the
• N2O4 - NO2 System at 100oC
• Original P (atm)
  Equilibrium P (atm)
• Expt 1 N2O4 1.00
  0.22
• NO2 0.00
  1.56
• Expt 2 N2O4 0.00
  0.07
• NO2 1.00
  0.86
• Expt 3 N2O4 1.00
  0.42
• NO2 1.00
  2.16

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The Equilibrium Constant

• The relationship
• (PNO2 )2/ PN2O2
• is a constant (P is the equilibrium partial
  pressure of the species)
• Experiment 1 (PNO2 )2/ PN2O2 (1.56)2/0.22 11
• Experiment 2 (PNO2 )2/ PN2O2 (0.86)2/0.07 11
• Experiment 3 (PNO2 )2/ PN2O2 (2.16)2/0.42 11
• The constant is called the equilibrium constant K
  for the system

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Characteristics of Equilibrium

• A dynamic equilibrium can only exist in a closed
  system neither reactants nor products can enter
  or leave the system
• At equilibrium, the concentrations of reactants
  and products remain constant
• At equilibrium, the forward and reverse reactions
  are taking place at equal and opposite rates
• Equilibrium can be approached from either side of
  the reaction equation

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Steady State vs Equilibrium

• When a process takes place in an open system, a
  steady state may be set up
• In a steady state, the concentration of a product
  may be constant, but this will be because it is
  being formed, and is leaving the system at the
  same rate
• Consider the evaporation of water in closed and
  open systems

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The Evaporation of Water

• Water in an open beaker
• Water will evaporate at a rate dependent on its
  temperature.
• The concentration of water vapour in the top of
  the beaker will be constant, but water molecules
  are entering the space and leaving it at the same
  rate
• This is an open system it is a steady state
• Eventually, all of the water will evaporate

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The Evaporation of Water

• Water in a closed container
• Water will evaporate at a rate dependent on its
  temperature
• As the concentration of water vapour rises, water
  molecules will condense
• Eventually a dynamic equilibrium will be
  established at that temperature
• The rate of evaporation will equal the rate of
  condensation
• H2O(l) H2O(g)