Chemistry 1011

1
Chemistry 1011

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

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12.3 Determination of K

• YOU ARE EXPECTED TO BE ABLE TO
• Calculate the value of K from experimental data
  for the system at equilibrium
• Calculate the value of K from experimental data
  for the system at the start of a reaction and at
  equilibrium

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Determination of K from Equilibrium Data

• Write a balanced equation for the equilibrium
• Write an expression for the equilibrium constant
• Input equilibrium partial pressures of components
• Calculate K

4
Determination of K from Equilibrium Data

• Ammonium chloride decomposes on heating
• At 400oC, 12.0g of NH4Cl is present in a closed
  system. The partial pressures of NH3 and HCl are
  3.0atm and 5.0atm.
• Calculate K
• NH4Cl(s) NH3(g) HCl(g)
• KP PNH3 x PHCl 3.0 x 5.0 15

5
Determination of K from Initial and Equilibrium
Data

• Write a balanced equation for the equilibrium
• Write an expression for the equilibrium constant
• Distinguish equilibrium from initial partial
  pressures
• Use stoichiometry to determine equilibrium
  partial pressures from initial partial pressure
  data
• Input equilibrium partial pressures of components
• Calculate K

6
Determination of K from Initial and Equilibrium
Data

• Consider the system
• 2HI(g) H2(g) I2(g)
• Initially, a system contains HI only, at a
  pressure of 1.00atm at 520oC.
• At equilibrium, the partial pressure of H2(g) is
  found to be 0.10atm
• Find
• PI2 at equilibrium
• PHI at equilibrium
• Kp

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Determination of K from Initial and Equilibrium
Data

• 2HI(g) H2(g) I2(g)
• Initially 1.00atm
  0.00atm 0.00atm
• At equilibrium ?
  0.10atm ?
• 2 moles of HI must react in order to produce 1
  mole of H2
• At the same time, 1 mole of I2 will be produced
• Partial pressure is proportional to the number of
  moles of gas
• DP -0.20atm
  0.10atm 0.10atm
• At equilibrium 0.80atm
  0.10atm 0.10atm
• Kp PH2 x PI2 0.10 x 0.10 0.016
• (PHI )2 (0.80)2

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Equilibrium Reaction of NOBr

• 2.00atm of NOBr(g) are placed into a closed
  container at 350oC
• NOBr(g) decomposes to NO(g) and Br2(g) and an
  equilibrium is established
• At equilibrium, the partial pressure of Br2 is
  0.25atm
• Write an equation for the equilibrium
• Write an expression for Kp
• Evaluate Kp for the reaction at 350oC

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Equilibrium Reaction of NOBr

• 2 NOBr(g) 2NO(g)
  Br2(g)
• Initial 2.00atm
  0.00atm 0.00atm
• Equilibrium ? ?
  0.25atm
• Each mole of NOBr that decomposes will produce 1
  mole of NO and ½ mole of Br2
• DP -0.50atm
  0.50atm 0.25atm
• Equilibrium 1.50atm 0.50atm
  0.25atm
• Kp (PNO)2 x PBr2 (0.50)2 x (0.25)
  2.8 x 10-2
• (PNOBr)2 (1.50)2