Chem 1310: Introduction to physical chemistry Part 3: Equilibria

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Chem 1310 Introduction to physical chemistry
Part 3 Equilibria

• Using ICE for equilibria

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Using ICE for equilibria

• Write out the chemical equation
• Setup the ICE tableone column for each reactant
  and productone row each for Initial/Change/Equili
  brium
• Define one change as x, express the other changes
  in it, fill the table
• Use stoichiometry or the rate constant expression
  to get an equation in x, solve it, calculate all
  fields

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• Given initial H2 I2 0.0175 M, final HI
  0.0270 M, calculate K.

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H2 I2 ? 2 HIstoichiometry question

• K HI2/(H2I2) (0.0276/0.0037)2 56

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H2 I2 ? 2 HIinitial composition question

• Given initial H2 I2 0.01 M, K 56, what
  is the final composition?

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H2 I2 ? 2 HIinitial composition question

• Given initial H2 I2 0.01 M, K 56, what
  is the final composition?

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H2 I2 ? 2 HIinitial composition question

• Given initial H2 I2 0.01 M, K 56, what
  is the final composition?

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H2 I2 ? 2 HIinitial composition question

• Use K HI2/(H2I2) (2x)2/(0.01-x)2 56
  ("trick")
• (2x)/(0.01-x) 7.48 Þ 2x 0.0748-7.48x Þ x
  0.00789

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H2 I2 ? 2 HIinitial composition question

• x 0.00789

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H2 I2 ? 2 HIinitial composition question 2

• Given initial H2 0.01 M, I2 0.02 M, K
  56, what is the final composition?

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H2 I2 ? 2 HIinitial composition question 2

• Given initial H2 0.01 M, I2 0.02 M, K
  56, what is the final composition?

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H2 I2 ? 2 HIinitial composition question 2

• Given initial H2 0.01 M, I2 0.02 M, K
  56, what is the final composition?

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H2 I2 ? 2 HIinitial composition question 2

• K (2x)2/(0.01-x)(0.02-x) 56
• Þ 4x2 56(0.01-x)(0.02-x) 56x2-1.68x0.0112
• Þ 52x2-1.68x0.0112 0 Þ x 0.0094

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H2 I2 ? 2 HIinitial composition question 2

• x 0.0094

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H2 I2 ? 2 HImixed composition question

• Given initial H2 0.01 M, K 56, which
  initial I2 should we choose to arrive at a
  final HI 0.015?

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H2 I2 ? 2 HImixed composition question

• Given initial H2 0.01 M, K 56, which
  initial I2 should we choose to arrive at a
  final HI 0.015?

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H2 I2 ? 2 HImixed composition question

• Given initial H2 0.01 M, K 56, which
  initial I2 should we choose to arrive at a
  final HI 0.015?

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H2 I2 ? 2 HImixed composition question

• K (0.015)2/(0.0025)(y-0.0075) 56
• Þ y 0.0091 Þ final I2 0.0016
• Check by re-calculating K !!!

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Ammonia formation

• Given a mixture of 1 mol N2 and 3 mol H2 in 10L,
  we add a catalyst to form ammonia at room
  temperature. What percentage of the starting
  material will react? KC 3.5108.
• N2 3 H2 ? 2 NH3

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Ammonia formation

• 1 mol N2 and 3 mol H2 in 10L

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Ammonia formation

• KC NH32/(N2H23) (2x)2/(.1-x)(.3-3x)3
  4x2/27(.1-x)4 3.5108 Þ 2x/5.196(.1-x)2
  18708Þ (.1-x)2 0.0000206x Þ x2-0.20002x0.01
  0Þ x 0.0986

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Ammonia formation

• x 0.0986. Conversion percentage (x/0.1)100
  98.6
• Try this for yourself what happens if the volume
  is increased to 100 L?

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Formation of water -using approximations

• O2 2 H2 ? 2 H2O
• We start with 1 mol of O2, 2 mol of H2 in 10 L.
  How many molecules of reactants remain?

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Formation of water -using approximations

• KC H2O2/(O2H22) (2x)2/(.1-x)(.2-2x)2
  (2x)2/4(.1-x)3 3.31081.
• Not easy to solve. Problem!

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Formation of water -using approximations

• KC (2x)2/4(.1-x)3 3.31081
• x must be very close to 0.1.
• There is a standard "trick" to solve problems
  like this. You want to neglect a small variable
  relative to a larger known quantity.
• That works best if we are trying to solve for a
  small variable. Since the reaction is virtually
  complete, it is better to solve for the fraction
  (y) that has not reacted.

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Formation of water -using approximations

• KC H2O2/(O2H22) (0.2-2y)2/y(2y)2
  (0.2-2y)2/4y3 (0.2)2/4y3 3.31081Þ y3
  (0.2)2/(43.31081) 3.0310-84 Þ y
  1.4510-28 mol/Lor a total of 0.00087 molecules.
  "Product-favoured"