Experimentally Determining Keq

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Experimentally Determining Keq
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Introduction

• At equilibrium, the concentration of all
  reactants remain constant with time
• Dynamic state exists the original reaction is
  still occurring but equally opposed by the
  reverse reaction
• The equilibrium constant, Keq, is the ratio of
  the concentration of all products present to the
  original reactants at the point of equilibrium

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The Equilibrium Constant

• For the general reaction
• aA bB ? cC dD
• Keq

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Determining Keq

• The main purpose of this lab is to determine Keq
  for the complex ion formed by the reaction of
  Fe3 with SCN-
• Fe3(aq) SCN-(aq) ? FeSCN2 (aq)
• orange clear
  wine red
• Where Keq

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Overview of Analyses

• Experimentally determine Transmittance
• Convert T to Absorbance
• Determine concentration of product formed by
  comparing abs values to a calibration curve
• Determine initial and equilibrium concentrations
  of reactants using an ICE table

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Reaction Concentrations

• Use a reaction (ICE) table, such as the one
  below, to determine concentrations of all species
  at equilibrium
• Fe3(aq) SCN-(aq) ? FeSCN2
  (aq)
• Initial .002M .002M
  0
• Change - x - x
  x
• Equilib. .002M - x .002M - x x

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Reactant Concentrations

• Use the dilution formula to determine initial
  concentrations of mixtures
• C1V1 C2V2
• C1 Original Concentration of Reactant
• V1 Initial Volume
• C2 Concentration after dilution
• V2 Total Volume after dilution

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Product Concentrations

• Using the Spectrophotometer, determine T of each
  mixture.
• Convert T to Abs (A 2 - log T)
• Find calculated absorbance point on the
  calibration curve and record the product
  concentration at that point
• Apply these values to the ICE table to
  determine all concentrations needed for
  calculating Keq

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Procedural Notes

• The cuvets wont hold the total dilution volume
  for each solution make your solutions in the
  large test tubes and transfer what you need to
  the cuvet
• Use a Kimwipe to clean the outside of each
  cuvet hold cuvets at the top so your
  fingerprints wont affect readings
• Take all readings at l 447nm