Exp 14B: Determining an Equilibrium Constant

1
Exp 14B Determining an Equilibrium Constant

• Le Chatelier's Principle
• In 1884, the French chemist Henri Le Chatelier
  suggested that equilibrium systems tend to
  compensate for the effects of stress or changes.
• When a system at equilibrium is disturbed, the
  equilibrium position will shift in the direction
  which tends to minimize, or counteract, the
  effect of the disturbance.
• If the concentration of a reactant is increased,
  the equilibrium position shifts to use up the
  added reactants by producing more products.
• Reaction between Fe3 and thiocyanate(SCN-)
  results in iron(III) thiocynate, Fe(SCN)2, a red
  complex, which represents an example of Le
  Chateliers Principle
• Fe3(aq) SCN-(aq) Fe(SCN)2(aq)
• (colourless) (red)

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Determining an Equilibrium Constant Le
Chatelier's Principle

• Changes in ConcentrationConsider the system at
  equilibrium
• Fe3(aq) SCN-(aq) Fe(SCN)2(aq)
• (colourless) (red)
• Increasing concentration of Fe3(aq) or SCN-(aq)
• results in the equilibrium position moving to the
  right
• use up some of the additional reactants and
  producing more Fe(SCN)2(aq)
• solution will become darker red (more Fe(SCN)2).
• Decreasing concentration of Fe3(aq) or SCN-(aq)
• results in the equilibrium position moving to the
  left
• produces more Fe3(aq) and SCN-(aq).
• the solution will become less red as
  Fe(SCN)2(aq) is consumed.

3
Determining an Equilibrium Constant Le
Chatelier's Principle

• Equilibrium constant Keq
• Fe3(aq) SCN-(aq) Fe(SCN)2(aq)
• (colourless) (red)
• Keq Fe(SCN)2eq
• Fe3eq SCN-eq
• How do we measure concentrations?
• Absorption of light
• Applying Beers Law
• absorption of light at a specific wavelength is
  proportional to the concentration of a solution

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Absorption of light by atoms and molecules
Transmission ratio of transmitted
light/incident light I/Io Beers
Law Absorption amount of light absorbed by
solution log Io/I ellc
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Beers Law

• Transmission I/Io
• Absorption -log T log Io/I
• Beers Law
• A el l c k c
• A absorption of light
• l length of light path
• c concentration
• el molar absorptivity or molar absorption
  coefficient
• k el l absorption constant

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Determining an Equilibrium Constant

• Fe3(aq) SCN-(aq) Fe(SCN)2(aq)
• (colourless) (red)
• Experimental
• Measure absorbance of a series of solutions with
  different known concentrations of the complex
  ion, Fe(SCN)2
• Problem
• Changing concentration of reactants changes
  concentration of complex product Fe(SCN)2 is
  participant in reaction!
• Solution
• Use excess of one of the reactants, so the other
  reactant becomes limiting
• Use excess SCN-, then Fe3 is limiting reactant
• ? Fe(SCN)2formed Fe3initial

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AnalysisDetermining absorption constant k

• Measure samples in spectrophotometer at 450 nm
  (absorption maximum for Fe(SCN)2)
• Plot absorption vs. Fe(SCN)2formed
• Determine absorption constant k slope of curve
• Use A k c, or c A/k

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AnalysisDetermining Equilibrium Constant K

• Measure A450 nm of samples with different
  concentrations of reactants
• Calculate Fe(SCN)2, Fe3i, Fe3eq, SCN-i
  and SCN-eq
• - Fe3i SCN-i 0.0025 M x 1.0 mL/7.0 mL
  3.6 x 10-4 M
• - Fe(SCN)2 A/k
• - Fe3eq SCN-eq Fe3i - Fe(SCN)2
• 3.6 x 10-4 M A/k X M
• ? Keq Fe(SCN)2eq/Fe3eq SCN-eq

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Exp 14B Determining an Equilibrium Constant
Part 1 Experimental - Determining k in Beers Law

• Step 1 make a dilution of 0.0025 M Fe(NO3)3 to
  0.0001 M 0.0025 M x (4.0 mL/100 mL)
• Use a 5-mL Mohr pipet to add 4.0 mL of 0.0025 M
  Fe(NO3)3 to a 100-mL volumetric flask
• Add 0.1 M HNO3 until exactly 100 mL. Mix
• Rinse the pipet with this solution
• Add the specified amounts from the table below to
  5 numbered test tubes

Test Tube No Diluted Fe(NO3)3 (mL) (0.0001 M) 1 M KSCN (ml) 0.1 M HNO3 (mL) Total Volume (mL) Concentration Fe(SCN)2
1 1.0 5.0 4.0 10.0 0.0001 M (1.0 mL/ 10 mL) 1.0 10-5 M
2 2.0 5.0 3.0 10.0
3 3.0 5.0 2.0 10.0
4 4.0 5.0 1.0 10.0
5 5.0 5.0 0 10.0
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Exp 14B Determining an Equilibrium Constant
Part 1 Analysis - Determining k (absorption
constant)
Test Tube No Fe(SCN)2 Absorption
1 1.0 10-5 M
2
3
4
5
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Exp 14B Determining an Equilibrium Constant
Part 1 Analysis - Determining k (absorption
constant)

• Plot Fe(SCN)2 vs Absorption
• Fe(SCN)2 on X-axis
• Absorption on Y-axis
• Slope k absorption constant

Line of best fit
Absorption
k slope Abs/Fe(SCN)2
Fe(SCN)2
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Exp 14B Determining an Equilibrium Constant
Part 2 Experimental - Determining equilibrium
constant Kc
Test Tube No 0.0025 M Fe(NO3)3 (mL) 0.0025 M KSCN (mL) 0.1 M HNO3 (mL) Total Volume (mL)
6 1.0 1.0 5.0 7.0
7 1.0 1.5 4.5 7.0
8 1.0 2.0 4.0 7.0
9 1.0 2.5 3.5 7.0
10 1.0 3.0 3.0 7.0
11 2.0 1.0 4.0 7.0
12 2.0 1.5 3.5 7.0
13 2.0 2.0 3.0 7.0
14 2.0 2.5 2.5 7.0
15 2.0 3.0 2.0 7.0
Total Vol. 5 10 20
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Exp 14B Determining an Equilibrium Constant
Part 2 Experimental - Determining equilibrium
constant Kc
Test Tube No Absorption
6
7
8
9
10
Test Tube No Absorption
11
12
13
14
15
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Exp 14B Determining an Equilibrium Constant
Part 2 Analysis - Determining equilibrium
constant Kc
Test Tube Starting Fe3 Starting SCN- Equilibrium Fe(SCN)2 Equilibrium Fe3 Equilibrium SCN- Kc
6
7
8
9
10
11
12
13
14
15
Average
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Exp 14B Determining an Equilibrium Constant
Part 2 Analysis - Determining equilibrium
constant Kc

• Calculation of concentration
• Tube 6
• starting Fe3 SCN-
• Fe(SCN)2 Absorption/slope Abs/k
• Equilibrium Fe3 Fe3i - Fe(SCN)2e
• Equilibrium SCN- equilibrium Fe3
• Equilibrium constant K Fe(SCN)2e / Fe3e
  SCN-e

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Exp 14B Determining an Equilibrium Constant
Part 2 Analysis - Determining equilibrium
constant Kc

• Calculation of concentration
• Tube 7
• starting Fe3
• starting SCN-
• Fe(SCN)2 Absorption/slope
• Equilibrium Fe3 Fe3i - Fe(SCN)2e
• Equilibrium SCN- SCN-i - Fe(SCN)2e
• Equilibrium constant K Fe(SCN)2e / Fe3e
  SCN-e

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• Next Week Oct 29
• Exp 14B Full lab report including graph for all
  the results
• Exp 15 The Relative Strength of Some Acids
• Lab preparations
• Read background and procedure
• Protocol
• Chemicals HCl, H3PO4, NaH2PO4, CH3COOH, NH4NO3 ,
  Al(NO3)3 , Zn(NO3)2
• Prelab assignment