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 Chatelier’s Principle
Fe3+(aq) + SCN-(aq)
(colourless)
Fe(SCN)2+(aq)
(red)
Determining an Equilibrium Constant
Le Chatelier's Principle
Changes in Concentration
Consider the system at equilibrium
Fe3+(aq) + SCN-(aq)
(colourless)
•
•
Fe(SCN)2+(aq)
(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.
Determining an Equilibrium Constant
Le Chatelier's Principle
Equilibrium constant Keq
Fe3+(aq) + SCN-(aq)
(colourless)
Keq =
Fe(SCN)2+(aq)
(red)
[Fe(SCN)2+]eq
[Fe3+]eq [SCN-]eq
•
How do we measure concentrations?
–
Absorption of light
–
Applying Beer’s Law
• absorption of light at a specific wavelength is proportional to
the concentration of a solution
Absorption of light by atoms and molecules
Transmission = ratio of transmitted light/incident light = I/Io
Beer’s Law
Absorption = amount of light absorbed by solution = log Io/I
= el*l*c
Beer’s Law
Transmission = I/Io
Absorption = -log T = log Io/I
Beer’s 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
Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
(colourless)
Fe(SCN)2+(aq)
(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)2+]formed = [Fe3+]initial
Analysis
Determining absorption constant k
1. Measure samples in spectrophotometer at 450
nm (absorption maximum for Fe(SCN)2+)
2. Plot absorption vs. [Fe(SCN)2+]formed
3. Determine absorption constant k = slope of
curve
4. Use A = k * c, or c = A/k
Analysis
Determining Equilibrium Constant K
1.
2.
3.
Measure A450 nm of samples with different concentrations of
reactants
Calculate [Fe(SCN)2+], [Fe3+]i, [Fe3+]eq, [SCN-]i and [SCN-]eq
- [Fe3+]i = [SCN-]i = 0.0025 M x 1.0 mL/7.0 mL = 3.6 x 10-4 M
- [Fe(SCN)2+] = A/k
- [Fe3+]eq = [SCN-]eq= [Fe3+]i - [Fe(SCN)2+] =
3.6 x 10-4 M – A/k = X M
 Keq = [Fe(SCN)2+]eq/[Fe3+]eq [SCN-]eq =
Exp 14B: Determining an Equilibrium Constant
Part 1: Experimental - Determining k in Beer’s Law
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
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)
1M
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
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
Part 1: Analysis - Determining k (absorption constant)
Test Tube No
[Fe(SCN)2+]
1
1.0× 10-5 M
2
3
4
5
Absorption
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
Part 1: Analysis - Determining k (absorption constant)
Absorption
Plot [Fe(SCN)2+] vs Absorption
• [Fe(SCN)2+] on X-axis
• Absorption on Y-axis
• Slope = k = absorption constant
Line of best fit
k = slope = Abs/[Fe(SCN)2+]
[Fe(SCN)2+]
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
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
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
Part 2: Experimental - Determining equilibrium constant Kc
Test Tube
No
Absorption
Test Tube
No
6
11
7
12
8
13
9
14
10
15
Absorption
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
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
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
Part 2: Analysis - Determining equilibrium constant Kc
Calculation of concentration
Tube 6:
• starting [Fe3+] = [SCN-]
• [Fe(SCN)2+] = Absorption/slope = Abs/k
• Equilibrium [Fe3+] = [Fe3+]i - [Fe(SCN)2+]e =
• Equilibrium [SCN-] = equilibrium [Fe3+]
• Equilibrium constant K = [Fe(SCN)2+]e / [Fe3+]e [SCN-]e
Exp 14B: Determining an Equilibrium Constant
Fe3+(aq) + SCN-(aq)
Fe(SCN)2+(aq)
Part 2: Analysis - Determining equilibrium constant Kc
Calculation of concentration
Tube 7:
• starting [Fe3+]
• starting [SCN-]
• [Fe(SCN)2+] = Absorption/slope
• Equilibrium [Fe3+] = [Fe3+]i - [Fe(SCN)2+]e
• Equilibrium [SCN-] = [SCN-]i - [Fe(SCN)2+]e
• Equilibrium constant K = [Fe(SCN)2+]e / [Fe3+]e [SCN-]e
• 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