Exp 14B: Determining an Equilibrium ConstantExp 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) Fe(SCN)2+(aq)(colourless) (red)
Determining an Equilibrium Constant 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.
Determining an Equilibrium Constant Determining an Equilibrium Constant Le Chatelier's Principle
Equilibrium constant Keq
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)(colourless) (red)
Keq = [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 moleculesAbsorption of light by atoms and molecules
Transmission = ratio of transmitted light/incident light = I/Io
Beer’s LawAbsorption = amount of light absorbed by solution = log Io/I = *l*c
Beer’s LawBeer’s Law
Transmission = I/Io
Absorption = -log T = log Io/I
Beer’s LawA= * l * c = k * c
A= absorption of lightl = length of light pathc = concentration
= molar absorptivity or molar absorption coefficientk = * l = absorption constant
Determining an Equilibrium ConstantDetermining 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)2+]formed = [Fe3+]initial
AnalysisAnalysisDetermining absorption constant Determining absorption constant kk
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
AnalysisAnalysisDetermining Equilibrium Constant Determining Equilibrium Constant KK
1. Measure A450 nm of samples with different concentrations of reactants
2. Calculate [Fe(SCN)2+], [Fe3+]i, [Fe3+]eq, [SCN-]i and [SCN-]eq
3. - [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 ConstantExp 14B: Determining an Equilibrium Constant
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 tubesTest 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
Part 1: Experimental - Determining Part 1: Experimental - Determining kk in Beer’s Law in Beer’s Law
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
Test Tube No [Fe(SCN)2+] Absorption
1 1.0× 10-5 M
2
3
4
5
Part 1: Analysis - Determining Part 1: Analysis - Determining k (absorption constant)k (absorption constant)
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
Plot [Fe(SCN)2+] vs Absorption
• [Fe(SCN)2+] on X-axis
• Absorption on Y-axis
• Slope = k = absorption constant
Part 1: Analysis - Determining Part 1: Analysis - Determining k (absorption constant)k (absorption constant)
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
[Fe(SCN)2+]
Abs
orpt
ion
k = slope = Abs/[Fe(SCN)2+]
Line of best fit
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
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
Part 2: Experimental - Determining Part 2: Experimental - Determining equilibrium constant Kequilibrium constant Kcc
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
Test Tube No
Absorption
6
7
8
9
10
Part 2: Experimental - Determining Part 2: Experimental - Determining equilibrium constant Kequilibrium constant Kcc
Test Tube No
Absorption
11
12
13
14
15
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
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
Part 2: Analysis - Determining Part 2: Analysis - Determining equilibrium constant Kequilibrium constant Kcc
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
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
Part 2: Analysis - Determining Part 2: Analysis - Determining equilibrium constant Kequilibrium constant Kcc
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
Exp 14B: Determining an Equilibrium ConstantExp 14B: Determining an Equilibrium Constant
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
Part 2: Analysis - Determining Part 2: Analysis - Determining equilibrium constant Kequilibrium constant Kcc
Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq)
• Next Week Oct 29 Exp 14B: Full lab report including graph for all the results
Exp 15: The Relative Strength of Some AcidsLab preparations
– Read background and procedure– Protocol– Chemicals: HCl, H3PO4, NaH2PO4, CH3COOH, NH4NO3 , Al(NO3)3 ,
Zn(NO3)2 • Prelab assignment