Exp 14B : Determining an Equilibrium Constant

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


• 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


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 =


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



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)



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)


[Fe(SCN)2+]

Abs

orpt

ion

k = slope = Abs/[Fe(SCN)2+]

Line of best fit


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



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



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



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




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 AcidsLab preparations

– Read background and procedure– Protocol– Chemicals: HCl, H3PO4, NaH2PO4, CH3COOH, NH4NO3 , Al(NO3)3 ,

Zn(NO3)2 • Prelab assignment

Documents

Exp 14B : Determining an Equilibrium Constant