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 Chatelier’s Principle Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) (colourless) (red)

2. 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)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

4. 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

5. 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

6. 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)2+]formed = [Fe3+]initial

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

8. AnalysisDetermining Equilibrium Constant K • 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 =

9. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) Exp 14B: Determining an Equilibrium Constant Part 1: Experimental - Determining k in Beer’s 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

10. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) Exp 14B: Determining an Equilibrium Constant Part 1: Analysis - Determining k (absorption constant)

11. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) 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+]

12. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) Exp 14B: Determining an Equilibrium Constant Part 2: Experimental - Determining equilibrium constant Kc

13. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) Exp 14B: Determining an Equilibrium Constant Part 2: Experimental - Determining equilibrium constant Kc

14. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) Exp 14B: Determining an Equilibrium Constant Part 2: Analysis - Determining equilibrium constant Kc

15. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) 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+] = [Fe3+]i - [Fe(SCN)2+]e = • Equilibrium [SCN-] = equilibrium [Fe3+] • Equilibrium constant K = [Fe(SCN)2+]e / [Fe3+]e [SCN-]e

16. Fe3+(aq) + SCN-(aq) Fe(SCN)2+(aq) 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+] = [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

17. 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