Equillibrium Lab

Purpose

To determine the equilibrium constant (Kc) for the reaction with a series of Iron III and SCN- reactions

What is Kc?

Kc = [FeSCN²⁺] / ([Fe³⁺][SCN⁻])

What does K tell you when it’s greater than one?

K>1 means products are favored

What does K tell you when it’s less than one?

K<1 means reactants are favored

Where are the iron thiocyanite equations thrown away?

Blue label waste container.

What’s the difference between the initial [SCN-] and [SCN-] consumed?

[SCN-] present at equillibrium

What’s the difference between the initial [Fe+3] and [Fe+3]

consumed?

[Iron III] present at equillibrium

What is plotted in Beer’s Law curve?

Absorbance vs. Concentration

Purpose of Part I?

Create a calibration curve to find K

Purpose of part II?

Determine equilibrium concentrations and calculate Kc

Procedure Part 1

1.Added 10.0mL of Iron III solution to 6 50mL volumetric flasks with a 10.0mL graduated cylinder

2.Pipetted 1.0-5.0mL of 0.0020M KSCN to flasks B-F

3.Added 0.1M HNO3 to each volumetric flask until 50.0mL, covered with parafilm, and mixed

4.Transferred solutions to cuvettes and measured absorbances at 447nm with the spectrophotometer

Procedure Part II

1.Added 5.0mL of 0.0020M Fe(NO₃)₃ to test tubes A-F

2.Added 1.0-5.0mL of 0.0020M KSCN to test tubes B-F in increasing order

3.Added HNO₃ to test tubes A-E to add to 10.0mL, covered with parafilm and inverted to mix

4.Transferred solutions to cuvette and measured absorbances at 447nm with the spectrophotometer

What settings is the spectrophotometer?

447nm

What was the purpose of KSCN (the source of SCN-)?

In part 1, determines the known concentrations of FeSCN+2 and

In part 2, establishes the diff equillibrium mixtures to calculate Kc

What was the concentration of the Fe(NO₃)₃ stock solution for the calibration curve(Part 1)?

0.2M

What was the concentration of the KSCN stock solution?

0.002M for part 1 and 2

What was the concentration of the Fe(NO₃)₃ stock solution part two?

0.002M, the same as KSCN

Why does initial [SCN-] = [FeSCN²⁺]?

There’s more iron III so all of SCN- reacts completely

Use dilution to calculate initial [SCN-]/[FeSCN⁺²] for calibration curve

C₁ = 0.002M KSCN       V₁ = Volume of KSCN added (mL)

C₂ = X [SCN-]                V₂ = 50.0mL

Calculating initial [Fe⁺³] for calibration curve

1.Molarity x Volume = Moles

2.Moles/Total Volume = Molarity

Molarity of Fe(NO₃)₃ Stock Part 1 (0.2M) x 10.0mL = molsFe(NO₃)₃

Fe(NO₃)₃ mols/ 50.0mL total volume solution = [Fe⁺³]

What calculations are for part 1?

Equilibrium [FeSCN⁺²] = initial [SCN-], initial [Fe⁺³]

Why is the calculated initial [Fe⁺³] constant for each table?

We added the same amount of Fe⁺³ to every solution each part of the experiment, so it’s the same concentration

Using 0.0020MFe⁺³, 10.0mL in each flask part 1 and then 5.0mL each tube part 2.

What calculations are in part II?

initial [Fe⁺³]

initial [SCN-]

equillibrium[FeSCN+2]

then calculate [Fe+3] and [SCN-] at equillibrium with an ICE box

How to find [FeSCN⁺²] with the graph for part 2?

Absorbance/slope

Equation for ice box

[Fe⁺³] + [SCN-] → [FeSCN⁺²]

How to find Fe⁺³] and [SCN-] with icebox

We know:

[FeSCN⁺²] at equilibrium and all the initial concentrations

then add/subtract to find the rest of the concentrations at equilibrium

Use K = FeSCN⁺²]     

[Fe⁺³] [SCN-]

Calculating initial [Fe⁺³] in part 2

[Fe⁺³] = (M stock x mL added/ Total Volume

0.0020M Fe(NO₃)₃ × 5.0mL/ 10.0mL = 0.0010M

Calculating initial [SCN-] in part 2

C1 = 0.002M KSCN

V1 = Volume of KSCN added(mL)

C2 = X [SCN-] we want to find

V2 = 10.0mL total volume

What does the spectrophotometer detect?

FeSCN⁺² complex

Role of HNO₃

To maintain an acidic environment for the reaction

Why keep total volume the same in each trial?

So concentration differences are only due to reactants, not dilution

Does K have units?

No