What percent of the SCN − present initially have been converted to FeSCN2 at equilibrium?

Lab 11 - Spectroscopic Determination of an Equilibrium Constant

Goal and Overview

The reaction of iron (Iii) with thiocyanate to yield the colored production, iron (III) thiocyanate, can exist described by the following equilibrium expression.

( one )

Fe^three+ + SCN⁻ FeSCNⁱⁱ⁺

You will written report this equilibrium using the Spec 20 UV-visible spectrometer. The wavelength of lite absorbed most strongly by the product will exist determined from the spectral contour of FeSCNⁱⁱ⁺. A Beer's Law plot volition be made for a serial of FeSCN²⁺ solutions of known concentration. Then, the concentrations of FeSCN²⁺ will be measured spectroscopically for a ready of solutions made with different initial concentrations of reactants. This data volition be used to decide

K _eq,

the equilibrium abiding for the reaction.

Objectives and Scientific discipline Skills

• •

  Perform volumetric dilutions and calculate resulting molarities.

• •

  Empathize and explain assimilation spectroscopy and the mathematical relationships betwixt percentage transmittance, absorbance, concentration, path length, and extinction coefficient.

• •

  Apply linear fitting methods to find relationships between dependent and independent variables, such as percent transmittance (absorbance) and concentration.

• •

  Explain and apply Beer's Constabulary; draw the assumptions and limitations imposed by the nature of the equilibrium on the calculation of FeSCN²⁺ associated with the absorption data.

• •

  Use absorption information to qualitatively and quantitatively analyze the concentration of FeSCN²⁺ in solution.

• •

  Identify and discuss factors or effects that may contribute to the uncertainties in values or assessments made from experimental data.

• •

  Analyze, quantify, and discuss the uncertainty in results when assumptions are used.

Suggested review and external reading

• •

  Reference information on spectroscopy (see Lab nine) and dilutions; relevant textbook information on spectroscopy and equilibrium

Background

This experiment investigates the equilibrium established by the reaction of the iron (Iii), Feⁱⁱⁱ⁺, and the thiocyanate, SCN^–, ions. See Eq. 1

Fe³⁺ + SCN⁻ FeSCN²⁺

. The reactants are colorless, only the FeSCN²⁺ ion is orangish-blood-red colored. At equilibrium, the concentrations of these three ions must be related to each other according to the equilibrium constant expression.

One goal in this experiment is to mensurate the value of Grand, using the spectrometer to quantitatively analyze the concentration of FeSCN^two+ ion. Absorption spectroscopy and Beer'due south Police were discussed in detail and were used in the Allura Scarlet Lab. This same method will be utilized to determine [FeSCN²⁺], the colored production.

( 4 )

T = = 10^{− ε bc} = 10^{− A}

Then, yous will use your understanding of equilibrium processes to deduce the equilibrium concentrations of the reactants. Knowing all three concentrations listed above in Eq. 2

Thousand =

 allows the equilibrium constant for this reaction to be calculated.

Procedure

Function 1: Qualitative Observations: Is the reaction exothermic or endothermic?

1

Using a 10 mL graduated cylinder, mensurate out approximately 2 mL of 2 × 10^–3 M NaNO₃ and put it in a exam tube.

2

Add together approximately 8 mL of 2 × 10^–3 1000 NaSCN.

3

Add approximately 10 mL of two × 10^–3 Chiliad Fe(NO_three)_iii. Note the color of the solution.

four

Fill up a Spec 20 cuvette no more than than 2/3 full, and split the remaining solution among iii test tubes.

five

Place one tube in an ice bath and i in the hot water bath on the hot plate.

six

Subsequently about ten minutes, compare them with the solution at room temperature.

7

Talk over the implications of your observations, basing your discussion on your noesis of Le Châtelier'south principle.

Do your observations imply an exothermic or endothermic reaction?

Part 2: Spectral Profile and

λ _max

(most sensitive wavelength) of FeSCN²⁺

1

Measure transmittance (%T) to 0.ane% of the mixtures in your cuvette in the range from 370 to 560 nm.

• a

  Utilize a cuvette containing 2 × 10^–iii G NaNO₃ as a blank to set 100% transmission.

• b

  Cheque the %T readings at 20 nm intervals.

• c

  When you attain the region of minimum transmittance, reduce the intervals to ten or fifty-fifty to 5 nm.

2

For the wavelength of minimum %T, calculate the absorbance, A, of the solution (A = –log T) to the appropriate number of significant figures.

three

Identify the wavelength of maximum absorbance, the experimental value of

λ _max.

How does your

λ _max

compare with your answer to Question 1 of the pre-lab assignment?

4

Set your spectrometer to

λ _max

for parts 3 and 4.

Function 3: Beer'southward Police Curve for FeSCN²⁺: determining ε of the production at

λ _max

To make solutions of known concentrations of FeSCN²⁺, you cannot merely dissolve a salt containing FeSCN^two+ in water because the ion will dissociate in social club to satisfy the equilibrium constant expression. If

M _eq

were known, solutions with known initial concentrations of FeSCN^two+ could be made, and algebra could be used to find equilibrium concentrations of Fe^three+, SCN^–, and FeSCN²⁺. Nonetheless, Yard is what you are trying to find in Role 4. In club to overcome this difficulty, y'all volition take spectral data for a fix of solutions made past mixing a very high Iron³⁺ concentration with extremely small SCN^– concentrations. Assume that SCN^– is the limiting reactant (i.e., that essentially all of it is used up to make FeSCN²⁺). By doing this, you can equate the equilibrium concentration of iron (III) thiocyanate,

[FeSCN²⁺]_eq

to the initial concentration of thiocyanate,

[SCN⁻]_initial.

Once you have actually determined

Grand _eq,

you must return to this assumption and verify its validity. Note for Parts 3 & four: You may wish to split the dilution work with your partner to salvage time. One person could do Part 3 while the other is doing Part 4.

1

Take well-nigh 100 mL of 0.i M Fe³⁺ solution ( solution B ) in a small labeled beaker.

2

Make the strongest colored solution of NaSCN and Fe(NO₃)₃ ( solution A ).

• a

  Using a volumetric pipet, put 5 mL of 2 × x^–three M NaSCN (concentration known to i%) into a 50 mL volumetric flask.

• b

  Fill up to the mark with solution B (higher up; 0.1 M Iron³⁺ solution).

three

Use only volumetric glassware, not graduated pipets or cylinders.

• •

  You lot accept the following volumetric pipets bachelor: 1, 2, 5, ten mL.

• •

  You have the following volumetric flasks available: ten, 50, 100 mL.

Accurately create x mL volumes of the post-obit dilutions of solution A with solution B. As each of these solutions is created, measure its %T to 0.1%.

• a

  Pure B for utilize as a blank (faint straw-colored, no colored complex); this is 0.1 K Iron(NO₃)_three.

• b

  1 mL A into 10 mL flask, filled to mark with B

• c

  3 mL A into 10 mL flask, filled to marker with B

• d

  5 mL A into x mL flask, filled to marker with B

• e

  vii mL A into 10 mL flask, filled to marker with B

• f

  ix mL A into ten mL flask, filled to mark with B

• grand

  Pure A, the pure, most cerise-orange solution

4

Calculate absorbance for each solution: A = –log(%T/100%).

5

Make a Beer'due south Law plot of absorption versus concentration of FeSCN^two+ (A vs [FeSCN²⁺]) for these 7 points. Use two meaning figures in your concentration values and iii for your absorbance values.

6

Draw the all-time-fit straight line to the points.

seven

This best-fit line mathematically has the form of Beer'due south Law: A = ε bc, with slope = ε b and y-intercept = 0; ε = molar extinction coefficient in L/mol · cm, b = pathlength in cm, and c = concentration in mol/L. Determine the gradient to the ones place.

8

Using the measured pathlength b of your cuvette (to 0.01 cm) and your slope, calculate the value of the extinction coefficient for FeSCN^two+ at its

λ _max

to the ones place.

9

Record which Spec twenty you used so you tin employ the same i for Part 4.

Summary of Desired Quantities Parts i – three.

• •

  Temperature Dependence: Is the reaction exothermic or endothermic? Explain.

• •

  Spectral Profile: What is

  λ _max?

• •

  Beer's Police Plot: Graph of Absorbance versus [FeSCN²⁺] noting ε at

  λ _max.

Part iv: Equilibrium Constant for the Formation of FeSCN²⁺

In this part of the experiment, you will gear up five solutions with the same initial concentration of Fe³⁺ ion but different initial concentrations of SCN^– ion. As y'all make each solution, mensurate its percentage transmittance at

λ _max

(Part two) and use your Beer's Law plot (Part 3) to institute the equilibrium FeSCN²⁺ concentration. From the initial concentrations of the reactants and the equilibrium concentration of the product, y'all can calculate the experimental value of

1000 _eq

for each of the five solutions using Eq. 2

G =

.

1

Use the solutions provided, each of which is ii × 10^–3 Yard: NaSCN, Fe(NO₃)₃, and NaNO_iii.

two

Use volumetric pipets and a 10 mL volumetric flask to ready each of the following five solutions.

• •

  NaSCN and Atomic number 26(NO₃)₃ deliver SCN^– and Fe^three+ to the solution.

• •

  NaNO_three is used to make full the flask to 10 mL.

• •

  The "Total used" row is designed to assist y'all judge how much of the stock solutions y'all should have in labeled beakers to your lab station.

• •

  Delight minimize waste – exercise not take actress and delight share leftovers.

Waste material disposal:		The solutions must exist put into the labeled waste material bottles in the back hood.
		Cipher tin can get down the sink.
		If you are e'er in doubtfulness, ask your TA.

3

Observe equilibrium concentrations of Fe^three+, SCN^–, and FeSCN²⁺ to two significant figures. (Run across Equations one

Feⁱⁱⁱ⁺ + SCN⁻ FeSCN²⁺

 and two

One thousand =

.)

ICE tables volition help yous decide these values. Initial amounts, changes in amounts, and final equilibrium amounts are shown. These values must be in moles/50.

• •

  initial amounts of reactants (amounts before the reaction begins): determined using the dilution equation

  • •

    To find the initial concentration of SCN^–, use the dilution equation: (Thousand ₁ Five ₁)/5 _two = M ₂, where

    V _two = x mL.

  • •

    To find the initial concentration of Fe³⁺, use the dilution equation: (Thou _ane Five ₁)/Five ₂ = Thousand ₂, where 5 ₂ = 10 mL.

• •

  initial amount of product (the corporeality before the reaction begins): zero

• •

  equilibrium concentration of product

  ([FeSCN²⁺]_eq):

  adamant spectroscopically

[FeSCN²⁺] at equilibrium is determined using Beer's Constabulary; x is the corporeality of FeSCN²⁺ created (adamant experimentally).

( five )

x = [FeSCN²⁺]_eq =  = A / slope

To complete your ICE tables, one for each trial in Part four (concentrations should have 2 significant figures):

• a

  Begin by filling out the product column from the bottom up. Because the stoichiometry is i:1:1, the amount of reactant consumed is equal to the amount of product formed.

• b

  Fill in the rest of the Water ice table box-by-box until the equilibrium reactant concentrations are adamant.

4

Employ Eq. two

Chiliad =

 and the equilibrium concentrations from the bottom row of each ICE tabular array to calculate K _eq to two meaning figures for the five trials in Part 4.

5

Detect the average value of K _eq, the standard deviation, and the relative error (standard deviation divided by the average). Are the values of K for all trials similar? Should they be?

Assumption Check Used in Making Beer's Law Plot

Given your value of 1000, go dorsum and check the validity of the assumption you made in Function iii.

1

Rewrite Eq. 2

K =

 then that the ratio of

[FeSCN^two+]_eq

to

[SCN⁻]_eq, [FeSCN²⁺]_eq/[SCN⁻]_eq,

is on one side.

2

Summate the guess ratio of

[FeSCN²⁺]_eq/[SCN⁻]_eq

using

K _eq

and [Iron³⁺]. Utilize your boilerplate value for

K _eq.

The Iron^three+ concentration was approximately 0.i K in Part 3; the change in its concentration should take been negligible.

3

The supposition that essentially all of the SCN^– reacted to form FeSCN²⁺ would mean that this ratio would need to be large. Is it? If the ratio is small, the supposition was conspicuously a bad 1 and the experiment is useless in determining the equilibrium FeSCNⁱⁱ⁺ concentration and

K _eq.

At least 95% of the initial SCN^– should react to grade FeSCN²⁺ at equilibrium.

four

Hash out how good the assumption was and how the assumption affected the calculated values of

K _eq.

• •

  For case, if

  K _eq

  is 350, then

  [FeSCNⁱⁱ⁺]_eq/[SCN⁻]_eq ≈ 35.

• •

  This means that 35 out of 36 SCN^– initially present have been converted to FeSCNⁱⁱ⁺ at equilibrium; 1 out of 36 is nowadays as SCN^–.

• •

  That is virtually a 3% error in the FeSCN²⁺ concentration due to simply the assumption.

• •

  And,

  [FeSCNⁱⁱ⁺]_eq

  is used to determine the other values at equilibrium; iii% error in concentrations translates into roughly ix% error in

  Thousand _eq

  (again, this is just from the assumption and does not consider other experimental variables).

5

Checking the assumption is only part of a thorough experimental analysis; information technology should non exist considered the main point of the lab.

Results

Complete your lab summary or write a written report (as instructed).

Abstract

Results

• Observations for Part ane
• λ _max and absorbance at λ _max for Part 2
• Beer's Police force plot for Office 3 including slope( ε b)
• Water ice tables, private and average

  K _eq

  values

Sample Calculations

• Absorbance from transmittance
• Concentration calculations

• K _eq

  calculation
• Assumption testing

Give-and-take/Conclusions

• What you institute out and how
• Relating results to predictions/theory
• How was Role 3 dependent on Role 2?
• Validity of the supposition
• What can you conclude from this experiment

Review Questions

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