   # Spectrophotometric Determination of Equilibrium Constant for a Reaction

Good Essays
DATE PERFORMED: JULY 20, 2007

SPECTROPHOTOMETRIC DETERMINATION OF EQUILIBRIUM CONSTANT FOR A REACTION

ABSTRACT

UV-VIS spectrophotometry is one of the most widely-used methods for determining and identifying many inorganic species. During this experiment, this spectrophotometry was used to determine the equilibrium constant, Keq, of the Fe3+(aq)+SCN-(aq)↔ FeSCN2+(aq) reaction. By determining the amount of light absorbed, the concentration of the colored FeSCN2+ solution was also quantitatively determined. From that data, the concentrations of the reagents at equilibrium may also be determined. This experiment should thus provide a Keq value without computing for the concentration of each of the species in the reaction. This experiment will only deal with the aspect of chemical equilibrium, particularly the aforementioned equilibrium constant, and not with associated topics such as thermodynamics or kinetics.

INTRODUCTION

Reactions strive to attain equilibrium or stability. In kinetics, stability is attained when the rate of formation of the products is equivalent to the rate of reactant re-formation. Rate is determined by the rate expression, r=k[A]x, where A is the reactant, x the order of reaction with respect to the reactant and also to the coefficient of the reactant if it is an elementary reaction, and k, the rate constant. In equilibrium, there is an equilibrium constant determined by kf[A]x=kr[B]y, which is equivalent to the aforementioned parameter of equilibrium, rate of formation (forward) is equal to the rate of reformation (reverse). Therefore, Keq=(kf/kr)=([B]y/[A]x). Concentration may be determined by taking the number of moles of the reagent or product and dividing it by the total volume of the solution (dimension: M, molarity). There is, however, another method of determining the equilibrium constant, Keq, of a reaction. Spectrophotometric analysis results in the quantitative determination of the concentration of the product, through the

References:  Skoog, D.A., West, D.M., et al. Fundamentals of Analytical Chemistry 8th edition. Brooks/Cole, Singapore. 2004.  Lothian, G.F. Absorption Spectrophotmetry. Hilge&Watts, London. 1958.  Kolthoff, I.M, Sandell E.B, et al. Quantitative Chemical Analysis: 4th Edition. Macmillan Co., New York. 1969.  Atkins, P.W., et al. Physical Chemistry 8th edition. Oxford University Press, United Kingdom. 2005.

## You May Also Find These Documents Helpful

• Powerful Essays

Date Performed: January 10 & 15, 2013 Spectrophotometric Determination of the Equilibrium Constant of a Reaction R.J.V. Ortega and J.C.V. Gatdula Institute of Chemistry, College of Science University of the Philippines, Diliman, Quezon City, Philippines Received January 22, 2013 ------------------------------------------------- ------------------------------------------------- ------------------------------------------------- ABSTRACT -------------------------------------------------…

• 2555 Words
• 11 Pages
Powerful Essays
• Powerful Essays

SPECTROPHOTOMETRIC DETERMINATION OF EQUILIBRIUM CONSTANT OF A REACTION UNIVERSITY OF THE PHILIPPINES, DILIMAN QUEZON CITY, PHILIPPINES August 2, 2013 ABSTRACT The objective of this experiment is to determine the equilibrium constant, denoted Keq, for the formation of [Fe(SCN)]2+ complex which is a product of the reaction between the ions Fe3+ and SCN-. In performing this experiment, solutions containing FeCl3 and KSCN, diluted in HCl, were measured for their absorbance using a UV-Vis…

• 2594 Words
• 11 Pages
Powerful Essays
• Good Essays

Spectrophotometric Determination of an Equilibrium Constant Introduction: In this experiment, you will study the reaction between aqueous iron (III) nitrate, Fe(NO3)3, and potassium thiocyanate, KSCN. They react to produce the blood-red complex [Fe(SCN)]2+. Fe3+ + SCN- ( [Fe(SCN)]2+ The equilibrium constant expression may be expressed as: K = [pic] You will prepare a series of standard solutions that contain known concentrations of [Fe(SCN)]2+ and will determine…

• 840 Words
• 4 Pages
Good Essays
• Better Essays

The Spectrophotometric Determination of an Equilibrium Constant. Abstract: The report presents determination of equilibrium constant for the formation of a complex ion FeSCN2+. This was accomplished using a colorimeter to measure absorbance of some known concentration solutions in order to generate the calibration curve. The equation of the graph was used to compute the equilibrium concentrations of the reactants and products, needed to calculate the equilibrium constant for…

• 1961 Words
• 8 Pages
Better Essays
• Good Essays

DATE PERFORMED: JANUARY 6, 2011 SPECTROPHOTOMETRIC DETERMINATION OF THE EQUILIBRIUM CONSTANT OF A REACTION ABSTRACT The objective of the experiment was to determine the equilibrium constant of the reaction forming ferric thiocyanate through the use of Spectrophotometry. For the calibration, five standard solutions were prepared, then their respective absorbance values that were obtained through the use of the spectrophotometer, were plotted versus the concentration of the analyte so that a calibration…

• 569 Words
• 3 Pages
Good Essays
• Satisfactory Essays

SPECTROPHOTOMETRIC DETERMINATION OF THE EQUILIBRIUM CONSTANT OF A REACTION Methodology The following stock solutions were prepared: 1.0 M HCl, 0.1 M HCl, 0.1 M KSCN, 0.002 M KSCN (in 0.1 M HCl), 0.2 M FeCl3 (in 0.1 M HCl), and 0.002 M FeCl3(in 0.1 M HCl). Using the prepared stock solutions, the standard (Table 1) and Unknown (Table 2) solutions were prepared. Table 2. Preparation of Standard Solutions 0.10 M KSCN 0.002 M FeCl3 0.1 M HCl Blank 2.0 ml 0.0 ml 8 ml S1 2.0 ml 0.1 ml 7…

• 293 Words
• 2 Pages
Satisfactory Essays
• Better Essays

Spectrophotometric Determination of the Equilibrium Constant of a Reaction DEPARTMENT OF MATERIALS, METALLURGICAL AND MINING ENGINEERING, COLLEGE OF ENGINEERING July 26, 2012 ------------------------------------------------- ABSTRACT ------------------------------------------------- Before proceeding to test the next solution, we must first clean the cuvette, using similar steps earlier. First, we rinse with water, then with the solution, and then we fill it up and test…

• 1062 Words
• 5 Pages
Better Essays
• Good Essays

CH 127 – Chem 2 Lab Determination of an Equilibrium constant Goals The purpose of this experiment is to determine the equilibrium constant for the reaction Fe3+(aq) + HSCN(aq) –>FeSCN2+(aq) + H+(aq). The equilibrium constant expression Kc for Reaction is kc=FeSCN2+[H+]Fe3++[HSCN] Procedure *Preparation of the Beer’s law plot Prepare five solutions of FeSCN2+(aq) of known concentrations between 1x10-5M and 1x10-4M by diluting various volumes of 4.62x10-4 HSCN. Calculate the Final concentration…

• 849 Words
• 4 Pages
Good Essays
• Good Essays

Determination of Equilibrium Constants Introduction Bromothymol blue is an indicator for many acid-base titrations. When adding different solutions within the indicator it is to react and change colors, in this experiment the different colors were blue, green, and yellow. In the following experiment, obtaining the absorbance levels for each one makes it possible to calculate the equilibrium constant. Materials and Methods For this specific experiment there are a few materials that are crucial…

• 754 Words
• 3 Pages
Good Essays
• Powerful Essays

Determination of an Equilibrium Constant Abstract: In this experiment, two reactions were run to determine the molar absorptivity and the equilibrium constant of FeSCN2+. The main principles used in this lab are equilibrium, LeChatlier’s Principle, Beer’s Law and Spectrocopy. The first reaction was run to completion using LeChatier’s Principle and the second reaction was run to equilibrium. A spectrophotometer was used to measure absorbances. Using a graph of absorbance versus concentration…

• 1810 Words
• 8 Pages
Powerful Essays