Spectrophotometric Determination of the Equilibrium Constant of a Reaction

Topics: Chemical equilibrium, Concentration, Chemistry Pages: 10 (2555 words) Published: July 4, 2013
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
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ABSTRACT
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The experiment is conducted in order to determine the equilibrium constant of the Iron (III) – Thiocyanate System through the use of a UV-Vis Spectrophotometer. It is achieved by inserting a series of solutions with varying concentrations of Fe3+ and SCN- into a cuvette, then to the spectrophotometer in order to determine the degree of absorbance of the [FeSCN2+].First, standards solutions are used in order to determine several critical values. Then, unknown solutions are used in order to determine their equilibrium constant. Results from the experiment showed that the average equilibrium constant of the reaction in the unknown solutions is479.27, which gave a 54.22% difference with the literature value of 1047.Although there is a high percentage of error, the spectrophotometer may prove to be a successful tool in determining the equilibrium constant of a given reaction. -------------------------------------------------

Introduction

Chemical equilibrium is defined as the moment when two opposing forces in a chemical reaction, the forward and the reverse reactions, occur at the same time or at equal rates. It is what all chemical systems try to achieve, considering other stresses and changes that might occur or affect the system. The rate at which the chemical equilibrium is achieved is called the equilibrium constant, denoted by Keq. It is affected mostly by temperature. Given a chemical equation:

A(aq) + 2 B(aq) ↔ C(aq)(1.1)

The Keq can be expressed as:

Keq = [C]eq / [A]eq[B]2eq(1.2)

When a system has reached chemical equilibrium, both reactants and products are present at the same time. Due to this fact, it is hard to determine the remaining concentration of the reaction and the formed products, hence, becoming a hindrance in determining the equilibrium constant.

Another way to determine the equilibrium constant is by the use of a spectrophotometer. In the experiment, it is used to determine the equilibrium constant of the reaction between Iron (III) and Thiocyanate. The chemical equation that occurred is:

Fe3+ + SCN- ↔ FeSCN2+(1.3)

The spectrophotometer is used to measure the amount of light that a solution absorbs, then transforms the data into numerical values, thus, studying the interaction of electromagnetic radiation with matter. Different substances possess different kinds of energies, sometimes manifesting itself by the color of the substance. Color is formed when several areas of the visible spectrum are reflected by the molecules of the substance while some are absorbed. Basically, the higher the concentration of the substance, the higher is the energy absorbed and the higher the rate of reflection of the color of the substance in the spectrum.

Spectrophotometers use a source of light, then focuses it on the cuvette, containing the solution, inside the sample holder. It measures the absorbance of the solution using a photosensitive cell inside the machine. The values given can then be used to determine the concentration of the pigmented substance. This is done by using the Beer-Lambert’s Law: A = εbc(1.4)

Where:
A = Absorbance
ε = Molar Absorptivity Coefficient (M/cm)
b = Path length (cm)
c = Molar Concentration of Analyte (M)

Basically, the Beer-Lambert’s Law states that the degree of absorbance of the substance is directly proportional to the path length and molar concentration of analyte, multiplied by the...
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