Spectrophotometric Determination of an Equilibrium Constant

Overview

Source: Laboratory of Dr. Michael Evans — Georgia Institute of Technology

The equilibrium constant, K, for a chemical system is the ratio of product concentrations to reactant concentrations at equilibrium, each raised to the power of their respective stoichiometric coefficients. Measurement of K involves determination of these concentrations for systems in chemical equilibrium.

Reaction systems containing a single colored component can be studied spectrophotometrically. The relation between absorbance and concentration for the colored component is measured and used to determine its concentration in the reaction system of interest. Concentrations of the colorless components can be calculated indirectly using the balanced chemical equation and the measured concentration of the colored component.

In this video, the Beer's law curve for Fe(SCN)²⁺ is determined empirically and applied to the measurement of K for the following reaction:

Equation 1

Four reaction systems with different initial concentrations of reactants are investigated to illustrate that K remains constant irrespective of initial concentrations.

Procedure

1. Determining the Beer's Law Curve for Fe(SCN)²⁺

Calibrate a visible spectrophotometer using distilled water as a blank.
Add 1.0 mL of 1.0 × 10^-4 M Fe(NO₃)₃ solution to a test tube.
To the same test tube, add 5.0 mL of 0.50 M KSCN solution.
To the same test tube, add 4.0 mL of 0.10 M HNO₃ solution. Cover the tube with a gloved finger and gently shake to mix.
Use a Pasteur pipette to transfer a small quanti

Results

Table 4 lists the absorbance and concentration data for solutions 1 – 5. Concentrations of Fe(SCN)²⁺ were determined from initial concentrations of Fe³⁺ under the assumption that all of the Fe³⁺ is converted to Fe(SCN)²⁺. A large excess of SCN^- was used in tubes 1 – 5 to ensure that this assumption holds true.

The molarity [Fe(SCN)²⁺]

Application and Summary

The equilibrium constant provides useful information about the extent to which a reaction will proceed to form products over time. Reactions with a large value of K, much larger than 1, will form products nearly complete given enough time (Figure 3). Reactions with a value of K less than 1 will not proceed forward to a significant degree. The equilibrium constant thus serves as a measure of the feasibility of a chemical reaction.