The Determination of an Equilibrium Constant

Recommended for grades 9–12.

Introduction

Chemical reactions occur to reach a state of equilibrium. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, K_eq. In this experiment, you will determine the value of K_eq for the reaction between iron (III) ions and thiocyanate ions, SCN^–.

${\text{F}}{{\text{e}}^{{\text{3}} + }}\left( {{\text{aq}}} \right) + {\text{ SC}}{{\text{N}}^-}\left( {{\text{aq}}} \right) \to {\text{FeSC}}{{\text{N}}^{{\text{2}} + }}\left( {{\text{aq}}} \right)$

The equilibrium constant, K_eq, is defined by the equation shown below.

${K_{eq}} = \frac{{[{\text{FeSC}}{{\text{N}}^{2 + }}]}} {{[{\text{F}}{{\text{e}}^{3 + }}][{\text{SC}}{{\text{N}}^ - }]}}$

To find the value of K_eq, which depends only upon temperature, it is necessary to determine the molar concentration of each of the three species in solution at equilibrium. You will determine the concentration by measuring light that passes through a sample of the equilibrium mixtures. The amount of light absorbed by a colored solution is proportional to its concentration. The red FeSCN²⁺ solution absorbs blue light, thus the Colorimeter users will be instructed to use the 470 nm (blue) LED. Spectrometer users will determine an appropriate wavelength based on the absorbance spectrum of the solution.

Objectives

In this experiment, you will

Prepare and test standard solutions of FeSCN²⁺ in equilibrium.
Test solutions of SCN⁻ of unknown molar concentration.
Determine the molar concentrations of the ions present in an equilibrium system.
Determine the value of the equilibrium constant, K_eq, for the reaction.

Sensors and Equipment

This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):

SpectroVis Plus (optional)
Colorimeter (optional)

Additional Requirements

You may also need an interface and software for data collection. What do I need for data collection?

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The student-version preview includes:

Step-by-step instructions for computer-based data collection
List of materials and equipment

Note: The experiment preview of the computer edition does not include essential teacher information, safety tips, or sample data. Instructions for Logger Pro and other software (such as LabQuest App or TI handheld software, where available) are on the CD that accompanies the book. We strongly recommend that you purchase the book before performing experiments.

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Standards Correlations

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