Chemical Equilibrium: Finding a Constant, Kc

Recommended for grades 9–12.

Figure from experiment 20 from Chemistry with Vernier

Introduction

The purpose of this lab is to experimentally determine the equilibrium constant, K_c, for the following chemical reaction:

${\text{F}}{{\text{e}}^{{\text{3 + }}}}{\text{(aq) + SC}}{{\text{N}}^{\text{ - }}}{\text{(aq)}} \leftrightarrow {\text{FeSC}}{{\text{N}}^{{\text{2 + }}}}{\text{(aq)}}$ ${\text{iron(III) + thiocyanate}} \leftrightarrow {\text{thiocyanoiron(III)}}$

When Fe³⁺ and SCN^- are combined, equilibrium is established between these two ions and the FeSCN²⁺ ion. In order to calculate K_c for the reaction, it is necessary to know the concentrations of all ions at equilibrium: [FeSCN²⁺]_eq, [SCN^–]_eq, and [Fe³⁺]_eq. You will prepare four equilibrium systems containing different concentrations of these three ions. The equilibrium concentrations of the three ions will then be experimentally determined. These values will be substituted into the equilibrium constant expression to see if K_c is indeed constant.

In order to determine [FeSCN²⁺]_eq, you will use the Colorimeter. The FeSCN²⁺ ion produces solutions with a red color. Because the red solutions absorb blue light very well, the blue LED setting on the Colorimeter is used. The computer-interfaced Colorimeter measures the amount of blue light absorbed by the colored solutions (absorbance, A). By comparing the absorbance of each equilibrium system, A_eq, to the absorbance of a standard solution, A_std, you can determine [FeSCN²⁺]_eq. The standard solution has a known FeSCN²⁺ concentration.

Objectives

In this experiment, you will determine the equilibrium constant, Kc, for the following chemical 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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