 Vernier Software & Technology

Chemical Equilibrium: Finding a Constant, Kc

Introduction

The purpose of this lab is to experimentally determine the equilibrium constant, Kc, 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 Kc 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 Kc 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, Aeq, to the absorbance of a standard solution, Astd, 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: ${\text{F}}{{\text{e}}^{{\text{3 + }}}}{\text{(aq) + SC}}{{\text{N}}^{\text{ - }}}{\text{(aq)}} \leftrightarrow {\text{FeSC}}{{\text{N}}^{{\text{2 + }}}}{\text{(aq)}}$ ${\text{iron(III) + thiocyanate}} \leftrightarrow {\text{thiocyanoiron(III)}}$

Sensors and Equipment

This experiment features the following Vernier sensors and equipment.

Option 4

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

Chemistry with Vernier

See other experiments from the lab book.

 1 Endothermic and Exothermic Reactions 2 Freezing and Melting of Water 3 Another Look at Freezing Temperature 4 Heat of Fusion of Ice 5 Find the Relationship: An Exercise in Graphing Analysis 6 Boyle's Law: Pressure-Volume Relationship in Gases 7 Pressure-Temperature Relationship in Gases 8 Fractional Distillation 9 Evaporation and Intermolecular Attractions 10 Vapor Pressure of Liquids 11 Determining the Concentration of a Solution: Beer's Law 12 Effect of Temperature on Solubility of a Salt 13 Properties of Solutions: Electrolytes and Non-Electrolytes 14 Conductivity of Solutions: The Effect of Concentration 15 Using Freezing Point Depression to Find Molecular Weight 16 Energy Content of Foods 17 Energy Content of Fuels 18 Additivity of Heats of Reaction: Hess's Law 19 Heat of Combustion: Magnesium 20 Chemical Equilibrium: Finding a Constant, Kc 21 Household Acids and Bases 22 Acid Rain 23 Titration Curves of Strong and Weak Acids and Bases 24 Acid-Base Titration 25 Titration of a Diprotic Acid: Identifying an Unknown 26 Using Conductivity to Find an Equivalence Point 27 Acid Dissociation Constant, Ka 28 Establishing a Table of Reduction Potentials: Micro-Voltaic Cells 29 Lead Storage Batteries 30 Rate Law Determination of the Crystal Violet Reaction 31 Time-Release Vitamin C Tablets 32 The Buffer in Lemonade 33 Determining the Free Chlorine Content of Swimming Pool Water 34 Determining the Quantity of Iron in a Vitamin Tablet 35 Determining the Phosphoric Acid Content in Soft Drinks 36 Microscale Acid-Base Titration

Experiment 20 from Chemistry with Vernier Lab Book Included in the Lab Book

Vernier lab books include word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.