Vernier Software and Technology
Vernier Software & Technology

Chemical Equilibrium: Finding a Constant, Kc

Figure from experiment 20 from Chemistry with Vernier


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.


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 1

Option 2

Option 3

Option 4

Additional Requirements

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

Standards Correlations

See all standards correlations for Chemistry with Vernier »

Chemistry with Vernier

See other experiments from the lab book.

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

Experiment 20 from Chemistry with Vernier Lab Book

<em>Chemistry with Vernier</em> book cover

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Dev Reference: VST0089

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