Vernier Software & Technology

# The Determination of an Equilibrium Constant

## Introduction

Chemical reactions occur to reach a state of equilibrium. The equilibrium state can be characterized by quantitatively defining its equilibrium constant, Keq. In this experiment, you will determine the value of Keq 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, Keq, 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 Keq, 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, Keq, for the reaction.

## 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?

## Advanced Chemistry with Vernier

See other experiments from the lab book.

 1 The Determination of a Chemical Formula 2 The Determination of the Percent Water in a Compound 3 The Molar Mass of a Volatile Liquid 4 Using Freezing-Point Depression to Find Molecular Weight 5 The Molar Volume of a Gas 6 Standardizing a Solution of Sodium Hydroxide 7 Acid-Base Titration 8 An Oxidation-Reduction Titration: The Reaction of Fe2+ and Ce4+ 9 Determining the Mole Ratios in a Chemical Reaction 10 The Determination of an Equilibrium Constant 11 Investigating Indicators 12 The Decomposition of Hydrogen Peroxide 13 Determining the Enthalpy of a Chemical Reaction 14A Separation and Qualitative Analysis of Cations 14B Separation and Qualitative Analysis of Anions 15A The Synthesis of Alum 15B The Analysis of Alum 16 Conductimetric Titration and Gravimetric Determination of a Precipitate 17 Determining the Concentration of a Solution: Beer's Law 18 Liquid Chromatography 19 Buffers 20 Electrochemistry: Voltaic Cells 21 Electroplating 22 The Synthesis and Analysis of Aspirin 23 Determining the Ksp of Calcium Hydroxide 24 Determining Ka by the Half-Titration of a Weak Acid 25 The Rate and Order of a Chemical Reaction 26 The Enthalpy of Neutralization of Phosphoric Acid 27 α, β, and γ 28 Radiation Shielding 29 The Base Hydrolysis of Ethyl Acetate 30 Exploring the Properties of Gases 31 Determining Avogadro's Number 32 Potentiometric Titration of Hydrogen Peroxide 33 Determining the Half-Life of an Isotope 34 Vapor Pressure and Heat of Vaporization 35 Rate Determination and Activation Energy

### Experiment 10 from Advanced 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.