   Chapter 21, Problem 59E

Chapter
Section
Textbook Problem

# The following test tubes each contain a different chromium complex ion. For each complex ion, predict the predominant color of light absorbed. If the complex ions are Cr(NH3)63+, Cr(H2O)63+, and Cr(H2O)4Cl2+, what is the identity of the complex ion in each test tube? (Hint: Reference the spectrochemical series.)

Interpretation Introduction

Interpretation: Three test tubes containing different complex ions are given. The color of light absorbed by each given ion is to be predicted. The identity of complex ion in each test tube is to be predicted.

Concept introduction: The capacity to absorb light by complex ion is directly proportional to crystal field splitting energy and energy absorbed is inversely proportional to the wavelength.

To determine: The color of light absorbed by each ion Cr(H2O)63+,Cr(NH3)63,Cr(H2O)4Cl2+ and the identity of complex ion in each test tube.

The order of increasing crystal field splitting energy for complex ions Cr(H2O)63+,Cr(NH3)63,Cr(H2O)4Cl2+ is,

Cr(NH3)63>Cr(H2O)63+>Cr(H2O)4Cl2+

Explanation

Explanation

The spectrochemical series (crystal field splitting energy) of strong to weak ligand is,

CN>NO2>en>NH3>H2O>OH>F>Cl>SCN>Br

Hence, order of increasing crystal field splitting energy for complex ions Cr(H2O)63+,Cr(NH3)63,Cr(H2O)4Cl2+ is,

Cr(NH3)63>Cr(H2O)63+>Cr(H2O)4Cl2+

The energy absorbed by light is given by the expression,

E=hcλ

Where,

• E is the energy absorbed.
• h is the Plank’s constant.
• c is the velocity of light.

It is clear from the above equation that, energy absorbed is inversely proportional to the wavelength. It means that higher the energy, lower will be the wavelength.

Since, order of increasing crystal field splitting energy for complex ions Cr(H2O)63+,Cr(NH3)63,Cr(H2O)4Cl2+ is,

Cr(NH3)63>Cr(H2O)63+>Cr(H2O)4Cl2+

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