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Ch.21 - Transition Elements and Coordination Chemistry
McMurry - Chemistry 8th Edition
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Tutti i libri di testoMcMurry 8th EditionCh.21 - Transition Elements and Coordination ChemistryProblema 21.137e
Capitolo 21, Problema 21.137e

The complete reaction of 2.60 g of chromium metal with 50.00 mL of 1.200 M H2SO4 in the absence of air gave a blue solution and a colorless gas that was collected at 25°C and a pressure of 735 mm Hg. (e) When an excess of KCN is added to the solution, the color changes, and the paramagnetism of the solution
decreases. Explain.

Guida verificata passo dopo passo
1
Identify the initial reaction: Chromium (Cr) reacts with sulfuric acid (H2SO4) to produce chromium sulfate (Cr2(SO4)3) and hydrogen gas (H2).
Recognize that the blue solution is likely due to the formation of a chromium(III) complex, such as [Cr(H2O)6]^{3+}.
Understand that the addition of an excess of KCN leads to the formation of a new complex, [Cr(CN)6]^{3-}, which is less paramagnetic.
Explain that the decrease in paramagnetism is due to the fact that the cyanide complex has a lower number of unpaired electrons compared to the initial chromium complex.
Conclude that the color change and decrease in paramagnetism are due to the formation of a different chromium complex with cyanide ligands, which alters the electronic structure and properties of the solution.

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Redox Reactions

Redox reactions involve the transfer of electrons between substances, leading to changes in oxidation states. In the given reaction, chromium metal is oxidized while sulfuric acid is reduced, resulting in the formation of a blue solution and a colorless gas, likely hydrogen. Understanding the electron transfer is crucial for analyzing the chemical changes occurring in the reaction.
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Identifying Redox Reactions

Paramagnetism

Paramagnetism is a form of magnetism that occurs in materials with unpaired electrons, which can be attracted by an external magnetic field. The presence of unpaired electrons in transition metal complexes, such as those formed with chromium, contributes to the solution's paramagnetic properties. The addition of KCN alters the electronic structure of the chromium complex, reducing the number of unpaired electrons and thus decreasing paramagnetism.
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Complex Ion Formation

Complex ions are formed when metal ions coordinate with ligands, which are molecules or ions that can donate electron pairs. In this case, KCN acts as a ligand that binds to chromium ions in the solution, leading to the formation of a new complex. This interaction not only changes the color of the solution but also affects its magnetic properties, illustrating the significance of ligand interactions in coordination chemistry.
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Complex Ions and Formation Constant
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In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)1Cl2]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

(d) Is the reaction product chiral or achiral? Explain.

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Locate on the periodic table the transition elements with the following electron configurations. Identify each element.

(c) [Kr] 4d25s2

(d) [Xe] 4f36s2

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Locate on the periodic table the transition elements with the following electron configurations. Identify each element.

(a) [Ar] 3d74s2

(b) [Ar] 3d54s1

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In acidic aqueous solution, the complex trans-[Co(en)2Cl1]2+(aq) undergoes the following substitution reaction:

trans-[Co(en)2Cl1]+(aq) + H2O(l) → trans-[Co(en)2(H2O)Cl]2+(aq) + Cl–(aq)

The reaction is first order in trans-[Co(en)2Cl2]+(aq), and the rate constant at 25°C is 3.2×10–5 s–1.

e. Draw a crystal field energy-level diagram for trans-[Co(en)2Cl2]+ that takes account of the fact that Cl– is a weaker-field ligand than ethylenediamine.

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Spinach contains a lot of iron but is not a good source of dietary iron because nearly all the iron is tied up in the oxalate complex [Fe(C2O4)3]3-.

(d) Draw the structure of [Fe(C2O4)3]3-. Is the complex chiral or achiral?

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Look at the location in the periodic table of elements A, B, C, and D. What is the electron configuration of the transition metal in each of the following ions?

(c) C3+

(d) DO42-

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