CAPE Chemistry Unit 1 - June 2016 Paper 2 Solutions - Question 6 - Transition Elements

Summary

This educational video walks through key concepts in transition metal chemistry, focusing on titanium and cobalt compounds. It covers the properties of transition metals, including their varying oxidation states, catalytic behavior, high density, and ability to form colored compounds. The script then explores the electronic configuration of the Ti³⁺ ion and the reason for the violet color in its aqueous solution, resulting from electron transitions. The video also demonstrates a ligand exchange in cobalt(II) ions, where the addition of hydrochloric acid changes the solution's color from pink to blue. The concept of stability constants and their role in these reactions is also discussed.

Takeaways

• 😀 Transition metals form compounds with different oxidation states, contributing to their diverse chemical properties.
• 😀 Due to their variable oxidation states, transition elements and their compounds often act as good catalysts in chemical reactions.
• 😀 Transition metals are characterized by very high densities, making them heavier and more durable than many other metals.
• 😀 Transition metal ions are known to form colored compounds, which is a distinctive feature of their chemical behavior.
• 😀 The electronic configuration of Ti³⁺ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹, with a loss of three electrons from the titanium atom.
• 😀 In aqueous solutions, the Ti³⁺ ion forms a violet color due to an electron transition from lower to higher energy levels, absorbing green light.
• 😀 When HCl is gradually added to a Co²⁺ aqueous solution, a color change occurs from pink to blue as chloride ions replace water ligands.
• 😀 The ionic equation for the cobalt color change involves the displacement of water molecules by chloride ions, resulting in a [CoCl₆]²⁻ complex.
• 😀 The stability constant of a complex indicates its stability, with higher stability constants leading to more stable complexes.
• 😀 In the case of Co²⁺, the stability constant for the Co²⁺-chloride complex is higher than that for the Co²⁺-aqua complex, driving the ligand exchange to form the blue [CoCl₆]²⁻ complex.

Q & A

What are the four properties of transition metals discussed in the script?

The four properties of transition metals discussed are: 1) They form compounds with different oxidation states, 2) They are often good catalysts, 3) They have very high density, and 4) Their ions form colored compounds.

How do transition metals act as catalysts?

Transition metals act as catalysts due to their ability to exist in multiple oxidation states, which allows them to facilitate chemical reactions by providing alternate pathways with lower activation energy.

What is the electronic configuration of the Ti³⁺ ion?

The electronic configuration of the Ti³⁺ ion (after losing three electrons) is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹.

Why does a titanium chloride solution appear violet when dissolved in water?

The violet color of the titanium chloride solution arises because the titanium ion has a partially filled d-orbital. When light is absorbed, an electron is promoted from a lower energy level to a higher energy level, with the absorption corresponding to green light. The complementary color, violet, is observed.

What happens when concentrated hydrochloric acid is added to a cobalt (II) solution?

When concentrated hydrochloric acid is added to a cobalt (II) solution, chloride ions replace water molecules as ligands, leading to a color change from pink to blue.

What is the ionic equation for the reaction between cobalt (II) and chloride ions in aqueous solution?

The ionic equation for the reaction is: [Co(H₂O)₆]²⁺ (aq) + 6 Cl⁻ (aq) → [CoCl₆]²⁻ (aq) + 6 H₂O (l).

How does the stability constant affect the formation of cobalt chloride complex?

The stability constant indicates the stability of the complex. A higher stability constant makes the formation of the cobalt chloride complex more favorable. In this case, the chloride ion complex has a higher stability constant than the water-ligand complex, driving the ligand exchange from water to chloride.

What is the significance of the stability constant in ligand exchange reactions?

The stability constant measures the likelihood of a metal-ligand complex forming. A higher stability constant indicates that the complex is more stable, which means the ligand exchange is more likely to occur, as seen with the cobalt (II) and chloride ions.

Why does the cobalt complex change color when chloride ions are added?

The color change occurs because the ligand exchange alters the coordination environment of the cobalt ion. Initially, the cobalt complex with water ligands is pink. After chloride ions replace water, the complex becomes blue due to changes in the electronic configuration and absorption of specific wavelengths of light.

What role do d-orbitals play in the color observed in transition metal complexes?

The d-orbitals in transition metal complexes are partially filled, allowing for electronic transitions. These transitions absorb specific wavelengths of light, and the color observed is the complementary color of the absorbed light, as seen with titanium and cobalt complexes.