9701/42/M/J/23/Q2 Cambridge International A Level Chemistry 9701 May/June 2023 Paper 42 Question 2

Summary

This video script delves into the key properties and reactions of transition elements, with a focus on cobalt and copper. It covers their ability to form multiple oxidation states, colorful compounds, and complex ions. The script explores the behavior of cobalt(II) in aqueous solutions, demonstrating ligand exchange reactions with ammonia, hydrochloric acid, and sodium hydroxide, leading to changes in color and the formation of new complexes. Additionally, it explains the concept of bent-dentate ligands, such as ethylenediamine, and their role in stereoisomerism, highlighting both cis/trans and optical isomerism. The oxidation state of cobalt in these reactions is also discussed.

Takeaways

• 😀 Transition elements can have multiple oxidation states, such as Fe²⁺, Fe³⁺, Mn²⁺ to Mn⁷⁺.
• 😀 Transition metals form colorful compounds, e.g., Cu²⁺ gives a blue solution and Co²⁺ forms a pink solution.
• 😀 Complex ions are formed when a central metal ion binds with one or more ligands, such as water molecules (e.g., [Co(H₂O)₆]²⁺).
• 😀 Ligand exchange reactions occur when ligands like ammonia, chloride, or hydroxide replace water molecules in complex ions.
• 😀 Adding ammonia to a cobalt(II) solution results in a brown or yellow-brown complex, [Co(NH₃)₆]²⁺.
• 😀 Adding HCl to a cobalt(II) solution forms the [CoCl₄]²⁻ complex, a blue solution.
• 😀 Adding sodium hydroxide causes cobalt(II) hydroxide to precipitate, forming a blue precipitate.
• 😀 A bidentate ligand, such as ethanedioate (C₂O₄²⁻), donates two lone pairs to form two dative bonds with the metal ion.
• 😀 Stereoisomerism in coordination compounds includes both geometrical (cis/trans) and optical isomerism.
• 😀 The oxidation state of cobalt in a complex involving ethanedioate and monodentate ligands is +3 (Co³⁺).
• 😀 Complex ions with bidentate and monodentate ligands can exist in multiple stereoisomeric forms, including cis/trans and optical isomers.

Q & A

What are two typical chemical properties of transition elements?

Transition elements can form multiple oxidation states (e.g., Fe²⁺, Fe³⁺, Mn²⁺, Mn³⁺) and they can form colored compounds (e.g., copper gives a blue solution, cobalt gives a pink solution).

What is a complex ion?

A complex ion is a metal ion bonded to one or more ligands via dative bonds, where the ligands donate electrons to the metal center.

How does the addition of ammonia affect a cobalt(II) complex?

Adding ammonia to a cobalt(II) complex leads to a ligand exchange, replacing the water molecules with ammonia. The resulting solution typically appears brown or yellow-brown.

What happens when excess hydrochloric acid is added to a cobalt(II) complex?

Excess hydrochloric acid replaces the water ligands in the cobalt(II) complex with chloride ions, forming a new complex ion, CoCl₄²⁻, which results in a blue solution.

What occurs when sodium hydroxide is added to a cobalt(II) complex?

Adding sodium hydroxide to a cobalt(II) complex results in a blue precipitate of Co(OH)₂, as hydroxide ions replace the water ligands.

What is a bidentate ligand?

A bidentate ligand is a species that donates two lone pairs of electrons to form two dative bonds with a metal ion. An example is the ethanedioate ion (C₂O₄²⁻).

What are cis-trans isomers in coordination chemistry?

Cis-trans isomerism refers to the arrangement of ligands in a complex, where 'cis' means the same ligands are on the same side, and 'trans' means they are on opposite sides.

What is optical isomerism in coordination complexes?

Optical isomerism occurs when two isomers of a complex are non-superimposable mirror images of each other, which means they cannot be perfectly aligned even when rotated.

What is the oxidation state of cobalt in a cobalt(III) complex with ethanedioate, water, and halide ligands?

In this complex, the oxidation state of cobalt is +3, because the ethanedioate ion (C₂O₄²⁻) is a -2 charge, and the halide ions (bromide and chloride) are -1 each, balancing the overall charge.

How can a cobalt complex form stereo-isomers?

A cobalt complex can form stereoisomers through cis-trans isomerism or optical isomerism, depending on how the ligands are arranged around the metal center. The presence of bidentate and monodentate ligands allows for these different isomeric forms.