Unit 9: Properties & Reactions of d-block Metal Compounds
Delving into the colorful and complex world of transition metal chemistry.
9.27 Compounds of Copper (CuCl₂, CuSO₄, CuO)
Compounds of copper, a typical d-block metal, are known for their distinctive colors and uses. Most common copper compounds contain the copper(II) ion, $Cu^{2+}$.
- Copper(II) Chloride (CuCl₂): A salt that is a bluish-green solid in its hydrated form. It is used as a catalyst in various industrial chemical reactions.
- Copper(II) Sulfate (CuSO₄): Perhaps the most well-known copper compound. In its hydrated form ($CuSO_4 \cdot 5H_2O$), it is a vibrant blue crystalline solid. It is toxic to fungi and microorganisms, making it useful as a fungicide in agriculture (e.g., in Bordeaux mixture) and as an algaecide in swimming pools. Anhydrous copper(II) sulfate is a white powder.
- Copper(II) Oxide (CuO): A black solid that is insoluble in water. It is a basic oxide and will react with acids to form copper(II) salts. It is used as a pigment in ceramics to produce blue, red, and green glazes. It also has applications as a weak oxidizing agent and a catalyst.
Solved Examples:
-
A farmer wants to protect their grapevines from fungal disease. Which copper
compound would be most suitable?
Solution: Copper(II) sulfate ($CuSO_4$) would be most suitable due to its properties as a fungicide. It is often mixed with lime to create Bordeaux mixture for this purpose. -
Write a balanced equation for the reaction of copper(II) oxide with sulfuric
acid.
Solution: As a basic oxide reacting with an acid, it will form a salt and water.
$CuO(s) + H_2SO_4(aq) \rightarrow CuSO_4(aq) + H_2O(l)$
9.28 Complex Ions & Ligands (Formation, Color)
A key feature of transition metal ions is their ability to form complex ions. A complex ion consists of a central metal ion bonded to one or more molecules or anions called ligands.
A ligand is a species that has at least one lone pair of electrons which it can donate to the central metal ion to form a dative covalent (or coordinate) bond. Common ligands include water ($H_2O$), ammonia ($NH_3$), and halide ions (e.g., $Cl^-$).
Formation and Color
The formation of complex ions is responsible for the characteristic colors of transition metal compounds. The d-orbitals in a free transition metal ion all have the same energy. When ligands bond to the ion, they cause the d-orbitals to split into different energy levels.
Electrons in the lower d-orbitals can then absorb energy from visible light to jump to the higher d-orbitals. The light that is not absorbed is transmitted or reflected, and this is the color we see. For example, a solution that absorbs red light will appear blue-green.
Ions without partially filled d-subshells (like $Zn^{2+}$ or $Na^+$) cannot do this, which is why their compounds are typically white or colorless.
Example: Copper(II) ions with Ammonia
When aqueous ammonia is added dropwise to a solution containing copper(II) ions (like
$CuSO_4$), a pale blue precipitate of copper(II) hydroxide is formed first.
$Cu^{2+}(aq) + 2OH^-(aq) \rightarrow Cu(OH)_2(s)$
However, if excess ammonia is added, the ammonia molecules act as ligands, displacing the
water and hydroxide ligands to form the deep blue tetraamminecopper(II) complex ion. The
precipitate redissolves.
$Cu(OH)_2(s) + 4NH_3(aq) \rightarrow [Cu(NH_3)_4]^{2+}(aq) + 2OH^-(aq)$
Solved Examples:
-
Define the term "ligand" and give an example.
Solution: A ligand is a molecule or ion with a lone pair of electrons that it can donate to a central metal ion to form a dative covalent bond. An example is the water molecule ($H_2O$). -
Why are solutions containing zinc ions ($Zn^{2+}$) colorless?
Solution: The $Zn^{2+}$ ion has an electron configuration of [Ar] $3d^{10}$. Its d-subshell is completely full. Since there are no empty or partially filled d-orbitals for electrons to jump to, it cannot absorb energy from visible light, and its solutions are colorless. -
What is a dative covalent bond?
Solution: A dative covalent bond (or coordinate bond) is a type of covalent bond where both electrons in the shared pair come from the same atom (the ligand). -
A solution containing iron(III) ions, $[Fe(H_2O)_6]^{3+}$, is yellow/brown.
Explain this observation.
Solution: The water ligands cause the d-orbitals of the $Fe^{3+}$ ion to split. Electrons in the lower d-orbitals absorb blue/violet light from the visible spectrum to jump to the higher d-orbitals. The remaining light, which is predominantly yellow/orange/red, is transmitted, making the solution appear yellow/brown.
9.29 Hydrated Salts & Test for Water
Many ionic compounds can incorporate a fixed number of water molecules into their crystal lattice structure to form hydrated salts. This incorporated water is known as the water of crystallization. In the case of transition metals, these water molecules often act as ligands, bonding to the central metal ion and giving the hydrated salt its color.
A compound without its water of crystallization is called anhydrous.
The Chemical Test for Water
The color difference between the hydrated and anhydrous forms of copper(II) sulfate provides a simple chemical test for the presence of water.
- Anhydrous copper(II) sulfate ($CuSO_4$) is a white powder.
- Hydrated copper(II) sulfate ($CuSO_4 \cdot 5H_2O$) is a blue crystalline solid.
When water is added to the white anhydrous powder, it turns blue as it becomes hydrated. This is a reversible reaction; heating the blue hydrated salt will drive off the water and turn it white again.
$$ \underset{\text{(white)}}{CuSO_4(s)} + 5H_2O(l) \rightleftharpoons \underset{\text{(blue)}}{CuSO_4 \cdot 5H_2O(s)} $$This test is specific to water and can be used to detect its presence in other liquids (e.g., to show that ethanol is not pure).
Solved Examples:
-
A student suspects a sample of ethanol is contaminated with water. How could
they use anhydrous copper(II) sulfate to check?
Solution: They would add a small amount of white anhydrous copper(II) sulfate powder to the ethanol sample. If water is present, the white powder will turn blue. If the ethanol is pure (anhydrous), the powder will remain white. -
Why is hydrated copper(II) sulfate blue, while anhydrous copper(II) sulfate
is white?
Solution: In the hydrated form, water molecules act as ligands, bonding to the $Cu^{2+}$ ion. This causes the d-orbitals to split, allowing the complex to absorb orange/red light and appear blue. In the anhydrous form, there are no ligands, so the d-orbitals are not split in the same way, and it does not absorb visible light, appearing white. -
How can you reverse the test for water?
Solution: You can gently heat the blue hydrated copper(II) sulfate. The heat provides the energy to drive off the water of crystallization, turning the solid back into white anhydrous copper(II) sulfate. You might observe steam (water vapor) being released. -
What is the name and color of the compound $FeSO_4 \cdot
7H_2O$?
Solution: The compound is hydrated iron(II) sulfate. Since it contains the $Fe^{2+}$ ion, it is typically a pale green solid.