CIE iGCSE Co-ordinated Sciences-C8.4 Transition elements- Study Notes- New Syllabus
CIE iGCSE Co-ordinated Sciences-C8.4 Transition elements – Study Notes
CIE iGCSE Co-ordinated Sciences-C8.4 Transition elements – Study Notes -CIE iGCSE Co-ordinated Sciences – per latest Syllabus.
Key Concepts:
Core
- Describe the transition elements as metals that:
(a) have high densities
(b) have high melting points
(c) form coloured compounds
(d) often act as catalysts as elements and in compounds
CIE iGCSE Co-Ordinated Sciences-Concise Summary Notes- All Topics
Transition Elements
Transition elements are metals found in the central d-block of the Periodic Table, between Groups 3 and 12. They are characterised by having partially filled d-orbitals, which give rise to their distinctive chemical and physical properties.
Key Physical Properties:
- High densities: Transition metals are generally very dense due to their closely packed metallic crystal structures and relatively high atomic masses. For example, iron has a density of 7.87 g/cm³, and platinum is 21.45 g/cm³.
- High melting and boiling points: Strong metallic bonding involving delocalised d-electrons leads to high melting and boiling points. The presence of partially filled d-orbitals contributes to stronger bonding. Mercury is an exception, with a low melting point (-39 °C) due to weak interatomic forces in its liquid state.
Chemical Properties:
Formation of coloured compounds:
The partially filled d-orbitals allow d–d electron transitions when light is absorbed. Different wavelengths of visible light are absorbed, producing coloured compounds. For example, copper(II) sulfate is blue, and chromium(III) chloride is green.
Variable oxidation states:
Many transition elements can exhibit more than one oxidation state in their compounds. For instance, iron can form Fe²⁺ and Fe³⁺ ions, while manganese can form Mn²⁺, Mn³⁺, Mn⁴⁺, Mn⁶⁺, and Mn⁷⁺. This is due to the involvement of d-electrons in bonding.
Catalytic activity:
Transition metals and their compounds frequently act as catalysts. This is due to their ability to lend and accept electrons via multiple oxidation states and to provide a surface for reactions (heterogeneous catalysis). For example,
- Iron is used as a catalyst in the Haber process for ammonia production.
- Vanadium(V) oxide (V₂O₅) is used as a catalyst in the contact process for sulfuric acid production.
Formation of complex ions:
Transition metals readily form complex ions with ligands due to the availability of empty d-orbitals, further contributing to coloured solutions and catalytic properties.
Example
Explain why copper(II) sulfate solution is blue.
▶️Answer/Explanation
Copper(II) ions have partially filled d-orbitals. Light absorption excites electrons between these d-orbitals (d–d transitions), absorbing certain wavelengths. The transmitted or reflected light appears blue, giving copper(II) sulfate its characteristic colour.
Example
Describe why iron is used as a catalyst in the Haber process.
▶️Answer/Explanation
Iron acts as a heterogeneous catalyst by providing a surface where nitrogen and hydrogen gases can adsorb. Its partially filled d-orbitals enable temporary bonding and electron transfer, weakening the strong N≡N triple bond. This accelerates the formation of ammonia without the iron being consumed in the reaction.
Example
Explain the trend in densities and melting points among transition metals.
▶️Answer/Explanation
Transition metals have strong metallic bonding due to delocalised d-electrons, which also allows tight atomic packing. This results in high densities and high melting/boiling points. Slight variations occur depending on the number of unpaired d-electrons and atomic radius, but in general, transition metals are denser and melt at higher temperatures than s-block metals.