what is a transition element?
A transition element is a d block element that can form an ion with an incomplete d sub shell.
- can form coloured compounds
- have variable oxidation states
- high melting and boiling points - this is due to the strong metallic bonds due to the 4s and 3d orbitals delocalised electrons.
- acts as a catalyst eg. in catalytic converters
- Form complex ions
General rule - fill 4s orbital first and then the 3d orbital. When losing electrons you lose electrons from the 4s sub shell first and then the 3d orbital.
- Zn2+ has an full d orbital
- Sc3+ has a empty d orbital
Cu electron configuration - [Ar]4s1 3d10
Cr - [Ar] 4s1 3d5
Catalysts speed up the rate of reaction by providing an alternate route with a lower activation energy(Ea). Transition metals are good catalysts due to their variable oxidation states as this means that they may allow an alternative reaction pathway with a lower activation energy. Transition metals can also act as a solid catalyst as they have partially filled d orbitals that allow dative bonds between the reactants and transition metal surface.
How a catalytic converter works?
- reactant molecules adsorb to the surface of catalyst and are held in place by weak dative bonds as the reaction occurs. this weakens the molecular bonds of the reactants.
- The reaction then occurs
- the molecules desorb of the surface of the catalyst as products.
Catalytic converters reduce the amount of pollutant gases released into the atmosphere.
Examples of Catalysts
- Fe - The Haber process
- Vandanium oxide used in the Contact process(converts SO2 into SO3)
- Nickel used Hydrogentation of alkene reactions
Transition elements can form coloured compounds due to the partial filling of electrons in the d orbital. Zinc(zn2+ full d orbital) and Scandium(sc3+ empty d orbital) are both colourless compounds.
- Each oxidation state of a transition metal ion has a different colour
A complex ion is when one or more ligands are attached to a transition metal ion by coordinate bonds. A lone prair of electrons from the ligand are donated into the empty orbital on the central metal ion.
The number of coordinate bonds formed by the central metal ion is - coordination number
Ligands in a complex ion dont have to be the same and H2O is a ligand
A ligand substitution reaction occurs when the ligands in the complex ion are replaced by another ligand. The new complex ion formed is more stable as it has stronger dative bonds.
Ligand Substitution reactions
Cu and NH3
small amount - produces Cu(OH)2
concentrated - [Cu(H20)6]2+ + 4NH3 ---> [Cu(NH3)4(H20)2] + 4H20 Pale blue(aq) deep blue(aq)
Cu and concentrated HCl
[Cu(H20)6]2+ + 4Cl- ---> [CuCl4]2- + 6H2O Pale blue(aq) Yellow(aq)
Co and concentrated HCl
[Co(H20)6]2+ + 4Cl- ---> [CoCl4]2- + 6H20 Pink(aq) blue(aq)
Cu2+ + OH- ---> Cu(OH)2 Pale blue(aq) Pale blue(s)
Fe2+ + OH- ---> Fe(OH)2 green(aq) green(s)
Fe3+ + OH- ---> Fe(OH)3 yellow(aq) rusty brown(s)
Co2+ + OH- ---> Co(OH)2 pink(aq) blue(s)
Stability of Complex ions
- This depends on the strength of the co-ordinate bonds formed
- It can be measured in terms of the equilibrium constant for its formation from its complex with water – Kstab