The D Block - Transition Metal
A transition metal is defined as a d-block element that forms at least one ion with a partially filled sub-shell of d electrons.
Transition metals have 4 important properties:
- Variable oxidation states - The differences between successive ionisation enthalpies in the 3d and 4s sub-shells are relatively small so multiple electron loss is possible.
- Ability to form coloured ions - Electron transitions occur when visible light is absorbed if the 3d sub-shell is partially filled.
- Ability to form complexes - 3d orbitals are able to accommodate electrons donated by ligands.
- Catalytic activity - Transition metals can either be homogeneous or heterogeneous catalysts. In heterogeneous catalysis the metals provide a surface onto which reactants can form weak interactions with. In homogeneous catalysis, the metals change from one oxidation state to another during the reaction before returning to their original oxidatino state, remaining unchanged at the end.
When a metal is placed in a solution of its ions, an equilibrium is established and a potential difference or electrode potential is created. Two half cells must be connected in order to create an electrochemical cell. When they're joined:
- The one with the more negative electrode potential becomes the negative terminal where oxidation occurs.
- The one with the more positive electrode potential becomes the positive terminal where reduction occurs.
The standard hydrogen half cell is used as the reference electrode which all other electrode potentials are measured against. All electrochemical cells must be set up under the following standard conditions:
- Temperature - 298K
- Pressure - 1atm
- Concentration - 1.00mol dm^-3
The direction of a particular reaction and thus the feasability of reactions can be determined by using the electrode potentials.
When visible light falls on a coloured compound, the light absorbed is the energy range that causes electronic transitions. This is when electrons move to higher energy levels the molecules become excited. They don't remain excited for long, and electrons beign to fall back to intermediate energy levels. When this occurs, the light absorbed is re-emitted in various forms including vibrational energy. The colorus we see are the colour wavelengths transmitted or reflected. The complementary colour to that absorbed is always the one we see.