transition elements

Transition element - a d block element that forms an ion with an incomplete d sub shell

d-block element - an element which has its highest energy sub-shell in a d sub shell

Complex ion - a transition metal ion bonded to one or more ligands by coordinate bonds

Ligand - a molecule or ion which donates a pair of electrons to the transition metal ion to form a coordinate bond

Coordination number - the total number of coordinate bonds formed between a central metal ion and its ligands

Bidentate ligand - can donate two lone pairs of electrons to the central metal ion to form two coordinate bonds

Ligand substitution - reaction in which one ligand in a complex is replaced by another ligand

Optical isomers - stereoisomers that are a pair of non superimposable mirror images of each other

Cr and Cu have full and half-full 4s and 3d sub-shells, which makes them more stable.

Cr = 1s2 2s2 2p6 3s2 3p6 4s1 3d5 

Cu = 1s2 2s2 2p6 3s2 3p6 4s1 3d10

One electron from the 4s sub shell has been transferred to 3d, giving the molecule extra stability.

Transition elements from Titanium to Copper all form ions with two or more oxidation states. They all form compounds with ions in the +2 oxidation state - which in most cases is due to losing the electrons in the 4s orbital, as these electrons have a higher energy than the 3d electrons.

As the 3d and 4s energy levels are quite close in energy, the 3d electrons can also be lost quite easily to form a stable ion.

This means that they can form effective catalysts as they are able to gain and lose electrons easily.

The oxidation state of the transition element is stated in the name of the compound.

E.g. Cu = +2 in copper (II) sulfate, Mn = +7 in potassium manganate(VII) and Cr = +6 in sodium dichromate(VI).

Compounds and salt solutions of transition elements are coloured.

When white light passes through a solution containing transition metal ions, some wavelengths of visible light are absorbed, the colour we observe is a mixture of the wavelengths of light that havent been absorbed.

This is due to the incompletely filled d sub-shell of the transition metal ion.

Copper(II) sulfate is blue

Iron(II) chloride is yellow

Chromium(III) sulfate is green

Homogeneous = catalyst is in the same state as reactants. Reacts to form an intermediate which breaks down to give products and the regenerated catalyst. e.g, making esters with concentrated H2SO4 from alcohol and carboxylic acids.

Heterogeneous = Catalyst is a different state as reactants. Reactants are adsorbed (weak bonds with catalysts surface) onto the catalyst surface where the reaction occurs, the products then desorb. E.g. hydrogenation of alkenes using a nickel catalyst 

Other examples:

Haber process - solid iron catalyst - heterogeneous

Contact process - solid vanadium(V) oxide (V2O5) - heterogeneous 

Decomposition of hydrogen peroxide - solid manganese(IV) oxide (MnO2) - heterogeneous

Precipitation reaction - two solutions of ions react together to make an insoluable compound

When an aqueous transition metal ion reacts with NaOH(aq), a coloured precipitate is formed.

Cu2+ = copper(II) hydroxide precipitate, Cu(OH)2(s) - blue solution to blue precipitate

Fe2+ = Iron(II) hydroxide, Fe(OH)2(s) - green solution to green precipitate

Fe3+ = Iron(III) hydroxide, Fe(OH)3(s) - pale yellow solution to rusty brown precipitate

Mn2+ = Manganese(II) hydroxide, Mn(OH)2(s) - pale pink solution to light brown precipitate - darkens when standing in air.

Cr3+ = chromium(III) hydroxide, Cr(OH)3(s) - violet solution to grey/green precipitate - SOLUABLE IN EXCESS NaOH forming a dark green solution forming a complex ion.

Fe2+, Fe3+ and Mn2+ react in the same way with NH3(aq) as they do with NaOH. NH3 produces OH- ions by acting as a base in water, and accepting H+ from water.

Cu2+(aq) with a small amount of NH3(aq) - blue solution to dark blue precipitate                           Cu2+(aq) + 2OH-(aq) ----> Cu(OH)2(s)

Cu2+(aq) with a large excess of NH3(aq) - pale blue solution to dark blue solution                         [Cu(H20)6]2+(aq) + 4NH3(aq) ---> [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l)

Cu2+(aq) with concentrated HCl(aq) - blue solution to green/yellow solution                                 [Cu(H20)6]2+(aq) + 4Cl-(aq) ---> [CuCl4]2-(aq) + 6H2O(l)

Cr3+(aq) with a small amount of NH3(aq) - pale purple solution to green precipitate                            Cr3+(aq) + 3OH-(aq) ----> Cr(OH)3(s)

Cr3+(aq) with an excess of NH3(aq) - pale purple solution to purple solution                                 [Cr(H2O)6]+3(aq) + 6NH3(aq) <---> [Cr(NH3)6]3+(aq) + 6H2O(l)