Transition Metals - Iandalgleish.co.uk

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ADVANCED HIGHER 2 TransMetsThe vacant d orbitals are the reason that many transition metalsare heterogeneous catalystse.g. Iron catalyses the Haber process :3H 2 + N 2 -> 2NH 3When the H 2 and N 2 become adsorbed on the catalyst surfaceelectrons flow out of the H-H and N N bonds into the vacant dorbitals on the Iron atoms. Thus the H-H and N N bonds areweakened lowering the activation energy.NNHHFeFeThe energies of the 3d and 4S orbitals are very similar. It is afact that electrons in a d orbital repel each other more thanelectrons in an S orbital. In a neutral atom the electrons in the3d orbital are therefore further from the nucleus (higher energy)than the electrons in the 4S orbital. When electrons are removedfrom the atom to form an ion the electron-electron repulsions arereduced and the 3d orbital actually falls below the 4S orbital inenergy.Electrons are, in effect, removed from the 4S orbitalbefore the 3d orbitals.e.g. the formation of the Mn2+ ion3d4S4S3dMn Mn 2+Mn (4S2 3d5) -> Mn2+ (4S0 3d5)The 2+ valency (oxidation state) is therefore the most common intransition elements but, because of the close proximity of the 3dorbitals, the third ionisation energy is low, resulting in theeasy removal of a third electron :Mn2+ (4S0 3d5) -> Mn3+ (4S0 3d4)
ADVANCED HIGHER 3 TransMetsIons with charges greater than 4+ are unknown. The energy requiredto pull 4 electrons completely off an atom is just too great.Subsequent electrons can however be released from the d orbitalsto form polar covalent bonds resulting in many compounds andions in which Manganese, like all the transition metals, exhibitsa variable valency:e.g. manganate(VII) ion MnO 4-An electrovalent structure for this ion would be impossible. Thehighly positive Mn7+ ion would attract electrons from the O2- ionsresulting in the formation of covalent bonds between Mn and O :O 2- O 2-O-O 2-Mn 7+O 2-OMnOOThe electrovalent model is nevertheless useful : we can imaginethe Mn7+ ion protected by a cage of O2- ions. Take away theprotection and you have a highly effective oxidising agent, Mn7+.e.g. Oxidation of chloride by acidified manganate(VII) :MnO 4- + H+ + Cl- -> Mn2+ + H 2 O + Cl 2The oxidation state of Mn decreases from 7 to 2 as it is reducedin this reaction.Manganese exhibits the greatest variety of oxidation states of allthe transition metals :Oxidation state Examples2 Mn2+, MnCl 2- 43 Mn3+4 MnO 2 , MnCl 2- 65 MnO 3- 46 MnO 2- 47 MnO 4-, MnO + 3
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