Topic 13 - Periodicity
From KstructIB
[edit] 13.1 Periodic trends Na-> Ar (the third period)
[edit] 13.1.1
(This seems very similar to the last bit of SL, but now with explanations)
Elements on the left are metallic, those on the right are non-metals.
Oxides : Non-metals produce acidic oxides, metals produce basic oxides and metalloids produce amphoteric (both acidic and basic) oxides.
(Sorry about the huge table)
| Adding H2O | Adding HCl | Adding NaOH | Nature | Conductivity | Melting Point | |
|---|---|---|---|---|---|---|
| Na2O | Na2O + H2O -> 2NaOH | Na2O + H+ -> 2Na+ + H2O | No reaction | Basic Oxide | Good | 1275 |
| MgO | MgO + H2O -> Mg(OH)2 | MgO + 2H+ -> Mg2+ + H2O | No reaction | Basic Oxide | Good | 2852 |
| Al2O3 | Insoluble | Al2O3 + 6H+ -> 2Al3+ + 3H2O | Al2O3 + 2OH- + 3H2O -> 2Al(OH)4 | Amphoteric Oxide | Good | 2027 |
| SiO2 | Insoluble | No reaction | SiO2 + 2OH- -> SiO32- + H2O | Acidic Oxide | None | 1610 |
| P4O10 (or P4O6) |
P4O10 + 6H2O -> 4H3PO4 P4O6+ 6H2O -> 4H3PO3 | No reaction | H3PO4 + OH- -> H2PO4- + H2O H3PO3 + OH- -> H2PO3- + H2O | Acidic Oxide | None | 24 |
| SO3 (or SO2) |
SO3 + H2O -> H2SO4 SO2 + H2O -> H2SO3 | No reaction | SO2 + OH- -> HSO4- SO2 + OH- -> HSO3- | Acidic Oxide | None | 17 |
| Cl2O7 Cl2O | Cl2O7 + H2O -> 2HClO4 Cl2O + H2O -> 2HOCl | No reaction | HCl2O7 + OH- -> Cl2O72- + H2O HOCl + OH- -> OCl- + H2O | Acidic Oxide | None | -92 |
Explaining the physical properties:
Conductivity : For ionic solutions (Na2O->Al2O3) conductivity is due to ions in solution/molten state. SiO2 is network covalent, hence has no free charged particles, therefore has no significant conductivity. Others are covalent molecules and do not conduct.
Melting point : Stronger bonds are created when atoms can be arranged in a simple structure. MgO is highest, followed by Al2O3 then Na2O (The ratio between the two atoms should be as close to 1 as possible). SiO2 is network covalent, hence has a high melting point (but not as high as for ionic bonding). The final 3 decrease in melting point due to decreasing polarity of molecules (i.e. smaller dipole-dipole interactions).
Halides (Cl could be replaced with Br, I, F etc.) : Ionic Chlorides dissolve in H2O with little reaction, covalent chlorides dissolve and react to form HCl.
NaCl : NaCl + H2O -> Na+ + Cl- + H2O
Good conductivity (ionic structure) MP = 801
MgCl2 : MgCl2 -> Mg2+ + 2Cl-
Good conductivity (ionic structure) MP = 714
Al2Cl6 : Al2Cl6 + 6H2O -> 2Al(OH)3 + 6HCl
Poor conductivity (Network covalent) MP = 178
SiCl4 : SiCl4 + H2O -> Si(OH)4 + 4HCl
No conductivity (Covalent molecular) MP = -70
PCl3 : PCl3 + 3H2O -> H3PO3 + 3HCl
PCl5 : 2PCl5 + 6H2O -> 2HPO3 + 10HCl
No conductivity (Covalent molecular) MP = -112
S2Cl2 : Not required
Cl2 : Cl2 + H2O -> HCl + HClO (Exception : F2 is such a strong oxidizer : 2F2 + 2H2O -> 4HF + O2)
No conductivity (Covalent molecular) MP = -101
Melting point : For NaCl and MgCl2 MP decreases due to packing (as above), and drops again for Al2Cl6 (which is network covalent). The others are covalent molecules, and the mp decreases due to decreasing polarity (Cl2 higher due to more electrons, resulting in greater LDF ?)
[edit] 13.2 D-block elements (first row)
[edit] 13.2.1
Typical d-block elements are generally those exhibiting multiple oxidation states (in period 4, not Sc or Zn) Form coloured compounds, form complex ions, and have catalytic properties
[edit] 13.2.2
The multiple oxidation states of the d-block (transition metal) elements are due to the proximity between the 4s and 3d sub shells (in terms of energy). All transition metals exhibit a 2+ oxidation state (both electrons being lost form the 4s and all have other oxidation states as in the following examples.
(Apparently we need to know only 2 of these. Weird if you ask me, but include Fe.)
- V : +2, +3, +4, +5
- Cr : +2, +3, +6
- Mn : +2, +4, +6, +7
- Fe : +2, +3
[edit] 13.2.3
Ligands are the molecules which donate an electron pair to form a dative covalent bond with the central atom (thus forming a complex ion).
[edit] 13.2.4
Complex ions are molecules which carry a charge. They are formed around a central atom, with other atoms (or molecules) donating an electron pair to form a covalent bond to this central atom.
[Fe(H2O)6]3+ : Fe is the central atom, H2O is the ligand.
[Fe(CN)6]3- : Fe is the central atom, CN is the ligand.
[CuCl4]3- : Cu is the central atom, Cl is the ligand.
[Cu(NH3)4]2+ : Cu is the central atom, NH3 is the ligand.
[Ag(NH3)2]+ : Ag is central atom, NH3 is the ligand.
[edit] 13.2.5
The colour in the transition metals (d-block) is predominantly due to the splitting of the d shell orbitals into slightly different energy levels. As a result, certain wavelengths of energy can be absorbed by the d-block elements (with electrons jumping between these slightly different energy levels), resulting in the complement colour being visible.
[edit] 13.2.6
d-block elements make good catalysts due to their multiple oxidation states. This gives them the ability to react with different species and produce a path of lower activation energy, and so cause the reaction to proceed at a faster rate.
- MnO2 in decomposition of hydrogen peroxide
- V2O5in the contact process
- Fe in Haber process
- Ni in conversion of alkenes to alkanes
