Main group chemistry Flashcards
What are the similarities in group 1?
One Electron in outer shell.
- Univalent – forms H+.
- Electron Affinity = -72 kJ mol-1
What are the similarities in group 17?
- Exists as homonuclear diatomic gas - One vacancy in outer shell
- Univalent – forms H–
- Ionisation energy = +1310 kJ mol-1
(Cl = +1260 kJ mol-1)
What are the differences in group 17?
Cl– exists in water but H- cannot as H2 forms spontaneously.
- H– is much harder to form than Cl-
- Electron affinity = -75 kJ mol-1 (Cl =
- 337 kJ mol-1)
What are the differences in group 1?
H has no inner, shielding electrons. - Hydrogen is not normally metallic. - Hydrogen is a gas. - HCl is a covalent gas, NaCl is an ionic solid. - Ionisation energy H = +1310 kJ mol-1 (Na = +494 kJ mol-1).
What do group 1 and 2 metals with helium produce?
Ionic hydrides
Why are hydrides strong bases?
They are not stable in water
What are saline hydrides?
strong reducing agents
What happens to thermal stability of saline hydrides?
Decreases down the group
Describe covalent bond strnegth
Decreases down each group. sigma bonding becomes weaker as orbital overlap becomes poorer
What affects acidity
- Polarity of the molecule –
electronegativity. - Bond strength of the hydride.
- Stabilisation in water of conjugate base – solvation effects.
Acidity increases across the period because FH electronegativity increases 3 and conjugate base is more strongly hydrated and thus stabilised.
Acidity increases down a group as the bond strength decreases.
Acidity vs number of oxo groups
Acidity decreases as number of oxo groups decreases. - HClO4 is stabilised through inductive effects: - extra O pull electron density away. - and through resonance effects.
Describe acidity changes with choice of halogen
Acidity decreases down the group.
- The Cl-O- conjugate base is smallest, most electronegative thus most stabilised – hydration effects.
Why are coordination compounds formed by d-block?
they are large and charge dense
Describe chelate and charge density effect
Group 2 cations are more charge dense and more likely to coordinate - form complexes with chelate ligands and binding is stronger than with unidentate ligands
More charge dense cations bind more strongly, stronger with chelate ligands
Rate of exchange is slower for more charge dense cations due to stronger binding
Describe macrocycle
S block metals form coordination compounds with cyclic ligands which are polydentate
What are crown ethers?
selective with respect to cation complexation, consider the size of cation and cavity
Describe cryptates
Cryptands are 3D macrocyclic ligands “molecular cages”
Selectivity for crypt ands is higher than for crown ethers
3D nature means that cavity geometry is more fixed, more spherical
Why do crypt ands bind more strongly?
There are more donor atoms and solvents have poor access to metal so decomplexation is difficult
How does choice of solvent also affects binding strength
KMnO4 is insoluble in toluene.
- Polar solvents e.g. H2O, MeOH compete strongly for cation binding.
- Less / non-polar solvents e.g. toluene do not compete much for cation binding therefore macrocycle binding is stronger.
- Strong complexation provides a driving force for solubility.
Describe the applications of crowns
Used in the environment to remove toxic metal ions from industrial waste e.g. cryptand selective for Hg2+ or Pb2+.
Crown ethers are used in phase transfer catalysis
Reaction does not generally proceed due to inefficient mixing but will go in presence of crown ether
This makes the complex more soluble in the organic layer [K(18- C-6)]+CN-
CN- is more ‘exposed’ i.e. separated from K+ therefore more nucleophilic
Antibiotics: Valinomycin is 12- member cyclic peptide - It uses six amide oxygens to bind K+.
What are the three ways in which boron alleviates electron deficiency
Intramolecular pie bonding - occurs where B is bonded to elements which have a lone pair
BX3 compounds act as Lewis acids, accept electron pair from a Lewis base - dative covalent bond
Dimer formation - known as boranes
Describe boron trihalides
Melting and boiling points increase down a group - molecules are non-polar so are held in liquid state by VdW
Bond length increases down the group - halide becomes larger the outer shells become more diffuse
Enthalpy of formation is endothermic - more difficult as you do down the group
