Main group chemistry

Question 1

What are the similarities in group 1?

Answer 1

One Electron in outer shell.

• Univalent – forms H+.
• Electron Affinity = -72 kJ mol-1

Question 2

What are the similarities in group 17?

Answer 2

• Exists as homonuclear diatomic gas - One vacancy in outer shell
• Univalent – forms H–
• Ionisation energy = +1310 kJ mol-1
  (Cl = +1260 kJ mol-1)

Question 3

What are the differences in group 17?

Answer 3

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)

Question 4

What are the differences in group 1?

Answer 4

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).

Question 5

What do group 1 and 2 metals with helium produce?

Answer 5

Ionic hydrides

Question 6

Why are hydrides strong bases?

Answer 6

They are not stable in water

Question 7

What are saline hydrides?

Answer 7

strong reducing agents

Question 8

What happens to thermal stability of saline hydrides?

Answer 8

Decreases down the group

Question 9

Describe covalent bond strnegth

Answer 9

Decreases down each group. sigma bonding becomes weaker as orbital overlap becomes poorer

Question 10

What affects acidity

Answer 10

1. Polarity of the molecule –
   electronegativity.
2. Bond strength of the hydride.
3. 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.

Question 11

Acidity vs number of oxo groups

Answer 11

Acidity decreases as number of
oxo groups decreases.
- HClO4 is stabilised through inductive effects:
- extra O pull electron density away.
- and through resonance effects.

Question 12

Describe acidity changes with choice of halogen

Answer 12

Acidity decreases down the group.

- The Cl-O- conjugate base is smallest, most electronegative thus most stabilised – hydration effects.

Question 13

Why are coordination compounds formed by d-block?

Answer 13

they are large and charge dense

Question 14

Describe chelate and charge density effect

Answer 14

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

Question 15

Describe macrocycle

Answer 15

S block metals form coordination compounds with cyclic ligands which are polydentate

Question 16

What are crown ethers?

Answer 16

selective with respect to cation complexation, consider the size of cation and cavity

Question 17

Describe cryptates

Answer 17

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

Question 18

Why do crypt ands bind more strongly?

Answer 18

There are more donor atoms and solvents have poor access to metal so decomplexation is difficult

Question 19

How does choice of solvent also affects binding strength

Answer 19

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.

Question 20

Describe the applications of crowns

Answer 20

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+.

Question 21

What are the three ways in which boron alleviates electron deficiency

Answer 21

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

Question 22

Describe boron trihalides

Answer 22

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