Physical - Macromolecules

Question 1

Silicon, silicon dioxide, diamond and graphite all have extremely high melting points. Suggest
why.

Answer 1

These substances all have giant covalent structures. To melt them, lots of strong covalent
bonds must be broken, which takes a lot of energy.

Question 2

Diamond mp, strength, conductor

Answer 2

MP VERY HIGH - many strong covalent bonds must be broken to
separate the Carbon atoms
STRONG - each carbon is joined to four others in a rigid
structure. Coordination Number = 4
NON-CONDUCTOR – No mobile charge carriers. No delocalised electrons - all four carbon electrons are
localised in covalent bonds. No mobile ions.

Question 3

Graphite

Answer 3

MELTING POINT HIGH because many strong covalent bonds must be broken to
separate the atoms
STRENGTH Soft because each carbon is joined to three others in a
layered structure, Coordination Number = 3. Layers are
held by weak van der Waals’ forces can slide over each other
ELECTRICAL CONDUCTIVITY CONDUCTOR because only three carbon electrons are used
for bonding. The fourth is delocalised, and can move freely.

Question 4

Silicon dioxide

Answer 4

MELTING POINT VERY HIGH because many strong covalent bonds must be broken to separate
the atoms
STRENGTH STRONG because each silicon atom is joined to four oxygen atoms - C No. =
4
each oxygen atom are joined to two silicon atoms - C No = 2
ELECTRICAL Conductivity NON-CONDUCTOR – No mobile charge carriers. No delocalised electro electrons - all
four silicon electrons are localised in covalent bonds. No mobile ions.