Grade 12 -chemistry explanations
Physical properties
giant structures such as diamond, graphite and silicon dioxide melting and boiling points are
high due to the large amount of energy needed to break the many strong covalent bonds
giant ionic solids (e.g. sodium chloride), the melting points and boiling points are determined by
electrostatic forces of attraction (ionic bonds) between the cations and the anions in the lattice
structure
For simple molecular substances the melting and boiling points are affected by the strength and
the relative number (for hydrogen bonds) of the intermolecular forces
,When comparing the melting and boiling points of substances in the same group on the Periodic
Table (e.g. halogens, hydrides of elements in groups 15, 16 and 17) or the same homologous
series (see organic chemistry section), the difference in melting points and boiling points is
mainly due to the London forces, the exception being those that experience hydrogen bonds
(e.g. H2S vs H2O)
- Melting vs boiling points
When comparing the melting and boiling points of substances of similar mass in different groups
on the Periodic Table or different homologous series (see organic chemistry section) the
difference in melting and/or boiling point is due to the presence of other intermolecular forces
(dipole-dipole and/or hydrogen bonds) besides the London forces which are present between all
particles
When comparing the melting and boiling points of substances that involve hydrogen bonding,
the difference in melting and/or boiling point is due to the relative number of hydrogen bonds
that occur between the particles (e.g. HF vs H2O or propanol vs ethanediol – see organic
chemistry section)
Metals and graphite conduct electricity due to the presence of free (mobile) valence electrons.
,Ionic compounds only conduct electricity when molten or in aqueous solution due to the
presence of free (mobile) ions, which are not present in the solid state.
, Chemical bonding
Cation - positive ion
Anion - negative ion
Bond order - The term bond order is to show the amount of electron pairs shared in the bond.
- Single order - is a single pair of shared electrons to make a single covalent bond.
- E.g. H:H single bond
- Double order - is where there are four shared electrons to make a double covalent
bond.
- E.g. O::O double bond
- Triple order - is where there are six shared electrons to make a triple covalent bond.
- E.g. :N:::N: Triple bond
Bond length - the average distance between the nuclei of two bonded atoms.
Bond energy - the energy needed to break a bond between two atoms into separate atoms.
Physical properties
giant structures such as diamond, graphite and silicon dioxide melting and boiling points are
high due to the large amount of energy needed to break the many strong covalent bonds
giant ionic solids (e.g. sodium chloride), the melting points and boiling points are determined by
electrostatic forces of attraction (ionic bonds) between the cations and the anions in the lattice
structure
For simple molecular substances the melting and boiling points are affected by the strength and
the relative number (for hydrogen bonds) of the intermolecular forces
,When comparing the melting and boiling points of substances in the same group on the Periodic
Table (e.g. halogens, hydrides of elements in groups 15, 16 and 17) or the same homologous
series (see organic chemistry section), the difference in melting points and boiling points is
mainly due to the London forces, the exception being those that experience hydrogen bonds
(e.g. H2S vs H2O)
- Melting vs boiling points
When comparing the melting and boiling points of substances of similar mass in different groups
on the Periodic Table or different homologous series (see organic chemistry section) the
difference in melting and/or boiling point is due to the presence of other intermolecular forces
(dipole-dipole and/or hydrogen bonds) besides the London forces which are present between all
particles
When comparing the melting and boiling points of substances that involve hydrogen bonding,
the difference in melting and/or boiling point is due to the relative number of hydrogen bonds
that occur between the particles (e.g. HF vs H2O or propanol vs ethanediol – see organic
chemistry section)
Metals and graphite conduct electricity due to the presence of free (mobile) valence electrons.
,Ionic compounds only conduct electricity when molten or in aqueous solution due to the
presence of free (mobile) ions, which are not present in the solid state.
, Chemical bonding
Cation - positive ion
Anion - negative ion
Bond order - The term bond order is to show the amount of electron pairs shared in the bond.
- Single order - is a single pair of shared electrons to make a single covalent bond.
- E.g. H:H single bond
- Double order - is where there are four shared electrons to make a double covalent
bond.
- E.g. O::O double bond
- Triple order - is where there are six shared electrons to make a triple covalent bond.
- E.g. :N:::N: Triple bond
Bond length - the average distance between the nuclei of two bonded atoms.
Bond energy - the energy needed to break a bond between two atoms into separate atoms.
