Assessment schedule
NZIC 2007
Chemistry 2.4 (AS90308)
Evidence
ONE
(a)
F
F C F
F
H C
TWO
(a) &
(b)
(c)
N
Achievement
Merit
Excellence
3 correct
O C
H
C
H
O
O
CH4 tetrahedral
NH3 trigonal pyramid
H2O bent
HCHO Trigonal planar
Three shapes
correct
CH4 non-polar
NH3 polar
H2O polar
HCHO polar
In CH4 there are
- four regions of negative charge around the central atom
- all these regions are bonding regions
- these will repel each other evenly (symmetry)
- will be distributed to the corners of a tetrahedron.
Three shapes
correct and
matching
polarities
Two of the
points
mentioned.
Full answer
(d)
The N and H have different attracting power for the
bonding electrons in the covalent bond
(electronegativities) The bonding electrons are pulled
closer to the nitrogen which gains a partial negative
charge while the other hydrogen gains a partial positive
charge.
N and H have
different
electronegativi
ties / bonding
electrons are
not shared
evenly.
Bonding
electrons are
closer to N than
H. This make N
– and N +
(e)
In NH3 there are
- four regions of negative charge (electron pairs) around
the central atom and
- these will be distributed to the corners of a tetrahedron.
- Three pairs are bonding pairs to hydrogen and the fourth
is a non-bonding pair so
- the shape is a trigonal pyramid.
Shape and
polarity stated.
Concept that
shape is related
to repulsion
between electron
pairs.
The molecule has 3 polar bonds on one side so is
unsymmetrical and hence is polar.
Merit plus
relates shape
and bond
polarity to a
polar
molecule.
Evidence
Achievement
magnesium
ions
ionic
chloride
diamond
atoms
covalent
silver
atoms
metallic
iodine
molecules weak intermolecular
(a) Copper is a metal. Metals conduct electricity
because they consist of an orderly array of
positive centres held together by a mobile sea of
electrons. The mobile / delocalized (free moving)
electrons conduct the electricity.
(c) Potassium bromide is an ionic compound with
ions held in fixed positions in the solid. There are
no free ions to conduct electricity in the solid.
When dissolved in water, the ions are free to
move so it conducts electricity.
Graphite has a layer structure with weak bonding
between layers. This allows the layers to move easily
over each other an act as a lubricant.
Three rows or
all particles
correct
All correct
Structure of
copper metal
described.
Links made
between
delocalized
electrons and
conductivity
Structure
described both
in the rings
and between
the layers.
(c)
Potassium bromide is an ionic compound with ions
held in fixed positions in the solid. There are no free
ions to conduct electricity in the solid. When dissolved
in water, the ions are free to move so it conducts
electricity.
Structure
described as a
solid and in
solution
FIVE
Iodine is a non-polar molecule with weak bonding
between molecules
Sodium iodide is ionic with strong bonding between
the ions.
Links
properties to
type of
bonding in
solid
Bonding
between layers is
weak, so layers
slide over each
other, so can be
used as a
lubricant
In a solid the
ions are in fixed
positions and
cannot move. In
solution, the ions
are free to move.
Correct
discussion for I2
and NaI of
melting point OR
solubility
All angles
correct
OR 1 angle
with an
description of
shape
2 bond angles
correct and
linked to regions
of charge or
shape around
central atom.
THREE
FOUR
(a)
(b)
Merit
Excellence
Links the free
ions to
conductivity
Discusses
melting point
and solubility
correctly for
both.
Melting point: The weak forces between molecules
of I2 are easily overcome on heating so I2 is easily
converted from solid to gas.
The forces between Na+ and I– ions are strong and
considerable energy is required to break them. Hence
a high melting point.
Solubility: I2 is non-polar and does not dissolve
readily in water (a polar solvent).
The polar water molecules are attracted to Na+ and I–
ions and surround them pulling the ions out of the
crystal and dispersing them through the water.
SIX
 = 109. There are 4 regions of negative charge
around the carbon atom so the bonds are directed to
the corners of a tetrahedral.
 = 120. There are three regions of negative charge
around the carbon atom and these will be directed to
the corners of a triangle.
 = 109. There are 4 regions of negative charge
around the carbon atom (2 bonding, 2 non-bonding)
so the bonds are directed to the corners of a
tetrahedral..
Sufficiency Statement.
Opportunities: 9 achieved, 8 Merit, 4 Excellence
Angles and
related
discussion
correct for all
3.
Achievement
Merit
Excellence
A total of FIVE opportunities correct at Achievement or higher
A total of SIX opportunities correct with FIVE at Merit or higher
A total of SIX opportunities correct with THREE at Merit or higher and TWO at Excellence