Example 1

Internal assessment resource Chemistry 2.7A for Achievement Standard 91167
PAGE FOR TEACHER USE
Assessment schedule: Chemistry 91167 Redox rules
Evidence/Judgements for Achievement Evidence/Judgements for Achievement with Merit Evidence/Judgements for Achievement
with Excellence
The student shows understanding:
relevant observations are recorded
For example:
Bubbles/fizzing, metal dissolves
Orange solution turns blue/green
Purple solution turns pale yellow-brown
key reactant and product species are correctly
identified for three reactions
For example:
Mg + H + Mg 2+ + H 2
Cr 2 O 2- -
7 + HSO 3 Cr 3+ 2-
+ SO 4
MnO - 4 + Fe 2+ Mn 2+ + Fe 3+
H 2 O 2 + I - I 2 + H 2 O
an oxidant or reductant is identified without
linking to oxidation number or electron transfer
for one reaction.
For example:
In the first reaction H 2 O 2 is the oxidant.
The student shows in-depth understanding:
balanced overall equations are written for three
reactions
For example:
Mg + 2H + Mg 2+ + H 2
Cr 2 O 2- 7 +3HSO - 3 +5H + 2Cr 3+ +3SO 2- 4 +4H 2 O
MnO - 4 + 8H + + 5Fe 2+ Mn 2+ + 5Fe 3+ + 4H 2 O
H 2 O 2 + 2H + + 2I - I 2 + 2H 2 O
observations are linked to species for three reactions
For example:
Metal dissolves because Mg turns to Mg 2+ .
Bubbles occur because H + turns to H 2 .
The solution turns from orange to green because
Cr 2 O 2- 7 is orange and Cr 3+ -
is blue/green. HSO 3
and SO 2- 4 are colourless.
MnO - 4 is purple, Fe 2+ is pale green. The resulting
solution is pale yellow-brown due to the formation
of Fe 3+ . Mn 2+ is colourless.
Brown colour forms due to the formation of I2. All
other species are colourless.
oxidants and reductants are identified in terms of
oxidation number or electron transfer for one reaction.
For example:
H 2 O 2 is the oxidant because each oxygen gains
one electron or oxidation number goes from -1 to -
2 in H 2 O.
I - is the reductant because it loses one electron or
oxidation number goes from -1 to 0.
The student shows comprehensive
understanding:
balanced overall equations are
consistently written with no errors
observations are linked to species for
all reactions
oxidants and reductants are identified
in terms of oxidation number or
electron transfer for both reactions.
For example:
When acidified H 2 O 2 reacts, it is the
oxidant because each oxygen gains
one electron or oxidation number goes
from -1 to -2 in H 2 O.
I - is the reductant because it loses one
electron or oxidation number goes
from -1 to 0.
When non-acidified H 2 O 2 reacts it is
the reductant because each oxygen
loses one electron or oxidation
number goes from -1 to 0 in O 2 . I 2 is
the oxidant because it gains one
electron or oxidation number goes
from 0 to -1.
Final grades will be decided using professional judgement based on a holistic examination of the evidence provided against the criteria in the
Achievement Standard.
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