CHEMISTRY 2.7
An assessment for AS90311
Describe oxidation-reduction reactions.
Credits: Three
INSTRUCTIONS
Answer ALL questions.
© New Zealand Institute of Chemistry 2006
2
You should spend about 30 minutes on this assessment
QUESTION ONE
Oxidation numbers
Determine the oxidation number of chromium in each of the following species
Species
Oxidation number of Cr
2–
CrSO4
CrO4
Cr2O3
Cr2O72–
Oxidation number of Cr
Working space
QUESTION TWO
Balancing half-equations
Balance each of the following half equations
(a)
(b)
(i)
Fe2+
Fe3+
(ii)
Cl2
Cl–
(iii) SO2
SO42–
(acidic conditions)
(iv) NO3–
NO
(acidic conditions)
Write a balanced equation for the reaction that occurs when SO2 gas is bubbled through dilute
nitric acid solution.
QUESTION THREE
When copper metal is added to a solution of silver nitrate the solution slowly turns blue and a grey
solid settles in the test tube.
(a)
Account for these observations in terms of the reactants and products.
_____________________________________________________________________________
_____________________________________________________________________________
(b)
Identify (i) the oxidant. ___________________________________________________
(ii) the reductant __________________________________________________
(iii) the ion that is not involved in the oxidation-reduction changes ___________
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3
(c)
Write the equations for the oxidation and reduction processes and the overall balanced ionic
equation.
Oxidation
Reduction
Overall
QUESTION FOUR
When acidified potassium permanganate (KMnO4) is added to hydrogen peroxide (H2O2) solution
the purple permanganate turns colourless and a colourless gas is produced.
(a)
Write the formula of the manganese product of the reaction.
_____________________________________________________________________________
(b)
(i)
Circle the word which best describes the role of potassium permanganate (KMnO4) in
this reaction.
Oxidant
(ii)
Reductant
Justify your choice. ____________________________________________________
_____________________________________________________________________________
(c)
(i)
Circle the formula of the gas produced in this reaction.
H2
(ii)
(d)
O2
Justify you’re your choice by considering the changes in oxidation states of the
elements involved.
Write balanced equations for the oxidation and reduction processes and the balanced overall
equation.
Oxidation
Reduction
Overall
Turn over
4
QUESTION FIVE
Electrolysis of zinc iodide solution.
An electric current is passed through a solution of zinc iodide (ZnI2). A grey deposit appears at one
electrode and a brown colour appears in the solution at the other electrode.
(a)
Identify (i) the grey deposit. ________________________________________________
(ii) the substance causing the brown colour ______________________________
(b)
Write the equation for the formation of the grey deposit.
_____________________________________________________________________________
(c)
(i)
Circle the word which describes the charge on the electrode at which the grey solid
forms
positive
(ii)
negative.
Justify your choice ____________________________________________________
_____________________________________________________________________________
(d)
Write the equation for the reaction at the anode.
_____________________________________________________________________________
(e)
(i)
Circle the word which describes the reaction at the anode.
oxidation
(ii)
reduction.
Justify your choice ____________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
End of Turn
Assessment
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