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903110
For Supervisor’s use only
Level 2 Chemistry, 2007
90311 Describe oxidation-reduction reactions
Credits: Three
2.00 pm Monday 19 November 2007
Check that the National Student Number (NSN) on your admission slip is the same as the number at the
top of this page.
A Periodic Table is provided on the RESOURCE SHEET in your Level 2 Chemistry package.
You should answer ALL the questions in this booklet.
If you need more space for any answer, use the page(s) provided at the back of this booklet and clearly
number the question.
Check that this booklet has pages 2–8 in the correct order and that none of these pages is blank.
YOU MUST HAND THIS BOOKLET TO THE SUPERVISOR AT THE END OF THE EXAMINATION.
For Assessor’s
use only
Achievement
Describe oxidation-reduction
reactions.
Achievement Criteria
Achievement
with Merit
Apply oxidation-reduction
principles.
Achievement
with Excellence
Discuss oxidation-reduction
processes.
Overall Level of Performance
© New Zealand Qualifications Authority, 2007
All rights reserved. No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
You are advised to spend 35 minutes answering the questions in this booklet.
QUESTION ONE
When sulfur dioxide gas is bubbled through a solution of acidified potassium dichromate, a colour
change from orange to green is observed.
Unbalanced equation: Cr2O72– + SO2 → Cr3+ + SO42–
(a)
Identify the species responsible for:
(i)
(ii) the green colour.
the orange colour
(b) Identify the oxidant:
Use oxidation numbers to justify your answer.
QUESTION TWO
One of the following reactions is an oxidation-reduction reaction.
Reaction A
CaCO3 → CO2 + CaO
Reaction B
PbO + H2 → Pb + H2O
Reaction C
SO3 + H2O → H2SO4
Identify the oxidation-reduction reaction by circling the letter of your choice.
A
B
Justify your answer.
Chemistry 90311, 2007
C
Assessor’s
use only
QUESTION THREE
Assessor’s
use only
Describe the expected observations for each of the following oxidation-reduction reactions.
Link the observations to the species involved in the reaction.
(a)
Aqueous chlorine, Cl2(aq), is added to a solution of sodium bromide, NaBr.
(b) A solution of iron(III) chloride, FeCl3, is added to a solution of potassium iodide, KI.
PLEASE TURN OVER TO PAGE 4 FOR QUESTION FOUR
Chemistry 90311, 2007
QUESTION FOUR
Assessor’s
use only
(a)
Determine the oxidation number of manganese (Mn) in each of the following species.
(i)
(ii) MnO2
(iii) Mn
(iv) MnO4–
MnO42–
(b)Potassium permanganate (KMnO4) is added to iron(II) sulfate (FeSO4) under acidic
conditions.
(i)
Write half-equations and the overall balanced equation for the reaction that occurs.
Oxidation half-equation
Reduction half-equation
Overall balanced equation
(ii)Describe the expected observations that you would make during this reaction and link
these to reactant and product species.
Chemistry 90311, 2007
QUESTION FIVE
Assessor’s
use only
Hydrogen peroxide decomposes at room temperature (25°C) according to the following equation.
2H2O2(aq) → 2H2O(ℓ) + O2(g)
The reaction involves both oxidation and reduction processes.
Discuss this statement using oxidation numbers to clearly explain why oxidation and reduction are
involved in the decomposition reaction.
Include in your answer the balanced half-equations for the processes that occur in the reaction.
QUESTION SIX
Write balanced half-equations for the oxidation and reduction reactions occurring, and then the
overall balanced equation, for the following reaction in acidic solution.
IO3– + SO32– → I2 + SO42–
(a)
Oxidation half-equation
(b) Reduction half-equation
(c)
Overall balanced equation
Chemistry 90311, 2007
QUESTION SEVEN
Assessor’s
use only
Magnesium can be produced by the electrolysis of molten magnesium chloride.
A simplified diagram of the cell used to manufacture magnesium is shown below.
+ electrode
– electrode
Molten
MgCl2
(a)
Label the electrodes as anode and cathode by writing in the boxes in the diagram above.
(b) Write half-equations for the reactions occurring at each electrode.
Cathode:
Anode:
Chemistry 90311, 2007
QUESTION EIGHT
Assessor’s
use only
The diagram below shows the electrolysis of molten lead bromide.
electrical energy
–
+
red / brown
fumes
Molten
PbBr2(ℓ)
molten beads
of grey / silver
Discuss the electrolysis of molten lead bromide by linking the observations in the above cell to the
reactions occurring at the anode and cathode.
Your answer should refer to:
•
•
•
•
•
reactant and product species
movement of species
oxidation and reduction processes
anode and cathode
equations for reactions occurring.
Chemistry 90311, 2007
Extra paper for continuation of answers if required.
Clearly number the question.
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Question
number
Chemistry 90311, 2007
Assessor’s
use only