9 0 3 11
2
903110
SUPERVISOR’S USE ONLY
Level 2 Chemistry, 2011
90311 Describe oxidation-reduction reactions
9.30 ��������������������������
am������������������������
Monday 21 November 2011
Credits: Three
Check that the National Student Number (NSN) on your admission slip is the same as the number at the
top of this page.
You should attempt ALL the questions in this booklet.
A periodic table is provided on the Resource Sheet L2–CHEMR.
If you need more room for any answer, use the extra space provided at the back of this booklet.
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.
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, 2011. All rights reserved.
No part of this publication may be reproduced by any means without the prior permission of the New Zealand Qualifications Authority.
2
You are advised to spend 35 minutes answering the questions in this booklet.
QUESTION ONE
(a)
The reaction between methane and oxygen, represented by the equation below, can be
classified as an oxidation-reduction reaction.
CH4 + 2O2 → CO2 + 2H2O
Use oxidation numbers to explain why the reaction is an oxidation-reduction reaction.
(b) The reaction between iron metal and copper sulfate solution, represented by the equation
below, can be classified as an oxidation-reduction reaction.
Fe + Cu2+ → Cu + Fe2+
Identify the reductant in this reaction.
Give reasons for your answer in terms of electron transfer.
(c)
(i)
Complete the table with the oxidation number of vanadium, V, in each species. The
first one has been done for you.
Species
Oxidation
number
VO2+
V3+
V2O5
+4
Chemistry 90311, 2011
VO3–
VO2+
ASSESSOR’S
USE ONLY
3
(ii) The reaction between an acidified solution containing VO2+ ions and zinc metal,
represented by the unbalanced equation below, can be classified as an oxidationreduction reaction.
VO2+ + Zn → Zn2+ + V3+
Discuss why this is an oxidation-reduction reaction in terms of oxidation-reduction
processes.
In your answer you must:
•
identify the species oxidised and reduced, justifying your answer using either
oxidation numbers or electron transfer
•
identify the oxidant and reductant
•
include half-equations
•
write the balanced overall equation.
Chemistry 90311, 2011
ASSESSOR’S
USE ONLY
4
QUESTION TWO
ASSESSOR’S
USE ONLY
(a)
Iron(II) sulfate, FeSO4, solution is added to a solution of acidified hydrogen peroxide, H2O2,
until no further change occurs. During the reaction the hydrogen peroxide is reduced to form
water, H2O.
(i)
(ii) Link this colour change to the species involved in the reaction.
(iii) Write the reduction half-equation in the box below.
Describe the colour change that occurs.
H2O2 →
(b) A solution containing HSO3– ions is added to an acidified potassium dichromate solution,
Cr2O72–. The hydrogen sulfite ions, HSO3–, are oxidised to colourless sulfate ions SO42–.
(i)
State the colour of the dichromate ion.
(ii) Give the colour and the formula of the species produced when acidified potassium
dichromate solution reacts in this way.
Colour:
Formula:
(iii) Write the dichromate half-equation in the box below.
→
Chemistry 90311, 2011
5
(c)
Two colourless solutions require identification. The solutions are potassium bromide,
KBr, and acidified potassium hydrogen sulfite, KHSO3. One reagent, iodine solution, I2, is
available.
Discuss how the solutions can be identified using only iodine solution.
Your answer must include:
•
experimental observations linked to the species
•
oxidation and reduction processes
•
balanced half-equations and the overall balanced equation for oxidation-reduction
reactions that occur.
Chemistry 90311, 2011
ASSESSOR’S
USE ONLY
6
QUESTION THREE
ASSESSOR’S
USE ONLY
A simplified diagram of a cell used in the electrolysis of molten aluminium oxide, Al2O3(ℓ), is
shown below.
The negative electrode is the carbon lining of the steel tank, and the positive electrodes are carbon
blocks, which can be lowered into the molten aluminium oxide.
electrical energy
carbon
molten
aluminium
dissolved aluminium oxide
Discuss what occurs in the electrolysis of molten aluminium oxide.
Your answer must include:
•
substances formed at the anode and the cathode
•
observations linked to the species involved at both the anode and the cathode
•
the movement of ions and electrons
•
oxidation and reduction processes
•
balanced equations for reactions at both the anode and the cathode.
Chemistry 90311, 2011
7
ASSESSOR’S
USE ONLY
Chemistry 90311, 2011
8
9 0 3 11
QUESTION
NUMBER
Extra space if required.
Write the question number(s) if applicable.
Chemistry 90311, 2011
ASSESSOR’S
USE ONLY