90696
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906960
SUPERVISOR’S USE ONLY
Level 3 Chemistry, 2012
90696 Describe oxidation-reduction processes
2.00 pm Tuesday 20 November 2012
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 L3–CHEMR.
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.
ASSESSOR’S USE ONLY
Achievement
Describe oxidation-reduction
processes.
Achievement Criteria
Achievement with Merit
Explain and apply oxidationreduction processes.
Achievement with Excellence
Discuss oxidation-reduction
processes.
Overall level of performance
© New Zealand Qualifications Authority, 2012. 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 copper half cell is connected to a standard hydrogen electrode, as shown below.
H2 gas at
100 kPa (1 bar)
–
V
+
Cu
Pt
salt bridge
1 mol L–1 CuSO4 (aq)
1 mol L–1 HCl (aq)
E° (Cu2+ / Cu) = +0.34 V
(a)
(i)
(ii) Write the balanced equation for the reaction at the negative electrode.
(iii) Write the balanced equation for the overall reaction.
(b) (i)
Write the balanced equation for the reaction at the positive electrode.
Complete the standard cell diagram for the cell shown above.
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(ii) Calculate the cell potential, E°cell.
(c)
Identify an appropriate substance which could be used in the salt bridge.
In your answer:
•
state why you have chosen this substance
•
discuss the movement of particles in the salt bridge.
(d) In the electrochemical cell opposite, the voltmeter is removed and replaced with a wire.
Discuss the electron movement within the cell, and describe changes that would occur in each
half cell.
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QUESTION TWO
ASSESSOR’S
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When a solution of acidified potassium permanganate is mixed with an iron(II) solution, the colour
of the resulting solution changes from purple to orange.
An electrochemical cell is set up to measure the E°cell value for the reaction. The cell is then
adjusted to enable the change in species to be observed.
(a)
Complete the diagram below to show how the standard electrochemical cell would be set up,
under standard conditions, to enable the change in species to be observed.
The diagram should include:
•
the solutions to be used
•
the type of electrode material
•
an external circuit
•
the direction of the electron flow.
Inert electrode:
Inert electrode:
salt bridge
(b) Write balanced half-equations to show the reactions occurring in each half cell.
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(c)
Justify the half-equations you have written.
Your discussion should include:
•
the oxidation-reduction processes occurring in each half cell
•
the colours associated with each species
•
an identification of the couple with the highest Eo value
•
an explanation for choosing the couple in the bullet point above.
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QUESTION THREE
(a)
ASSESSOR’S
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Complete the table below by giving the oxidation numbers of the species underlined.
Species
Oxidation
Number
SO42–
SO2
I2
(b) Three possible reactions are represented by the unbalanced equations below:
HOCl + I2 → Cl2 + IO3–
HOCl + SO2 → SO42– + Cl2
I2 + SO42– → no reaction
(i)
Write balanced half-equations for the oxidation and reduction reactions between HOCl
and I2, and write a balanced equation for the overall reaction occurring.
HOCl half-equation
I2 half-equation
Balanced equation for the overall reaction
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(ii) Use the reactions on the previous page to determine which E° value from the list below
should be assigned to each couple.
Justify your answer.
E° values:
+1.21 V
Couple
+1.63 V
+0.16 V
E° value / (V)
HOCl / Cl2
SO42– / SO2
IO3– / I2
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90696
QUESTION
NUMBER
Extra paper if required.
Write the question number(s) if applicable.
Chemistry 90696, 2012
ASSESSOR’S
USE ONLY