9.1 Oxidation and Reduction
1. [1 mark]
Applying IUPAC rules, what is the name of MnO2?
A. Magnesium(II) oxide
B. Manganese(II) oxide
C. Magnesium(IV) oxide
D. Manganese(IV) oxide
Markscheme
D
Examiners report
[N/A]
2. [1 mark]
Which is a correct statement for the reaction below?
2MnO4-(aq) + 6H+(aq) + 5NO2-(aq) → 2Mn2+(aq) + 5NO3-(aq) + 3H2O(l)
A. MnO4- is the reducing agent and the oxidation number of Mn increases.
B. MnO4- is the oxidizing agent and the oxidation number of Mn decreases.
C. NO2- is the reducing agent and the oxidation number of N decreases.
D. NO2- is the oxidizing agent and the oxidation number of N increases.
Markscheme
B
Examiners report
[N/A]
3. [1 mark]
Applying IUPAC rules, what is the name of MnO2?
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A. Magnesium(II) oxide
B. Manganese(II) oxide
C. Magnesium(IV) oxide
D. Manganese(IV) oxide
Markscheme
D
Examiners report
[N/A]
4. [1 mark]
Which represents a redox reaction?
A.
B.
C.
D.
Markscheme
A
Examiners report
[N/A]
5. [1 mark]
Which species can oxidize ethanol to ethanoic acid?
A.
B.
Fe
C.
2
D.
Acidified
Markscheme
D
Examiners report
[N/A]
6. [1 mark]
What are the oxidation states of each element in
?
Markscheme
A
Examiners report
[N/A]
7. [1 mark]
What is the coefficient for I– when the following equation is balanced using the smallest possible whole
numbers?
___
A.
1
B.
2
___
___
___
3
C.
3
D.
5
Markscheme
D
Examiners report
We accept that this is a difficult question but many candidates will have come across the reaction in
their laboratory work – and they should know that an equation must be balanced for charge as well as
for species. It was the most difficult question on the paper but there do need to be questions that will
differentiate the grade 7 candidates from those who achieve grade 6.
8. [1 mark]
Which represents a redox reaction?
A.
B.
C.
D.
Markscheme
A
Examiners report
[N/A]
9. [1 mark]
Which is a redox reaction?
A.
B.
C.
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D.
Markscheme
C
Examiners report
[N/A]
10. [1 mark]
Which species of vanadium has a different oxidation number from the rest?
A.
B.
C.
D.
Markscheme
D
Examiners report
[N/A]
11. [1 mark]
Which statement is correct for the following reaction?
A.
is the oxidizing agent and it undergoes reduction.
B.
is the reducing agent and it undergoes oxidation.
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C.
is the oxidizing agent and it undergoes oxidation.
D.
is the reducing agent and it undergoes reduction.
Markscheme
A
Examiners report
[N/A]
12. [1 mark]
At which side of the equation are electrons,
ions and
Markscheme
A
Examiners report
[N/A]
13. [1 mark]
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needed to complete the half-equation?
What are the correct oxidation numbers of chromium in
and manganese in
Markscheme
B
Examiners report
[N/A]
14. [1 mark]
Which species are the oxidizing and reducing agents in the following reaction?
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?
Markscheme
D
Examiners report
[N/A]
15. [1 mark]
Which equation shows oxygen undergoing reduction?
A.
2F2 + O2 → 2F2O
B.
Na2O + H2O → 2NaOH
C.
H2O2 + 2HI → 2H2O + I2
D.
2CrO42− + 2H+
Cr2O72− + H2O
Markscheme
C
Examiners report
[N/A]
16a. [2 marks]
Vanadium has a number of different oxidation states.
Determine the oxidation state of vanadium in each of the following species.
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Markscheme
Do not penalize incorrect notation twice.
[2 marks]
Examiners report
[N/A]
16b. [1 mark]
Electrode potentials for the reactions of vanadium and other species are shown below.
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Identify, from the table, a non-vanadium species that can reduce VO2+(aq) to V3+(aq) but no further.
Markscheme
H2SO3(aq)
OR
Pb(s)
[1 mark]
Examiners report
[N/A]
16c. [1 mark]
Identify, from the table, a non-vanadium species that could convert
Markscheme
Zn(s)
[1 mark]
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to V2+(aq).
Examiners report
[N/A]
16d. [1 mark]
Formulate an equation for the reaction between VO2+(aq) and V2+(aq) in acidic solution to form V3+(aq).
Markscheme
Accept equilibrium sign.
[1 mark]
Examiners report
[N/A]
17a. [5 marks]
Iron rusts in the presence of oxygen and water. Rusting is a redox process involving several steps that
produces hydrated iron(III) oxide,
, as the final product.
The half-equations involved for the first step of rusting are given below.
Half-equation 1:
Half-equation 2:
(i)
Identify whether half-equation 1 represents oxidation or reduction, giving a reason for your
answer.
(ii)
Identify the oxidation number of each atom in the three species in half-equation 2.
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(iii)
Deduce the overall redox equation for the first step of rusting by combining half-equations 1 and
2.
(iv)
Identify the reducing agent in the redox equation in part (iii).
Markscheme
(i)
oxidation and (iron/Fe) loses electrons/increases in oxidation number/state;
(ii)
;
Award [2] for five correct.
Award [1] for four correct.
Accept use of oxidation states (0, +1, –2, –2, +1) for oxidation numbers.
Penalize once for incorrect notation (eg, 2, 2–).
(iii)
;
Ignore state symbols.
(iv)
Fe/iron;
Examiners report
In Part (a) almost all candidates could correctly identify the equation as an oxidation reaction and
justify their choice. Assigning oxidation numbers to particular species proved slightly trickier, with
many not knowing that elements always have an oxidation state of zero.
Combining the half equations also provided a bit of challenge with many equations having residual
electrons, though most students could correctly identify the reducing agent. The aqueous solubility of
oxygen gas in Part (b) was poorly explained, with the discussion being most frequently in terms of
polarity rather than invoking hydrogen bonding. The electrolysis question in Part (c) was generally
well answered, though most relied on “completing the circuit” to obtain the salt bridge mark with few
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showing any comprehension of the way in which this was achieved. Both the property responsible for
the ordering of the periodic table and the relationship of electronic structure to position in the periodic
table, required for Part (d), were well known and it was rare for a student not to gain full marks.
Similarly in Part (e), most students correctly drew the curve that would result from a more effective
catalyst. Many also seemed to be aware of the basic idea of how to find the reaction rate, though correct
use of the terms “tangent” and “gradient” was rare and many failed to note it referred to “initial rate”.
Most students could also identify an appropriate alternative method for monitoring the rate. In the final
section most students could accurately label the axes of a Maxwell-Boltzmann curve and many could
also use it to explain the effect of a catalyst, though some weaker students confused this with the effect
of temperature and constructed a second curve.
17b. [2 marks]
The oxygen in half-equation 2 is atmospheric oxygen that is found dissolved in water in very small
concentrations. Explain, in terms of intermolecular forces, why oxygen is not very soluble in water.
This question is from chapter 4 but it is good practice to do a topic you may not have revised in a while.
Markscheme
oxygen is non-polar;
needs to break strong hydrogen bonds/H–bonds between water molecules (to dissolve) / oxygen
cannot form hydrogen bonds/H–bonds with water;
oxygen can only form (weak) van der Waals’/vdW/LDF/London/dispersion forces with water;
Examiners report
In Part (a) almost all candidates could correctly identify the equation as an oxidation reaction and
justify their choice. Assigning oxidation numbers to particular species proved slightly trickier, with
many not knowing that elements always have an oxidation state of zero.
Combining the half equations also provided a bit of challenge with many equations having residual
electrons, though most students could correctly identify the reducing agent. The aqueous solubility of
oxygen gas in Part (b) was poorly explained, with the discussion being most frequently in terms of
polarity rather than invoking hydrogen bonding. The electrolysis question in Part (c) was generally
well answered, though most relied on “completing the circuit” to obtain the salt bridge mark with few
showing any comprehension of the way in which this was achieved. Both the property responsible for
the ordering of the periodic table and the relationship of electronic structure to position in the periodic
table, required for Part (d), were well known and it was rare for a student not to gain full marks.
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Similarly in Part (e), most students correctly drew the curve that would result from a more effective
catalyst. Many also seemed to be aware of the basic idea of how to find the reaction rate, though correct
use of the terms “tangent” and “gradient” was rare and many failed to note it referred to “initial rate”.
Most students could also identify an appropriate alternative method for monitoring the rate. In the final
section most students could accurately label the axes of a Maxwell-Boltzmann curve and many could
also use it to explain the effect of a catalyst, though some weaker students confused this with the effect
of temperature and constructed a second curve.
18a. [1 mark]
An acidic sample of a waste solution containing Sn2+(aq) reacted completely with K2Cr2O7 solution to
form Sn4+(aq).
State the oxidation half-equation.
Markscheme
Sn2+(aq) → Sn4+(aq) + 2e–
Accept equilibrium sign.
Accept Sn2+(aq) – 2e– → Sn4+(aq).
[1 mark]
Examiners report
[N/A]
18b. [1 mark]
Deduce the overall redox equation for the reaction between acidic Sn2+(aq) and Cr2O72–(aq), using
section 24 of the data booklet.
Markscheme
Cr2O72–(aq) + 14H+(aq) + 3Sn2+(aq) → 2Cr3+(aq) + 7H2O(l) + 3Sn4+(aq)
Accept equilibrium sign.
[1 mark]
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Examiners report
[N/A]
18c. [1 mark]
Calculate the percentage uncertainty for the mass of K2Cr2O7(s) from the given data.
This question is from chapter 11. You will learn more about errors as you do the IA and it may be worth
taking a quick look at chapter 11 to find out how to do this question.
Markscheme
«13.239 g ± 0.002 g so percentage uncertainty» 0.02 «%»
Accept answers given to greater precision, such as 0.0151%.
[1 mark]
Examiners report
[N/A]
18d. [1 mark]
The sample of K2Cr2O7(s) in (i) was dissolved in distilled water to form 0.100 dm3 solution. Calculate its
molar concentration.
Markscheme
« [K2Cr2O7] =
=» 0.450 «mol dm–3»
[1 mark]
Examiners report
[N/A]
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18e. [2 marks]
10.0 cm3 of the waste sample required 13.24 cm3 of the K2Cr2O7 solution. Calculate the molar
concentration of Sn2+(aq) in the waste sample.
Markscheme
n(Sn2+) = «0.450 mol dm–3 x 0.01324 dm3 x
«[Sn2+] =
=» 0.0179 «mol»
=» 1.79 «mol dm–3»
Award [2] for correct final answer.
[2 marks]
Examiners report
[N/A]
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