Oxidation State Exam-style Questions | GCSE Chemistry CIE Extended Triple

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Question 1

Zinc is extracted from zinc blende by roasting it in air to form zinc oxide.

The zinc oxide is then heated with carbon to form zinc.

The equations for the reactions are shown.

2ZnS + 3O₂ ➔ 2ZnO + 2SO₂

ZnO + C ➔ Zn + CO

Which statement about the reactions above is not correct?

in reaction 1 the oxidation state of sulfur increases and it is oxidised

in reaction 1 the oxidation state of zinc increases and it is oxidised

in reaction 2 the carbon acts as a reducing agent and it is oxidised

in reaction 2 the oxidation state of zinc decreases and it is reduced

(0/1 marks)

Define the term 'oxidising agent'.

(0/1 marks)

Give the oxidation number of carbon in C.

(0/1 marks)

The oxidation number of oxygen in CO is -2.

Give the oxidation number of carbon in CO.

(0/1 marks)

Identify the oxidising agent in reaction 2.

Give a reason for your answer.

(0/2 marks)

Question 2

When chlorine gas dissolves in water a reaction occurs.

Cl₂ + H₂O ➔ HCl + HClO

Which of the following options correctly identifies the oxidation number for chlorine in the chlorine-containing species?

-1 in Cl₂, -1 in HCl and -1 in HClO

0 in Cl₂, -1 in HCl and -1 in HClO

-1 in Cl₂, +1 in HCl and +1 in HClO

0 in Cl₂, -1 in HCl and +1 in HClO

(0/1 marks)

Give the ionic half-equation for the reduction of chlorine, Cl₂, to chloride ions, Cl^-.

(0/1 marks)

Define oxidation and reduction in terms of:

loss or gain of oxygen
loss or gain of electrons

(0/2 marks)

Question 3

Titanium can be extracted from its ore in a three-stage process.

The second stage of the extraction of titanium converts titanium dioxide into titanium chloride.

The equation for this chemical reaction is:

TiO₂ + 2Cl₂ + C ➔ TiCl₄ + CO₂

Explain which element has been oxidised in this reaction.

Give the change in oxidation number.

(0/3 marks)

In stage 3, the titanium chloride produced in stage 2 reacts with magnesium in an atmosphere of argon.

The products of this reaction are magnesium chloride and titanium.

b) i)

Write a balanced symbol equation for the reaction between titanium chloride and magnesium.

(0/2 marks)

b) ii)

Suggest why this reaction cannot be successfully carried out in an atmosphere of air.

(0/1 marks)

Question 4

Blue copper sulfate and zinc react to form zinc sulfate and copper in a displacement reaction.

The equation for the reaction between copper sulfate and zinc is shown below.

CuSO₄ + Zn ➔ ZnSO₄ + Cu

Displacement reactions are redox reactions.

Describe oxidation and reduction in terms of electrons.

(0/2 marks)

The ionic equation for the reaction between zinc and aqueous copper ions is shown below.

Zn + Cu²⁺ ➔ Zn²⁺ + Cu

Which statement about this reaction is correct?

copper ions are oxidised and their oxidation number decreases from +2 to 0

copper ions are reduced because they lose electrons

zinc atoms are oxidised and their oxidation number increases from 0 to +2

zinc atoms are reduced because they gain electrons

(0/1 marks)

For the reaction above, write two ionic half-equations.

State which one is reduction and which one is oxidation.

(0/2 marks)

Name the oxidising agent and the reducing agent in the reaction above.

(0/2 marks)

Explain why there is a colour change in the solution as the reaction occurs.

(0/2 marks)

Question 5

The ionic half-equations for the reactions at the two electrodes in the electrolysis of copper(II) sulfate using copper electrodes are shown below.

Anode:

Cu(s) ➔ Cu²⁺(aq) + 2e^–

Cathode:

Cu²⁺(aq) + 2e^– ➔ Cu(s)

Which species is reduced during the electrolysis?

Explain your answer.

(0/2 marks)

Give the oxidation number of solid copper.

(0/1 marks)

Give the oxidation number of copper ions in solution.

(0/1 marks)

During electrolysis, electricity passes through the copper(II) sulfate solution.

Solid copper(II) sulfate does not conduct electricity.

Explain why.

(0/2 marks)

Question 6

When magnesium is added to aqueous copper(II) sulfate a redox reaction occurs.

The ionic equation for the reaction is shown below.

Mg + Cu²⁺ ➔ Mg²⁺ + Cu

Explain why this is a redox reaction.

(0/1 marks)

Give one change you would observe during this reaction.

(0/1 marks)

Identify the oxidising agent in this reaction.

Justify your answer.

(0/2 marks)

Question 7

Give the oxidation numbers of the named elements in the following substances.

H in H₂.

(0/1 marks)

H in NH₃.

The oxidation number of N is -3.

(0/1 marks)

Ca in Ca(OH)₂.

The oxidation number of the hydroxide ion, OH^-, is -1.

(0/1 marks)

Ar.

(0/1 marks)

Cl in NaCl.

-1

(0/1 marks)

C in CO₂.

The oxidation number of O is -2.

(0/1 marks)

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