Redox TC

A series of reactions are shown. What is the oxide that undergoes oxidation?

1. 2 CO + 2 NO → 2 CO₂ + N₂

2. Fe₂O₃ + 2 Al → Al₂O₃ + Fe

3. 2 ZnO + C →2 Zn + CO₂

4. Fe₂O₃ + 3 CO → 2 Fe + 3 CO₂

• CO

• C

• NO

• Fe2O3

Answer explanation

The correct answer is A because oxygen has been added to CO in reactions 1 and 4.

In which reaction is the reactant being reduced?

• Mg → Mg ²⁺ + 2 e^-

• Mg ²⁺ + 2 e^- → Mg

• CrO → Cr₂O₃

• C → CO₂

Answer explanation

The correct answer is B because:

• Reduction is the gain of electrons.

• Mg²⁺ gains 2 electrons and its oxidation number changes from +2 to 0, hence it has been reduced.

Vanadium is a transition element and forms several oxides. In which of the following does vanadium undergo oxidation?

• V₂O₃ → VO

• V₂O₅ → VO₂

• V₂O₃ → V₂O₅

• VO₂ → V₂O₃

Answer explanation

The correct answer is C because the oxidation state of V in V₂O₃ is +3 and in V₂O₅ it is +5, hence its oxidation number has increased, so oxidation has occurred.

Manganese is a transition element and forms several oxides. In which of the following changes does manganese undergo reduction?

• Mn₂O₃ → Mn₂O₇

• MnO → MnO₃

• Mn₂O₂ → MnO

• MnO → Mn

Answer explanation

The correct answer is D because:

• Oxygen has been removed from MnO, hence it has been reduced.

• Its oxidation number has decreased from +2 to 0.

Potassium manganate (VII) is used to test the presence of reducing agents. Which row correctly describes what happens when acidified potassium manganate (VII) is added to a solution containing a reducing agent?

• A

• B

• C

• D

Answer explanation

• When acidified potassium manganate (VII) is added to a reducing agent its colour changes from pink-purple to colourless.

• The MnO₄^- ions undergo reduction by electron gain to form Mn²⁺ ions which are colourless.

Which option correctly describes an oxidising agent and a reducing agent?

• An oxidising agent adds oxygen to a substance an a reducing agent removes oxygen from a substance

• An oxidising agent adds oxygen to a substance and a reducing agent increases the oxidation number of a substance

• An oxidising agent removes oxygen to a substance and a reducing agent adds oxygen to a substance

• An oxidising agent decreases the oxidation number of a substance and a reducing agent increases the oxidation number of a substance

Answer explanation

• An oxidising agent is an element or a compound that oxidises another substance.

• In so doing the oxidising agent is itself reduced.

• A reducing agent is an element or a compound that reduces another substance.

• In so doing the reducing agent is itself oxidised.

Which one of the following substances is commonly used as a reducing agent?

• H₂SO₄

• KMnO₄

• H₂O₂

• CO

Answer explanation

• Carbon monoxide is an important reducing agent which is used in the extraction of iron from iron ore.

• It removes the oxygen from the iron ore:

Fe₂O₃ + 3CO → 2Fe + 3CO₂

• In the reaction the carbon monoxide is itself oxidised:

3CO → 3CO₂

In which two of the following redox reactions is the substance in bold acting as a reducing agent?

CuO + H₂ → Cu + H₂O

2 Ca + O₂ → 2 CaO

CO + CuO → Cu + CO₂

CO₂ + C → 2 CO

• 1 and 2 only

• 1 and 3 only

• 2 and 3 only

• All of them

Answer explanation

The correct answer is C because:

• Carbon monoxide and calcium removes the oxygen from the other reactant, therefore acting as a reducing agent.

• The CO and Ca molecule is then itself oxidised as it gains an oxygen atom.

Hydrogen and copper (II) oxide react to produce copper metal and water

CuO + H₂ → Cu + H₂O

• Oxidising agent H₂O

  Reducing agent Cu

• Oxidising agent H₂

  Reducing agent CuO

• Oxidising agent Cu

  Reducing agent H₂O

• Oxidising agent CuO

  Reducing agent H₂

Answer explanation

The correct answer is D because:

• In the reaction the hydrogen removes the oxygen atom from the copper oxide molecule, hence the hydrogen is the reducing agent.

• In doing so, the hydrogen has itself now become oxidised by the copper oxide which is therefore the oxidising agent.

• C is incorrect as water is not an oxidising agent.