5 - Oxidation and Reduction

Practice exam questions following revision of these cards as there are calculations etc to do. These cards are just definitions and short questions.

Define reduction in terms of (i) electron transfer and (ii) change in oxidation number

i) gain of (increase in) electrons

ii) reduction (decrease) in oxidation number

Define oxidation in terms of (i) electron transfer, (ii) change in oxidation number.

i) Loss of electrons

ii) Increase in oxidation number

Reducing agent

Brings about reduction, it itself is oxidised

Oxidising agent

brings about oxidation, it itself is reduced

Oxidation number

Charge that an atom has or appears to have when electrons are distributed according to certain rules

Use oxidation numbers to identify the reducing agent in the following reactions.

Equation 1: SO₃^2- is Reducing agent // S⁺⁴ to S⁺⁶

Equation 2: Cl^- is Reducing agent // Cl^-1 to Cl₂⁰

Hence, or otherwise, balance both the equations.

State the oxidation number of phosphorus in P₄ and PO₄^3- .

0, +5

State the oxidation number of nitrogen in NO₃^- and NO.

+5, +2

Hence or otherwise, balance the chemical equation

Identify the reducing agent in the reaction above

P₄

What are the oxidation numbers of iron in FeO, Fe₃O₄ and Fe₂O₃ respectively?

FeO : 2(+2, II) //

Fe₃O₄ : 8/3 (+8/3) //

Fe₂O_{3 : 3 (+3, III)}

The oxidation number of hydrogen in most of its compounds is +1. Explain why the oxidation number of hydrogen in the metal hydrides is -1

Hydrogen (H) more electronegative / metals less electronegative / metals more electropositive / more electronegative atom assigned gets negative charge

The oxidation number of oxygen in most of its compounds is -2. What is the oxidation number of oxygen in OF₂?

2

The following unbalanced equation describes a reaction that occurs in aqueous solution. Assign oxidation numbers and, hence or otherwise, balance this equation.

Identify the reducing agent in this reaction.

Mn²⁺ / Mn(II)

What is the oxidation number of sulphur in (i) SO₂ (ii) H₂SO₄ (iii)Na₂S₂O₃?

i) +4 / 4 / IV //

ii) +6 / 6 / VI

iii) +2

Write the oxidation number for

(i) oxygen in OF₂

(i) xenon in XeF₄

i) 2 / +2 //

ii) 4 / +4

i) Explain with the aid of a clearly-labelled diagram how impure copper is purified by electrolysis.

Copper sulphate electrolyte / battery (power supply) //

impure copper oxidised / impure copper attaches to anode //

pure copper attaches to cathode //

copper ions in solution reduced / pure copper plated out on cathode

ii) Write balanced equations for the reactions that take place at the electrodes

At anode: Cu_(s) - 2e^- → Cu²⁺ _(aq)

At cathode: Cu_(aq) + 2e^- → Cu_(s)

iii) What happens to impurities in the copper during this electrolysis?

Falls to bottom of container / found under anode

Electrolysis: Acidified H₂O

Give the

i) oxidation reaction (at the anode)

ii) reduction reaction (at the cathode)

i) H₂O - 2e^- → 2H⁺ + ^½ O₂

ii) 2H⁺ +2e^- → H₂

Electrolysis: Acidified H₂O

What is added to improve the conductivity of water?

Dilute H₂SO₄

Electrolysis: Sodium Sulphate

Give the

i) oxidation reaction (at the anode)

ii) reduction reaction (at the cathode)

i) 2H₂O - 4e^- → O₂ + 4H⁺

ii) 2H₂O + 2e^- → H₂ + 2OH^-

Electrolysis: Sodium Sulphate

Name a suitable material for the electrodes

Platinum

Electrolysis: Potassium Iodide

Give the

i) oxidation reaction (at the anode)

ii) reduction reaction (at the cathode)

i) 2I^- - 2e^- → I₂

ii) 2H₂O + 2e^- → H₂ + 2OH^-

Electrolysis: Potassium Iodide

Name a suitable material for the electrodes

Platinum

Electrolysis: Potassium Iodide

Explain the colour change observed at the anode

Colour of the iodine produced

Electrolysis: Potassium Iodide

State and explain the colour observed at the negative electrode in the electrolysis of aqueous potassium iodide, containing a little phenolphthalein indicator, using inert electrodes.

pink (purple) - hydroxyl ions produced / pH increases / alkaline

Complete and balance the equation for the chemical reaction that occurs when a piece of aluminium is placed in a solution of copper (II) ions: Cu²⁺ + Al →

Cu²⁺ + Al → Cu + Al³⁺

{Balance}

3Cu²⁺ + 2Al → 3Cu + 2Al³⁺