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IGCSEChemistryCambridge (CIE)Revision Notes6. Chemical ReactionsRedoxRedox & Electron Transfer

Redox & Electron Transfer (Cambridge (CIE) IGCSE Chemistry)

Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

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Redox & electron transfer

Extended tier only

  • Redox reactions can also be defined in terms of electron transfer

  • Oxidation is a reaction in which an element, ion or compound loses electrons

    • The oxidation number of the element is increased

    • This can be shown in a half-equation, e.g. when silver reacts with chlorine, silver is oxidised to silver ions:

Ag → Ag+ + e-

  • Reduction is a reaction in which an element, ion or compound gains electrons

    • The oxidation number of the element is decreased

    • This can be shown in a half-equation, e.g. when oxygen reacts with magnesium, oxygen is reduced to oxide ions:

O2 + 4e- → 2O2-

  • For example, when iron reacts with a compound of copper such as copper sulfate, a displacement reaction occurs

iron + copper sulfate → iron(II) sulfate + copper 

Fe + CuSO4 → FeSO4 + Cu

Fe + Cu2+ + SO42– → Fe2+ + SO42– + Cu

  • We can then remove the spectator ions to see the overall change

Fe + Cu2+→ Fe2+ + Cu

  • The iron atom has lost electrons to become a positive ion, so has been oxidised

  • The positive copper ion has gained electrons to become an atom, so have been reduced

The redox reaction between Fe and Cu2+

The redox reaction of iron and copper(II)

The Fe atom is oxidised (loses electrons) and the Cu2+ ion is reduced (gains electrons)

Worked Example

Which change in the following equation is oxidation?

V3+ + Fe3+  → V4+ + Fe2+

Answer:

  • Step 1 - Identify the changes for each species:

    • V3+ to V4+ 

      • V3+ has lost 1 electron 

    • Fe3+ to Fe2+

      • Fe3+ has gained 1 electron

  • Step 2 - Identify each change as either oxidation and reduction

    • V3+ to V4+ is oxidation

    • Fe3+ to Fe2+ is reduction

  • Therefore, V3+ has been oxidised

Examiner Tips and Tricks

Use the mnemonic OIL-RIG to remember oxidation and reduction in terms of the movement of electrons:

  • Oxidation Is Loss 

  • Reduction Is Gain.

Identifying redox reactions

Extended tier only

Identifying redox reactions using oxidation numbers

  • The oxidation number is a number assigned to an atom or ion in a compound 

  • It shows the number of electrons that an atom has lost, gained or shared in forming a compound

    • So, the oxidation number helps you to keep track of the movement of electrons in a redox process

  • It is written as a +/- sign followed by a number

    • Positive oxidation number = loss of electrons

    • Negative oxidation number = gain of electrons

    • For example, aluminium in a compound usually has the oxidation number of +3 indicating it has lost 3 electrons

    • Careful: It is easy to confuse oxidation number with charge which is written by a number followed by a +/- sign)

  • A few simple rules help guide you through the process of determining the oxidation number of any element

Rules for assigning oxidation numbers

 

Rule

Example

1

The oxidation number of any uncombined element is zero

H2 
Zn
O2 

2

Many atoms or ions have fixed oxidation numbers in compounds

Group 1 elements are always +1
Group 2 elements are always +2
Fluorine is always –1
Hydrogen is +1, except in hydrides like NaH where it is –1
Oxygen is –2, except in peroxides where it is in –1 and in F2O where it is +2

3

The oxidation number of an element in a monoatomic ion is always the same as the charge

Zn2+ = +2
Fe3+ = +3
Cl– = –1

4

The sum of the oxidation numbers in a compound is zero

NaCl
Na = +1
Cl = –1
Sum of oxidation numbers = 1 – 1 = 0

5

 The sum of the oxidation numbers in an ion is equal to the charge on the ion

SO42–
S = +6
Four O atoms = 4 x (–2) = –8
Sum of oxidation numbers = 6 – 8 = –2

6

In either a compound or an ion, the more electronegative element is given the negative oxidation number

F2O
Two F atoms = 2 x (–1) = –2
O = +2

  • Redox reactions can be identified by the changes in the oxidation number when a reactant goes to a product

Worked Example

The equation for the reaction between chlorine and potassium iodide is shown below. 

Cl2 + 2KI → 2KCl + I2

Identify which species has been:

  1. Oxidised

  2. Reduced 

Answer: 

  1. The species that has been oxidised is iodine 

    • 2I- → I2 +2e-

    • The oxidation number of I- is -1

    • The oxidation number of iodine in I2 is 0

    • The oxidation number has increased so the iodide ions have been oxidised / lost electrons

  2. The species that has been reduced is chloride ions

    • Cl2 + 2e- → 2Cl- 

    • The oxidation number of chlorine as Cl2 is 0

    • The oxidation number of Cl- is -1

    • The oxidation number has decreased so the Cl2 has been reduced / gained electrons

Identifying redox reactions by colour changes

  • The tests for redox reactions involve the observation of a colour change in the solution being analysed

  • Two common examples are acidified potassium manganate(VII), and potassium iodide

  • Potassium manganate(VII), KMnO4, is an oxidising agent which is often used to test for the presence of reducing agents

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

Testing for reducing agents with acidifed potassium manganate(VII)

Diagram to show the colour change when potassium manganate(VII) is added to a reducing agent

  • Potassium iodide, KI, is a reducing agent which is often used to test for the presence of oxidising agents

  • When added to an acidified solution of an oxidising agent such as aqueous chlorine or hydrogen peroxide (H2O2), the solution turns a red-brown colour due to the formation of iodine, I2

Testing for oxidising agents with potassium iodide

Diagram to show the colour change when potassium iodide is added to an oxidising agent

  • The potassium iodide is oxidised as it loses electrons

  • The hydrogen peroxide is reduced

  • Therefore, potassium iodide is acting as a reducing agent 

Oxidising & reducing agents

Extended tier only

What is an an oxidising agent?

  • An oxidising agent is a substance that oxidises another substance, and becomes reduced in the process

  • An oxidising agent gains electrons as another substance loses electrons

  • Common examples include hydrogen peroxide, fluorine and chlorine

What is a reducing agent?

  • A reducing agent is a substance that reduces another substance, and becomes oxidised in the process

  • A reducing agent loses electrons as another substance gains electrons

  • Common examples include carbon and hydrogen

  • The process of reduction is very important in the chemical industry as a means of extracting metals from their ores 

Identifying oxidising and reducing agents

CuO + H2 → Cu + H2O

  • Hydrogen is reducing the CuO

  • Hydrogen is itself oxidised as it has gained oxygen / lost electrons

  • So, the reducing agent is hydrogen:

H2 → 2H+ + 2e-

  • CuO is reduced by hydrogen

    • This means that the hydrogen is oxidised by CuO

  • CuO is reduced as it has lost oxygen / gained electrons

  • So, the oxidising agent is copper oxide

Cu2+ +2e- →  Cu

Worked Example

When iron reacts with bromine to form iron(II) bromide, a redox reaction reaction occurs:

Fe + Br2 →  FeBr2

Which species is acting as the reducing agent in this reaction?

Answer

  1. Step 1 - Write half equations to work out what has gained/lost electrons

    • Fe → Fe2+ + 2e-

    • Br2 + 2e- → 2Br-

    • Fe loses electrons; Br2 gains electrons

  2. Step 2 - Deduce what has been oxidised/reduced (remember OIL RIG)

    • Fe has been oxidised as it has lost electrons

    • Br2 has been reduced as it has gained electrons

  3. Step 3 - Identify the reducing agent

    • Fe is the reducing agent as it has been oxidised by losing electrons and caused Br2 to be reduced as it gained electrons

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    Author
    Reviewer
    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.