Redox & Electron Transfer (CIE IGCSE Chemistry)
Revision Note
Author
AlexandraExpertise
Chemistry
Redox & Electron Transfer
EXTENDED
Oxidation & Reduction
- 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-
Example: Identifying Redox Reactions
zinc + copper sulphate → zinc sulphate + copper
Zn + CuSO4 → ZnSO4 + Cu
- The ions present (with state symbols) in the equation are:
Zn(s) + Cu2+(aq) + SO42-(aq) →Zn2+(aq) + SO42-(aq) + Cu(s)
- The spectator ions (those that do not change) are SO42-(aq)
- These can be removed and the ionic equation written as:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
- By analysing the ionic equation, we can split the reaction into two half equations by adding in the electrons to show how the changes in charge have occurred:
Zn(s) → Zn2+(aq) + 2e-
Cu2+(aq) +2e- → Cu(s)
- It then becomes clear that zinc has been oxidised as it has lost electrons
- Copper ions have been reduced as they have gained electrons
Exam Tip
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
Oxidation Number
- The oxidation number (also called oxidation state) is a number assigned to an atom or ion in a compound which indicates the degree of oxidation (or reduction)
- It shows the number of electrons that an atom has lost, gained or shared in forming a compound
- 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 (not to be confused with charge which is written by a number followed by a +/- sign)
- E.g. aluminium in a compound usually has the oxidation state +3
- A few simple rules help guide you through the process of determining the oxidation number of any element
Table of Rules for Assigning Oxidation Numbers
- 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:
a) Oxidised
b) Reduced
Answer:
-
- The species that has been oxidised is iodine
- The oxidation number of I- is -1
- The oxidation number of iodine in I2 is 0
- The oxidation number has increased so the iodine has been oxidised (lost electrons)
- 2I-(aq) → I2(s) +2e-
- The species that has been reduced is chloride ions
- The oxidation number of chlorine as Cl2 is 0.
- The oxidation number of Cl- is -1
- The oxidation number has decreased so the Cl- has been reduced (gained electrons)
- Cl2(g) + 2e- → 2Cl-(aq)
- The species that has been oxidised is iodine
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
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:
2KI (aq) + H2SO4 (aq) + H2O2 (aq) → I2 (aq) + K2SO4 (aq) + 2H20 (l)
- The potassium iodide is oxidised as it loses electrons and hydrogen peroxide is reduced, therefore potassium iodide is acting as a reducing agent as it will itself be oxidised:
2I- → I2 + 2e-
Diagram to show the colour change when potassium iodide is added to an oxidising agent
Oxidising & Reducing Agents
EXTENDED
Oxidising agent
- 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
Reducing agent
- 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
Example
CuO + H2 → Cu + H2O
- In the above reaction, hydrogen is reducing the CuO and is itself oxidised as it has lost electrons, so the reducing agent is therefore hydrogen:
H2 → 2H+ + 2e-
- The CuO is reduced to Cu by gaining electrons and has oxidised the hydrogen, so the oxidising agent is therefore 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
What is acting as the reducing agent in this reaction?
Answer
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
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
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
You've read 0 of your 0 free revision notes
Get unlimited access
to absolutely everything:
- Downloadable PDFs
- Unlimited Revision Notes
- Topic Questions
- Past Papers
- Model Answers
- Videos (Maths and Science)
Did this page help you?