Ch 4 · Redox · O Level Chemistry 5070
Topic 4 of 4 in Chemical Reactions — O Level Chemistry 5070 — 48 lessons.
Introduction
Rusting, burning, extracting iron in a blast furnace and every electrolysis reaction are all examples of redox. The idea begins simply, as gaining or losing oxygen, and then widens to the transfer of electrons and to oxidation numbers. Learn the definitions exactly and practise deciding which substance is the agent and which is the substance being changed.
Key ideas — Oxidation and reduction as gain and loss of oxygen
Oxidation is gain of oxygen. Reduction is loss of oxygen. A redox reaction is one in which oxidation and reduction happen together: one substance gains the oxygen that another loses.
🧑🏫 Teacher's explanation
Think of a chemical reaction like a game of hot potato. In some reactions, oxygen atoms are the 'potatoes' being passed around. When a substance receives an oxygen atom, it has been 'oxidized'. When a substance gives its oxygen atom away to someone else, it has been 'reduced'.
📖 Definitions
🔍 Worked example — Easy
In the reaction 2Mg(s) + O_2(g) arrow 2MgO(s), identify which element is oxidised and which is reduced.
🔍 Worked example — Standard
Consider the reaction: CuO(s) + H_2(g) arrow Cu(s) + H_2O(g). Identify the oxidising agent.
🔍 Worked example — Exam-style
In the reaction Fe_2O_3(s) + 3CO(g) arrow 2Fe(s) + 3CO_2(g), identify the reducing agent and explain why.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
$2Mg(s) + O_2(g) arrow 2MgO(s) \n(a) Identify the substance that is oxidised. [1]\n(b) Identify the substance that is reduced. [1]\n(c) Identify the oxidising agent. [1] [3 marks]
Key ideas — Electron transfer and oxidation numbers
electrons. A wider definition covers reactions with no oxygen at all. Oxidation Is Loss of electrons, Reduction Is Gain (OIL RIG). When zinc displaces copper:
🧑🏫 Teacher's explanation
Think of electrons like currency in a marketplace. In a redox reaction, atoms are constantly trading these 'electron coins'. Oxidation is when an atom 'spends' or loses its electrons, making it more positive. Reduction is when an atom 'receives' or gains electrons, making it more negative.
📖 Definitions
🔍 Worked example — Easy
In the reaction Mg(s) + Cl_2(g) arrow MgCl_2(s), identify which species is oxidised and write the half-equation for its oxidation.
🔍 Worked example — Standard
Calculate the oxidation number of Sulfur (S) in the sulfate ion, SO_4^2-.
🔍 Worked example — Exam-style
In the reaction MnO_4^-(aq) + 8H^+(aq) + 5e^- arrow Mn^2+(aq) + 4H_2O(l), identify the oxidising agent and explain why.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
CuO(s) + H_2(g) arrow Cu(s) + H_2O(g) [3 marks]
Key ideas — Oxidising and reducing agents
definitions. An oxidising agent is a substance that oxidises another substance and is itself reduced. A reducing agent is a substance that reduces another substance and is itself oxidised.
🧑🏫 Teacher's explanation
Think of a redox reaction as a game of 'pass the ball'. In this game, the 'ball' is an electron. If a substance gives its electron away, it is being oxidized. If a substance takes an electron, it is being reduced. This might seem counter-intuitive, but remember: losing a negative charge makes your overall charge more positive (oxidation), while gaining a negative charge makes your charge more negative (reduction).
📖 Definitions
🔍 Worked example — Easy
In the reaction CuO + H_2 arrow Cu + H_2O, identify the oxidising agent.
🔍 Worked example — Standard
In the reaction Cl_2 + 2KBr arrow 2KCl + Br_2, identify the reducing agent.
🔍 Worked example — Exam-style
In the reaction MnO_4^- + 8H^+ + 5e^- arrow Mn^2+ + 4H_2O, identify the oxidising agent.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
CuO + H_2 arrow Cu + H_2O (a) Identify the substance that is oxidised. [1] (b) Identify the oxidising agent. [1] (c) In the reaction C + O_2 arrow CO_2, determine the change in oxidation state for Carbon. [1] [3 marks]
Key ideas — Tests for oxidising and reducing agents
two colour tests. Acidified aqueous potassium manganate(VII) is a powerful oxidising agent. A reducing agent turns it from purple to colourless, because the manganate(VII) ion is reduced to the almost colourless manganese(II) ion: MnO_4^- + 8H^+ + 5e^- arrow mathrmMn^2+ + 4H_2O.
🧑🏫 Teacher's explanation
Think of redox reactions like a game of hot potato involving electrons or oxygen. In this game, one substance is 'giving away' something (oxidation) and another is 'taking' it (reduction). Because these processes always happen together, we call them redox reactions.
📖 Definitions
🔍 Worked example — Easy
Identify the oxidising agent in the following reaction: MnO_4^-(aq) + 8H^+(aq) + 5e^- arrow Mn^2+(aq) + 4\ extH_2 extO(l)
🔍 Worked example — Standard
A student adds acidified potassium manganate(VII) to a solution. The purple color disappears, leaving a colorless solution. What has been added to the solution?
🔍 Worked example — Exam-style
In the reaction 2I^-(aq) + Cl_2(aq) arrow I_2(s) + 2Cl^-(aq), identify the reducing agent and the colour change observed if I^- is the analyte.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) State what this observation tells the student about the unknown solution. [1] (b) Identify whether the unknown solution contains an oxidising agent or a reducing agent. [1] [2 marks]
Worked example
Question. Chlorine is bubbled through aqueous iron(II) chloride and the solution changes from pale green to yellow-brown.
🔍 Worked example — Easy
In the reaction CuO + H_2 arrow Cu + H_2O, identify which substance is oxidized and which is reduced using the oxygen definition.
🔍 Worked example — Standard
Consider the reaction: Zn + Cu^2+(aq) arrow Zn^2+(aq) + Cu(s). Determine the change in oxidation number for Zinc and state if it is oxidized or reduced.
🔍 Worked example — Exam-style
Chlorine gas reacts with aqueous iron(II) chloride: 2FeCl_2(aq) + Cl_2(g) arrow 2FeCl_3(aq) + 2Cl^-(aq). (a) Identify the oxidising agent. (b) Write the half-equation for the reduction of chlorine.
⚠️ Common errors
Common mistakes
Practice
Answers Oxidation is gain of oxygen [B1]; reduction is loss of oxygen [B1]; carbon is oxidised (it gains oxygen to form CO) [B1]. (a) +4 [B1]; (b) +6 [B1]; (c) +5 [B1]. Magnesium is the reducing agent [B1]; it loses electrons (Mg arrow mathrmMg^2+ + 2e^-), reducing the hydrogen ions to hydrogen [B1]. Add acidified aqueous potassium manganate(VII) [B1]; it turns from purple to colourless [B1]. The solution contains an oxidising agent [B1]; the iodide ions have been oxidised (lost electrons) to form iodine [B1].