Chapter 2.2: Redox, Rusting and Iron
Welcome to your study guide for Redox, Rusting and Iron! This chapter is a core part of Chemistry Unit C2 in your CCEA Double Award Science course. Chemical reactions involving the transfer of oxygen, hydrogen, and electrons are happening all around us—from the corrosion of a steel bridge to the massive industrial blast furnaces used to extract iron from rock. Let's break these ideas down into simple, step-by-step concepts so you can master them with confidence!
---1. Understanding Redox Reactions
The word redox is simply a combination of two words: REDuction and OXidation. In chemical reactions, these two processes always take place at the same time.
A. Redox in Terms of Oxygen and Hydrogen
At the most fundamental level, oxidation and reduction describe what happens to oxygen or hydrogen during a chemical reaction:
• Oxidation is the gain of oxygen OR the loss of hydrogen.
• Reduction is the loss of oxygen OR the gain of hydrogen.
• A redox reaction is a reaction where oxidation and reduction happen simultaneously.
Analogy: Think of oxygen like a ball in a game of catch. One player loses the ball (reduction) while the other catches it (oxidation). You cannot have a throw without a catch!
B. Redox in Terms of Electrons
In many reactions, especially displacement reactions and reactions involving ions, oxygen might not even be present. Here, we define redox in terms of electrons (\(\text{e}^-\)).
• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.
Memory Trick: Always remember the classic mnemonic OIL RIG:
Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
C. Oxidising Agents and Reducing Agents
These terms can sometimes trip students up, but here is an easy way to think about them:
• An oxidising agent oxidises something else by taking electrons away from it (or giving it oxygen). In doing so, the oxidising agent is itself reduced.
• A reducing agent reduces something else by donating electrons to it (or removing oxygen from it). In doing so, the reducing agent is itself oxidised.
D. Worked Example: Displacement Reaction
Let's look at what happens when iron metal is added to copper(II) sulfate solution:
Full equation: \(\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}\)
Ionic equation: \(\text{Fe(s)} + \text{Cu}^{2+}\text{(aq)} \rightarrow \text{Fe}^{2+}\text{(aq)} + \text{Cu(s)}\)
Let's split this into two electron half-equations:
1. \(\text{Fe} \rightarrow \text{Fe}^{2+} + 2\text{e}^-\)
Iron loses 2 electrons. Loss of electrons is oxidation (OIL). Iron is oxidised and acts as the reducing agent.
2. \(\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}\)
Copper ions gain 2 electrons. Gain of electrons is reduction (RIG). Copper(II) ions are reduced and act as the oxidising agent.
Key Takeaway: Oxidation is the gain of oxygen, loss of hydrogen, or loss of electrons. Reduction is the loss of oxygen, gain of hydrogen, or gain of electrons (OIL RIG).
---2. The Rusting of Iron
Rusting is a specific type of corrosion that destroys iron structures, costing billions of pounds worldwide each year.
A. What is Rust?
• Rust is chemically known as hydrated iron(III) oxide.
• Its chemical formula is represented as \(\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}\) (where \(x\) represents a variable number of water molecules).
• Important Examiner Distinction: The word rusting applies only to iron and alloys containing iron (such as steel). All other metals corrode, but they do not "rust".
B. Conditions Required for Rusting
For iron to rust, both oxygen (from air) and water must be present at the same time. If either is missing, iron will not rust.
C. Investigating the Conditions (The Classic 4-Tube Experiment)
In the laboratory, this is investigated using four sealed test tubes containing clean iron nails:
Tube 1 (Water + Air):
• Setup: Nail in unboiled tap water open to the air.
• Result: Rust forms. Both oxygen and water are present.
Tube 2 (Water Only — No Oxygen):
• Setup: Nail in boiled distilled water covered with a layer of oil.
• Why this works: Boiling water expels all dissolved oxygen/air. The layer of oil floating on top acts as a physical barrier preventing oxygen from re-entering.
• Result: No rust forms.
Tube 3 (Oxygen/Air Only — No Water):
• Setup: Nail in a dry tube containing granules of anhydrous calcium chloride (\(\text{CaCl}_2\)) with a rubber stopper.
• Why this works: Anhydrous calcium chloride absorbs moisture and water vapour from the air inside the tube.
• Result: No rust forms.
Tube 4 (Accelerated Rusting):
• Setup: Nail placed in a salt solution (e.g., sodium chloride in water) exposed to air.
• Result: Rust forms much faster. Electrolytes such as dissolved salts speed up the rate of the rusting reaction.
Key Takeaway: Rust is hydrated iron(III) oxide (\(\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}\)). Both water and oxygen are essential for iron to rust, and dissolved salts accelerate the process.
---3. Methods of Rust Prevention
Because rusting weakens iron and steel structures, we use different methods to prevent it depending on what the object is used for.
A. Barrier Methods
Barrier methods work by placing a physical coating over the surface of the iron to exclude both water and oxygen.
• Painting: Ideal for large, fixed structures like bridges, gates, and car body panels.
• Oiling and Greasing: Ideal for moving mechanical parts where paint would rub off, such as bicycle chains and engine components.
• Plastic Coating: Used for items like wire mesh fencing, garden furniture, and dish drainers.
• Electroplating / Tin Plating: Coating steel with a thin, unreactive metal layer (e.g., steel food cans coated with a layer of tin).
Limitation of Barrier Methods: If the barrier is scratched or chipped, water and oxygen reach the iron beneath, and it will begin to rust.
B. Sacrificial Protection
In sacrificial protection, blocks of a more reactive metal (such as zinc or magnesium) are attached directly to the iron or steel structure.
• How it works: Because the attached metal is more reactive than iron, it oxidises and corrodes preferentially. It loses electrons in place of the iron, protecting the iron from oxidation.
• Applications: Ships' hulls, underground steel oil pipelines, and offshore oil rigs.
• Maintenance: The sacrificial blocks slowly corrode away and must be replaced periodically.
C. Galvanising (The Dual-Action Method)
Galvanising is the process of coating iron or steel with a layer of zinc (usually by dipping the object into molten zinc).
• Why galvanising is special: It provides protection in two ways:
1. Physical Barrier: The continuous zinc layer keeps out oxygen and water.
2. Sacrificial Protection: Even if the zinc layer is scratched or cracked, the exposed iron still will not rust! Because zinc is higher than iron in the reactivity series, zinc continues to sacrifice itself and corrode instead of the iron.
Key Takeaway: Barrier methods stop water and oxygen from touching iron. Sacrificial protection uses a more reactive metal (like zinc or magnesium) that corrodes instead of the iron. Galvanising uses zinc to provide both barrier and sacrificial protection.
---4. Extraction of Iron in the Blast Furnace
Iron is extracted from its main ore, haematite, inside an enormous industrial reactor called a blast furnace.
A. The Raw Materials (The "Charge")
The solid raw materials are added continuously into the top of the furnace:
1. Haematite: The iron ore containing iron(III) oxide (\(\text{Fe}_2\text{O}_3\)).
2. Coke: A cheap, impure form of carbon (\(\text{C}\)) that acts as both a fuel and the source of the reducing agent.
3. Limestone: Calcium carbonate (\(\text{CaCO}_3\)), added to remove acidic impurities (mainly sand/silicon dioxide).
4. Hot Air: Blown into the bottom through nozzles (tuyeres) to supply oxygen for combustion.
B. Step-by-Step Chemical Reactions in the Furnace
Zone 1: Combustion of Coke (Heat Production)
Coke burns vigorously in the blast of hot air. This is a strongly exothermic reaction that generates temperatures up to \(1900^\circ\text{C}\):
\(\text{C(s)} + \text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)}\)
Zone 2: Formation of the Reducing Agent
As the carbon dioxide rises, it reacts with more hot coke to produce carbon monoxide gas:
\(\text{CO}_2\text{(g)} + \text{C(s)} \rightarrow 2\text{CO(g)}\)
Carbon monoxide (\(\text{CO}\)) is the main reducing agent in the blast furnace.
Zone 3: Reduction of Iron(III) Oxide to Iron
The carbon monoxide reduces the iron(III) oxide in haematite to molten iron:
\(\text{Fe}_2\text{O}_3\text{(s)} + 3\text{CO(g)} \rightarrow 2\text{Fe(l)} + 3\text{CO}_2\text{(g)}\)
Molten iron is very dense, so it flows down to the bottom of the furnace and is periodically tapped off.
Zone 4: Removal of Impurities (Slag Formation)
Haematite naturally contains sand, which is silicon dioxide (\(\text{SiO}_2\)), an acidic impurity that would clog the furnace if not removed.
1. The high heat decomposes the limestone into calcium oxide (thermal decomposition):
\(\text{CaCO}_3\text{(s)} \rightarrow \text{CaO(s)} + \text{CO}_2\text{(g)}\)
2. Calcium oxide (a basic metal oxide) reacts with silicon dioxide (an acidic non-metal oxide) in a neutralisation reaction to produce molten calcium silicate, known as slag:
\(\text{CaO(s)} + \text{SiO}_2\text{(s)} \rightarrow \text{CaSiO}_3\text{(l)}\)
• Why does slag not mix with iron? Molten slag is less dense than molten iron, so it floats on top of the iron layer and is tapped off through a separate upper outlet. (Slag is cooled and reused for road building and construction!).
Key Takeaway: Haematite (\(\text{Fe}_2\text{O}_3\)) is reduced by carbon monoxide (\(\text{CO}\)) to produce molten iron. Limestone decomposes into \(\text{CaO}\) to react with sand (\(\text{SiO}_2\)), forming molten slag (\(\text{CaSiO}_3\)), which floats on top of the denser iron.
---5. Common Exam Pitfalls & Tips
• Don't call all corrosion "rusting": Copper turns green and aluminium forms an oxide layer—they corrode. Only iron and steel rust.
• Name rust precisely: If asked for the chemical name of rust, always write hydrated iron(III) oxide. Writing just "iron oxide" will lose marks.
• Explain the boiling water tube correctly: In the rusting investigation, state clearly that boiling removes dissolved air/oxygen and the oil layer prevents oxygen from re-dissolving.
• Identify the correct reducing agent: When asked what reduces haematite in the blast furnace, name carbon monoxide (\(\text{CO}\)), not solid carbon/coke.
• Layer order at the blast furnace base: Remember that molten iron is at the very bottom (highest density) and molten slag floats on top (lower density).
Quick Revision Checklist
• Can you define oxidation and reduction in terms of oxygen, hydrogen, and electrons (OIL RIG)?
• Can you write half-equations for displacement reactions and identify the oxidising and reducing agents?
• Can you state the chemical name and formula for rust (\(\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}\)) and the two conditions needed for it to form?
• Can you explain how barrier methods, sacrificial protection, and galvanising work?
• Can you write the 4 key balanced equations taking place in the blast furnace and explain the role of limestone?