What You Need to Know
The Big Idea
Redox reactions involve electron transfer. Oxidation is electron loss, reduction is electron gain—they always happen together.
Oxidation Numbers
Track electrons moving in reactions. Positive number = electrons lost. Negative number = electrons gained.
OIL RIG
Oxidation Is Loss, Reduction Is Gain. Your most powerful exam tool.
Agents
Oxidising agents cause oxidation (gain electrons). Reducing agents cause reduction (lose electrons).
Roman Numerals & Oxidation Numbers
Transition metals can form different ions with different charges. When we name these compounds, we use Roman numerals to show which ion we're talking about.
How Roman Numerals Work in Names
The Roman numeral represents the oxidation number of the metal ion. Here's the pattern:
Roman Numeral = Oxidation Number
The Roman numeral in brackets after the element's name tells you the charge on that ion:
- Silver(I) chloride, AgCl → the silver is Ag+
- Iron(II) oxide, FeO → the iron is Fe2+
- Iron(III) oxide, Fe₂O₃ → the iron is Fe3+
- Cobalt(II) chloride, CoCl₂ → the cobalt is Co2+
💡 Naming Tip: The Roman numeral always goes straight after the element name, in brackets. It describes only the oxidation state of that one element, not the whole compound.
Why This Matters
Different oxidation states create different compounds with different properties. Iron can form two common oxides:
- FeO (iron(II) oxide) — contains Fe2+ ions
- Fe₂O₃ (iron(III) oxide) — contains Fe3+ ions
Same element, different formula, different names. This is your first clue that electron transfer is happening.
Worked Example: Naming Compounds with Roman Numerals
Question:
What is the oxidation number of the transition metal ion in silver(I) chloride, AgCl?
Answer:
The Roman numeral (I) tells us the silver ion is Ag+. So the oxidation number is +1.
Practice Question 1
Name the compound with the formula CoCl₂, including the Roman numeral for the metal ion.
What Is a Redox Reaction?
A redox reaction is any reaction where oxidation and reduction happen at the same time in the same reaction. You cannot have one without the other—they're like dance partners.
Two Ways to Think About Redox: Oxygen Transfer vs Electron Transfer
You'll see redox defined in two ways. Both are correct; they just describe the same thing from different angles.
Definition 1: In Terms of Oxygen
- Oxidation = oxygen is added to a substance
- Reduction = oxygen is removed from a substance
- When both happen in one reaction → redox reaction
This definition is older, simpler, and works perfectly for many reactions (like burning and metal extraction).
A Classic Redox Reaction: Zinc Oxide + Carbon
ZnO + C → Zn + CO₂
Let's trace what happens:
- Zinc oxide loses oxygen → it is reduced
- Carbon gains oxygen (becomes CO₂) → it is oxidised
- Both changes happen in the same reaction → this is a redox reaction
Key point:
Worked Example: Identifying Oxidation & Reduction
Question:
Explain which chemicals are oxidised and reduced in:
Fe₂O₃ + 3CO → 2Fe + 3CO₂
Step 1 — Track oxygen:
Fe₂O₃ loses oxygen → becomes Fe with no oxygen. Fe₂O₃ is reduced.
CO