Redox Reactions
Master oxidation and reduction concepts, learn to assign oxidation states, write half equations, and explore real-world applications of redox chemistry.
Oxidation and Reduction
Redox reactions involve the simultaneous transfer of electrons between substances. Oxidation is the loss of electrons, while reduction is the gain of electrons. These always occur together -- you cannot have one without the other.
OIL RIG -- Remembering Redox
OIL
Oxidation Is Loss
Loss of electrons
Increase in oxidation state
The species that loses electrons is oxidised
RIG
Reduction Is Gain
Gain of electrons
Decrease in oxidation state
The species that gains electrons is reduced
Oxidising and Reducing Agents
Oxidising agent: The substance that causes oxidation in another species. It gains electrons and is itself reduced.
Reducing agent: The substance that causes reduction in another species. It loses electrons and is itself oxidised.
Key insight: The oxidising agent is reduced, and the reducing agent is oxidised. This often confuses students -- focus on what happens to the electrons.
Oxidation States (Oxidation Numbers)
Oxidation states are numbers assigned to atoms in compounds to track electron transfer. They help identify which atoms are oxidised and which are reduced in a reaction.
Rules for Assigning Oxidation States
Elements in their elemental form have an oxidation state of 0 (e.g. Na, O2, Fe)
Monatomic ions have an oxidation state equal to their charge (e.g. Na+ = +1, Cl− = −1)
Oxygen is usually −2 (except in peroxides where it is −1)
Hydrogen is usually +1 (except in metal hydrides where it is −1)
The sum of oxidation states in a compound equals 0; in a polyatomic ion, it equals the ion charge
Half Equations and Real-World Applications
A redox reaction can be split into two half equations: one showing oxidation and the other showing reduction. The electrons lost in oxidation must equal the electrons gained in reduction.
Example: Zinc reacting with copper sulfate
Oxidation half equation:
Zn → Zn2+ + 2e−
Zinc loses 2 electrons (is oxidised)
Reduction half equation:
Cu2+ + 2e− → Cu
Copper ions gain 2 electrons (are reduced)
Overall equation:
Zn + Cu2+ → Zn2+ + Cu
Batteries (Electrochemical Cells)
Batteries convert chemical energy to electrical energy via redox reactions. Electrons flow from the anode (oxidation) through an external circuit to the cathode (reduction).
Corrosion (Rusting)
Iron rusting is a redox reaction: iron is oxidised (Fe → Fe2+ + 2e−) and oxygen is reduced. Water and electrolytes accelerate the process. Galvanising (zinc coating) protects iron.
Key Vocabulary
Oxidation
The loss of electrons by a species in a chemical reaction, resulting in an increase in oxidation state.
Reduction
The gain of electrons by a species in a chemical reaction, resulting in a decrease in oxidation state.
Oxidation State
A number assigned to an atom representing the number of electrons it has lost, gained, or shared in a compound.
Half Equation
An equation showing either the oxidation or reduction part of a redox reaction, explicitly showing the electrons transferred.
Worked Examples
Determine the oxidation state of sulfur in H2SO4.
Step 1: H is +1 (x2 = +2), O is −2 (x4 = −8).
Step 2: The compound is neutral, so all oxidation states sum to 0.
Step 3: +2 + S + (−8) = 0, so S = +6.
Answer: The oxidation state of sulfur in H2SO4 is +6.
In the reaction 2Mg + O2 → 2MgO, identify what is oxidised and what is reduced.
Step 1: Mg goes from 0 to +2 (loses electrons) -- Mg is oxidised.
Step 2: O goes from 0 to −2 (gains electrons) -- O is reduced.
Step 3: Mg is the reducing agent (it causes reduction of O2). O2 is the oxidising agent.
Half equations: Mg → Mg2+ + 2e− (oxidation) and O2 + 4e− → 2O2− (reduction).
Explain why zinc is used to galvanise iron.
Step 1: Zinc is more reactive than iron (higher on the activity series).
Step 2: If the iron surface is scratched, zinc will preferentially oxidise (Zn → Zn2+ + 2e−) instead of the iron.
Answer: This is called sacrificial protection. Zinc acts as a sacrificial anode, corroding in place of iron and protecting it from rusting.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
In a redox reaction, oxidation is defined as:
Question 2
What is the oxidation state of oxygen in most compounds?
Question 3
In the reaction Zn + Cu2+ → Zn2+ + Cu, which species is the reducing agent?
Question 4
What is the oxidation state of nitrogen in NO3−?
Question 5
Which of the following is an example of a redox reaction?
Key Concepts Summary
- ●Oxidation = loss of electrons (increase in oxidation state); reduction = gain of electrons (decrease in oxidation state).
- ●OIL RIG: Oxidation Is Loss, Reduction Is Gain -- of electrons.
- ●Oxidation states track electron transfer; rules include O = −2, H = +1, elements = 0, and sum = charge.
- ●Half equations show oxidation and reduction separately; electrons lost must equal electrons gained.
- ●Real-world redox: batteries (electrochemical cells), corrosion (rusting), and galvanising (sacrificial protection).