Half-equations are one of the most important tools in IB Chemistry Topic 9 (Redox Processes). They show how electrons are transferred in oxidation and reduction reactions. Because redox reactions often involve multiple species and complex balancing, half-equations simplify the process by breaking the overall reaction into two manageable parts. Mastering half-equations makes it far easier to balance redox reactions, calculate electron flow, and build galvanic or electrolytic cells.
What Is a Half-Equation?
A half-equation (half-reaction) is an equation that shows either the oxidation or the reduction part of a redox reaction, including electron transfer.
There are two types:
- Oxidation half-equation: electrons are lost
- Reduction half-equation: electrons are gained
When combined, they produce the overall redox reaction.
Half-equations show clearly:
- Which species is oxidized
- Which species is reduced
- How many electrons move
- How to ensure atom and charge balance
Oxidation and Reduction Reminder
Use the classic IB mnemonic:
OIL RIG
- Oxidation Is Loss of electrons
- Reduction Is Gain of electrons
In all redox processes:
- Oxidation and reduction occur simultaneously
- One species donates electrons, the other accepts
Half-equations represent each change separately.
Example: Simple Half-Equations
Oxidation example:
Zn(s) → Zn²⁺(aq) + 2e⁻
Zinc loses electrons, so this is oxidation.
Reduction example:
Cu²⁺(aq) + 2e⁻ → Cu(s)
Copper ions gain electrons.
These two half-equations together form the Daniell cell reaction.
How to Write a Half-Equation (Step-by-Step)
IB Chemistry requires students to balance half-equations systematically.
Here is the standard method used for aqueous reactions:
Step 1: Balance atoms other than O and H
Write down the species and balance non-O/H atoms first.
Step 2: Balance oxygen using H₂O
Add water molecules to the side that needs oxygen.
Step 3: Balance hydrogen using H⁺
Add H⁺ ions to the side that needs hydrogen.
Step 4: Balance charge using electrons
Add e⁻ to the more positive side to equalize total charge.
Step 5: Verify mass and charge balance
Atoms and charges must be equal on both sides.
This method works for acidic solutions (H⁺ present).
Writing Half-Equations in Basic Conditions
For reactions in alkaline (basic) solutions:
- Write the half-equation as if it were acidic
- Add OH⁻ to both sides to cancel any H⁺
- Combine H⁺ + OH⁻ → H₂O
- Simplify waters if needed
This process is frequently tested in HL redox questions.
Combining Half-Equations
To form a full redox reaction:
1. Ensure electron numbers match
Multiply one or both half-equations so electrons cancel.
2. Add the equations together
Electrons must disappear completely.
3. Cancel species that appear on both sides
This gives the final balanced overall reaction.
Example: Balancing a Full Redox Reaction
Given:
Oxidation:
Fe²⁺ → Fe³⁺ + e⁻
Reduction:
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Step 1: Multiply the iron half-reaction by 5
(So both reactions have 5 electrons)
5Fe²⁺ → 5Fe³⁺ + 5e⁻
Step 2: Add them together
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
This is the classic IB permanganate–iron(II) titration equation.
Why Half-Equations Matter in IB Chemistry
Half-equations are essential for:
1. Balancing redox reactions
Especially in acidic or basic solutions.
2. Understanding galvanic/electrolytic cells
Electrode processes are written as half-equations.
3. Determining electron transfer
IB exams frequently ask for the number of electrons moved.
4. Calculating E°cell
Half-equations provide standard reduction potentials.
5. Redox titrations
All IB redox titrations require half-equation analysis.
Common IB Misunderstandings
“Electrons can be on both sides.”
Never—electrons must cancel completely in the full equation.
“You balance charge after multiplying equations.”
Charges must be balanced before combining.
“Water appears only on one side instantly.”
It’s added only to balance oxygen or hydrogen.
“Oxidation half-equations contain H⁺ or H₂O automatically.”
Only if needed for balancing.
FAQs
Do half-equations always include electrons?
Yes—electron transfer defines redox reactions.
Are half-equations the same in galvanic and electrolytic cells?
The reactions may reverse, but the method is identical.
What if both sides already have electrons?
That indicates incorrect balancing; electrons must appear on only one side of each half-equation.
Conclusion
A half-equation shows either oxidation or reduction in a redox reaction, including electrons. They allow IB students to balance complex reactions, build electrochemical cells, and predict electron flow. Mastering half-equations not only strengthens your redox understanding but also prepares you for some of the most common IB exam questions.
