GCSE · Chemistry · Edexcel · Spec 1CH0

Half-equations at electrodes

Electrolysis moves electrons, and a half-equation is how chemists write down exactly where they go. One electrode, one story. Today you set the numbers yourself.

Chemistry · Reaction balancer

Set the numbers: balance each electrode's half-equation

Tap + and − to change a number. The atom row counts each element. The ± row adds up the charge on each side (an electron, e⁻, has a charge of 1−). Get every row ticked.

Reactants
1
Al³⁺
1
e⁻
Products
1
Al
Al1→1✓
±2→0✗
Conservation check: count atoms on both sides.Not yet.

This is the cathode in molten aluminium oxide. Positive ions are pulled to the cathode and gain electrons to become atoms, so the electrons are on the left. The atoms already match. Now fix the ± row.

Tap + or − under each molecule to set its coefficient.
Watch out: Electrons are always added, on the side where they appear. You never subtract them. If the charges don't match, add more electrons until they do.

Why the anode product is a molecule

Chlorine, bromine, iodine and oxygen exist in their standard form as Cl₂, Br₂, I₂ and O₂. Tap each part of the equation to see its job.

Tap a molecule to see what it does in the reaction.

Worked example: copper sulfate solution

Problem

Copper sulfate solution is electrolysed. Copper forms at the cathode and oxygen forms at the anode. Write the half-equation for each electrode.

Chemistry · Electrolysis

Spot the pattern

For each half-equation, tick every statement that is true. Then check your grid and look down the columns.

Na⁺ + e⁻ → Na
Cu²⁺ + 2e⁻ → Cu
2H⁺ + 2e⁻ → H₂
2Br⁻ → Br₂ + 2e⁻
2I⁻ → I₂ + 2e⁻
4OH⁻ → 2H₂O + O₂ + 4e⁻

Chemistry · Electrolysis

Find the faulty line

A student writes the anode half-equation for bromide ions forming bromine. Which line is the first one to go wrong?

A student's answer — which line goes wrong?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

A half-equation is one electrode's story, written down. Set the numbers below and see it balance.

What you need to know

  • A half-equation shows what happens to one reactant, such as the change at a single electrode.
  • In equations an electron is written e⁻: the e stands for electron and the minus sign for its negative charge.
  • At the cathode, the negative electrode, positive ions gain electrons. That is reduction, and the electrons go on the left.
  • The anode, the positive electrode, attracts negative ions, which lose electrons there: oxidation, electrons on the right.
  • Have a goMia says the anode is where positive ions arrive, "because it's the positive electrode". She is very sure. Is she right?

    No. The positive anode attracts negative ions, and those are the ions that lose electrons there.

    Mia has matched positive with positive. It is the opposite way round: the anode attracts negative ions, which lose electrons there.

  • Cathode examples: Mg²⁺ + 2e⁻ → Mg, Al³⁺ + 3e⁻ → Al and 2H⁺ + 2e⁻ → H₂.
  • A half-equation shows each substance in its standard chemical form, so diatomic elements are molecules: 2Cl⁻ → Cl₂ + 2e⁻.
  • Molten aluminium oxide gives Al³⁺ + 3e⁻ → Al at the cathode and 2O²⁻ → O₂ + 4e⁻ at the anode.
  • Have a goDev bets that 2O²⁻ → O₂ + 4e⁻ and 2Cl⁻ → Cl₂ + 2e⁻ release the same number of electrons. Does Dev win?

    No. The oxide half-equation releases 4 electrons and the chloride one releases 2.

    The electrons are not a fixed number per anode. They depend on how many ions go in and how big their negative charge is.

  • When oxygen is formed at the anode from an aqueous solution, hydroxide ions are oxidised: 4OH⁻ → 2H₂O + O₂ + 4e⁻.
  • Balance atoms and charges. Add electrons, never take them away, on the side where they appear, until all charges cancel.
  • Have a goA potassium ion, K⁺, becomes a potassium atom at the cathode. How many electrons do you add, and on which side?

    One electron, on the left: K⁺ + e⁻ → K.

    K⁺ has a charge of 1+ and the atom has none, so one added electron cancels the charge. You add electrons on the left and never take them away.

  • Electrons on the left show reduction, on the right oxidation. OIL RIG: oxidation is loss, reduction is gain.

The big picture

A half-equation describes what happens to one reactant, such as the change at a single electrode. At the cathode, positive ions gain electrons (reduction), so the electrons are on the left. At the anode, negative ions lose electrons (oxidation), so the electrons are on the right. Balance the atoms first, remembering that a diatomic product such as Cl₂ needs two ions, then add electrons, never take them away, until the charges cancel too.

Key points

1Cathode: positive ions gain electrons (reduction), electrons on the left. Anode: negative ions lose electrons (oxidation), electrons on the right.
2Balance in two passes: atoms first, remembering that a diatomic product needs two ions, then charge.
3Electrons are added on the side where they appear. They are never taken away.
4Use 4OH⁻ → 2H₂O + O₂ + 4e⁻ when oxygen is formed at the anode from an aqueous solution.

Worked example

Problem

Magnesium ions, Mg²⁺, form magnesium atoms at the cathode. Write the half-equation, check it balances, and say whether it shows oxidation or reduction.

⚠ Watch out

Writing single atoms at the anode (Cl instead of Cl₂) and fixing the charges by taking electrons away. Chlorine, bromine, iodine and oxygen are written as molecules, so two ions go in, and electrons are always added on the side where they appear.

🧠

Memory hook

OIL RIG: oxidation is loss, reduction is gain. Electrons on the right have been lost (anode). Electrons on the left have been gained (cathode). And for oxygen: two oxide ions, two electrons each, four in total.

✓

Check yourself

Pick one electrode from this lesson. Which ions go there, are electrons gained or lost, and is it oxidation or reduction? Write its half-equation and check atoms and charge.

Flashcards

(14)
What is a half-equation?
An equation for what happens to one reactant, such as the change at a single electrode. It shows electrons lost in oxidation or gained in reduction.
How is an electron written in an equation?
e⁻. The e stands for electron and the minus sign for its negative charge.
What happens at the cathode?
Reduction. The cathode is the negative electrode, and positive ions gain electrons there. The electrons go on the left.
What happens at the anode?
Oxidation. The anode is the positive electrode, so it attracts negative ions, which lose electrons there. The electrons go on the right.
What are the cathode half-equations for Mg²⁺ and Al³⁺?
Mg²⁺ + 2e⁻ → Mg and Al³⁺ + 3e⁻ → Al.
What is the cathode half-equation for hydrogen ions?
2H⁺ + 2e⁻ → H₂.
What are the anode half-equations for chloride, bromide and iodide ions?
2Cl⁻ → Cl₂ + 2e⁻, 2Br⁻ → Br₂ + 2e⁻ and 2I⁻ → I₂ + 2e⁻.
What is the anode half-equation in molten aluminium oxide?
2O²⁻ → O₂ + 4e⁻. Oxide ions are oxidised to oxygen gas, because oxygen exists in its standard state as the O₂ molecule.
Why is chlorine written Cl₂ and not Cl in an anode half-equation?
A half-equation shows the substance in its standard chemical form, and a diatomic element is a molecule. So two chloride ions are needed.
Which half-equation applies when oxygen is formed at the anode from an aqueous solution?
4OH⁻ → 2H₂O + O₂ + 4e⁻. Hydroxide ions are oxidised.
What does a balanced half-equation show?
Balanced atoms on each side and charges that cancel each other out.
Where do you put electrons when balancing, and can you take them away?
Add them on the side where they appear. They are added and never taken away.
What does OIL RIG stand for?
Oxidation Is Loss, Reduction Is Gain (of electrons).
Electrons on the left of a half-equation: oxidation or reduction?
Reduction, because the electrons are gained. Electrons on the right show oxidation.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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