Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂

Q: How do I revise Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂ for GCSE?

Read the notes on Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂, then test yourself without looking. Answering questions and checking what you missed builds recall faster than re-reading.

Q: Is this Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂ guide aligned to the AQA specification?

Yes. This guide is linked to the Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂ topic in the AQA GCSE specification.

This half equation shows hydrogen ions, H⁺, being reduced to hydrogen gas, H₂, at the cathode during electrolysis.

Study notes

From the checked teaching notes for this specification point.

The idea

What earns the marks

Identifies the reactant as hydrogen ions, H⁺, and the product as hydrogen molecules, H₂.
States that reduction occurs at the cathode because electrons are gained.
Balances atoms: two H⁺ ions produce one H₂ molecule, so the equation has 2H⁺ and H₂.
Balances charge: two H⁺ ions carry a total charge of 2⁺, so two electrons, 2e⁻, are needed to give zero charge on both sides.
Explains that the electrons come from the external circuit and are supplied at the negative electrode.
Recognises that this half equation represents the cathode reaction in electrolysis of aqueous solutions containing H⁺ ions.

Common mistakes and how to put them right

Writing H⁺ + e⁻ → H₂, which is incorrect because it does not balance hydrogen atoms or charge.
Put it right: The correct equation is 2H⁺ + 2e⁻ → H₂.
Writing 2H⁺ + 2e⁻ → H₂²⁺ or H₂⁺, which incorrectly assigns a charge to the hydrogen molecule; H₂ is neutral.
Placing this reaction at the anode instead of the cathode.
Put it right: Oxidation occurs at the anode, while reduction of H⁺ occurs at the cathode.

Exam tips

Check both atom balance and charge balance before writing the final half equation.
Remember the mnemonic 'reduction at the cathode' because both reduction and cathode begin with consonants, while oxidation and anode begin with vowels.
If asked to identify the electrode, link the gain of electrons to the negative electrode and state that hydrogen gas is formed.
Note that writing and balancing half equations is assessed at Higher Tier only.

Try the lesson

The start of the Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂ lesson, made from checked teaching notes for this specification point.

Have a go first

In the half equation 2H⁺ + 2e⁻ → H₂, what is the total charge on the left-hand side?

Tip: press 1 to 4 to choose

Listen

This half equation shows hydrogen ions, H⁺, being reduced to hydrogen gas, H₂, at the cathode during electrolysis. The equation must balance in both charge and atoms. Two H⁺ ions each gain one electron, giving two electrons in total, so the charge on the left is 2⁺ plus 2⁻, which is zero, matching the neutral H₂ on the right. The two hydrogen atoms on the left become the two hydrogen atoms in the H₂ molecule on the right. In aqueous solutions, H⁺ ions are attracted to the negative cathode, where they gain electrons. For example, in the electrolysis of dilute sulfuric acid, hydrogen is produced at the cathode. Representing these reactions as half equations is a Higher Tier only skill. A strong answer earns marks for these points. Identifies the reactant as hydrogen ions, H⁺, and the product as hydrogen molecules, H₂. States that reduction occurs at the cathode because electrons are gained. Balances atoms: two H⁺ ions produce one H₂ molecule, so the equation has 2H⁺ and H₂. Balances charge: two H⁺ ions carry a total charge of 2⁺, so two electrons, 2e⁻, are needed to give zero charge on both sides. Exam tip: Check both atom balance and charge balance before writing the final half equation.

See it

A flowchart running from top to bottom. Hydrogen ions, H⁺, lead to gaining electrons, 2e⁻, which leads to reduction at the cathode, which leads to hydrogen molecules, H₂. A separate box, electrons from the external circuit, also points to the gaining electrons step. A further chain shows checking atoms balance: 2H⁺ gives one H₂, leading to checking charge balance: 2⁺ plus 2⁻ is zero, leading to the half equation: 2H⁺ + 2e⁻ → H₂. — It shows hydrogen ions gaining electrons at the cathode to form hydrogen molecules, with the atom and charge balance checks leading to the half equation.

Label the diagram

Some boxes have lost their names. Pick the right name for each number.

The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. Hydrogen ions, H⁺, lead to gaining electrons, 2e⁻, which leads to reduction at the cathode, which leads to hydrogen molecules, H₂. A separate box, electrons from the external circuit, also points to the gaining electrons step. A further chain shows checking atoms balance: 2H⁺ gives one H₂, leading to checking charge balance: 2⁺ plus 2⁻ is zero, leading to the half equation: 2H⁺ + 2e⁻ → H₂.

1Box 1
2Box 2
3Box 3

0 of 3 labelled

There is more to this lesson

The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

Carry on with the full lesson

A week of Premium from your first lesson · Then the free plan continues · No card, no charge

Test your understanding of Representation of reactions at electrodes as half equations (HT only): 2H+ + 2e⁻ → H₂

Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

Take topic quiz