AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Electrolysis of aqueous solutions: At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen.
In aqueous electrolysis, the negative electrode (cathode) attracts positive ions.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous electrolysis, the negative electrode (cathode) attracts positive ions. Water provides H⁺ ions alongside metal ions such as Cu²⁺, Na⁺ or Zn²⁺. The ion discharged depends on relative reactivity: if the metal is more reactive than hydrogen, its ions are too stable to be reduced easily, so H⁺ ions gain electrons instead and hydrogen gas forms. For example, in sodium chloride solution, sodium is more reactive than hydrogen, so the cathode reaction is 2H⁺ + 2e⁻ → H₂. In copper chloride solution, copper is less reactive than hydrogen, so Cu²⁺ + 2e⁻ → Cu deposits copper metal.
What earns the marks
- Identify the negative electrode as the cathode, where positively charged ions gain electrons (reduction).
- Compare the reactivity of the metal in the solution with hydrogen using the reactivity series.
- If the metal is more reactive than hydrogen, state that hydrogen gas, not the metal, is produced at the cathode.
- Write the cathode half-equation 2H⁺ + 2e⁻ → H₂ when hydrogen is discharged.
- If the metal is less reactive than hydrogen, state that the metal is deposited at the cathode.
- Link the product to an observation, such as bubbles of gas for hydrogen or a coloured solid deposit for a metal.
Common mistakes and how to put them right
Thinking the more reactive metal is always discharged at the cathode.
Put it right by explaining that more reactive metals form stable ions that are harder to reduce, so hydrogen is released instead.
Confusing the cathode with the anode.
Put it right by recalling that the cathode is the negative electrode and attracts positive ions.
Writing the hydrogen half-equation as H⁺ + e⁻ → H₂ without balancing.
Put it right: Correct this to 2H⁺ + 2e⁻ → H₂ so atoms and charge balance.
Exam tips
- State the rule first, then apply it to the named solution, for example 'sodium is more reactive than hydrogen, so hydrogen is produced'.
- Give the half-equation as well as the product name to show the electron transfer clearly.
- Use the reactivity series explicitly in your answer rather than saying 'it depends on the metal'.
Try the lesson
The start of the Electrolysis of aqueous solutions: At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen. lesson, made from checked teaching notes for this specification point.
Have a go first
In the electrolysis of aqueous sodium chloride, sodium is more reactive than hydrogen. Which product is formed at the negative electrode?
Listen
In aqueous electrolysis, the negative electrode (cathode) attracts positive ions. Water provides H⁺ ions alongside metal ions such as Cu²⁺, Na⁺ or Zn²⁺. The ion discharged depends on relative reactivity: if the metal is more reactive than hydrogen, its ions are too stable to be reduced easily, so H⁺ ions gain electrons instead and hydrogen gas forms. For example, in sodium chloride solution, sodium is more reactive than hydrogen, so the cathode reaction is 2H⁺ + 2e⁻ → H₂. In copper chloride solution, copper is less reactive than hydrogen, so Cu²⁺ + 2e⁻ → Cu deposits copper metal. A strong answer earns marks for these points. Identify the negative electrode as the cathode, where positively charged ions gain electrons (reduction). Compare the reactivity of the metal in the solution with hydrogen using the reactivity series. If the metal is more reactive than hydrogen, state that hydrogen gas, not the metal, is produced at the cathode. Write the cathode half-equation 2H⁺ + 2e⁻ → H₂ when hydrogen is discharged. A common slip: Thinking the more reactive metal is always discharged at the cathode. Instead, explaining that more reactive metals form stable ions that are harder to reduce, so hydrogen is released instead. Exam tip: State the rule first, then apply it to the named solution, for example 'sodium is more reactive than hydrogen, so hydrogen is produced'.
See it
Key words
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Reactivity series and selective discharge
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Reduction at the cathode
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Writing and balancing half-equations
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See every definition
- Reactivity series and selective discharge
- Comparing the metal in the solution with hydrogen using the reactivity series decides which ion is discharged at the cathode.
- Reduction at the cathode
- At the negative electrode, positively charged ions gain electrons.
- Writing and balancing half-equations
- The cathode half-equation 2H⁺ + 2e⁻ → H₂ is written so atoms and charge balance when hydrogen is discharged.
Label the diagram
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1Box 1
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3Box 3
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