AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
The ions discharged when an aqueous solution is electrolysed using inert…
Full specification wording
Electrolysis of aqueous solutions: The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved.
In aqueous solution, water molecules dissociate slightly to give H⁺ and OH⁻ ions alongside the ions from the dissolved compound.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 1 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In aqueous solution, water molecules dissociate slightly to give H⁺ and OH⁻ ions alongside the ions from the dissolved compound. At the cathode, the more easily reduced positive ion is discharged. If the metal is more reactive than hydrogen, hydrogen is produced instead of the metal; if the metal is less reactive than hydrogen, the metal is produced. At the anode, oxygen is usually produced unless a halide ion is present, in which case the halide is discharged. Reactivity therefore decides which ions are discharged.
For example: In sodium chloride solution, hydrogen forms at the cathode and chlorine at the anode; in copper chloride solution, copper forms at the cathode and chlorine at the anode.
What earns the marks
- Aqueous solutions contain H⁺ and OH⁻ ions from water in addition to the ions from the dissolved substance.
- At the cathode, the ion that is discharged depends on the relative reactivity of the metal compared with hydrogen.
- If the metal is more reactive than hydrogen, hydrogen gas is produced at the cathode rather than the metal.
- If the metal is less reactive than hydrogen, the metal is produced at the cathode.
- At the anode, oxygen is usually produced from OH⁻ ions unless a halide ion is present, when the halide is discharged.
- The products can be identified by testing gases, for example a lighted splint for hydrogen and damp litmus for chlorine.
Common mistakes and how to put them right
Assuming the metal is always produced at the cathode.
Put it right by comparing the metal's reactivity with hydrogen.
Writing that hydrogen is produced at the anode.
Put it right by placing hydrogen production at the negative electrode.
Ignoring water as a source of ions.
Put it right by including H⁺ and OH⁻ from water in the explanation.
Exam tips
- State the rule first, then apply it to the specific solution given.
- Name the products at each electrode and give a simple test for each gas.
- Use the reactivity series to justify why a metal or hydrogen is discharged.
Try the lesson
The start of the Electrolysis of aqueous solutions: The ions discharged when an aqueous solution is electrolysed using inert electrodes depend on the relative reactivity of the elements involved. lesson, made from checked teaching notes for this specification point.
Have a go first
During the electrolysis of an aqueous solution using inert electrodes, hydrogen gas is produced at the negative electrode (cathode) instead of a metal. What determines that hydrogen is discharged rather than the metal?
Listen
In aqueous solution, water molecules dissociate slightly to give H⁺ and OH⁻ ions alongside the ions from the dissolved compound. At the cathode, the more easily reduced positive ion is discharged. If the metal is more reactive than hydrogen, hydrogen is produced instead of the metal; if the metal is less reactive than hydrogen, the metal is produced. At the anode, oxygen is usually produced unless a halide ion is present, in which case the halide is discharged. Reactivity therefore decides which ions are discharged. For example, in sodium chloride solution, hydrogen forms at the cathode and chlorine at the anode; in copper chloride solution, copper forms at the cathode and chlorine at the anode. A strong answer earns marks for these points. Aqueous solutions contain H⁺ and OH⁻ ions from water in addition to the ions from the dissolved substance. At the cathode, the ion that is discharged depends on the relative reactivity of the metal compared with hydrogen. If the metal is more reactive than hydrogen, hydrogen gas is produced at the cathode rather than the metal. If the metal is less reactive than hydrogen, the metal is produced at the cathode. A common slip: Assuming the metal is always produced at the cathode. Instead, comparing the metal's reactivity with hydrogen. Exam tip: State the rule first, then apply it to the specific solution given.
Key words
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Electrolysis of aqueous solutions
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Reactivity series and selective discharge
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Identification of electrode products
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See every definition
- Electrolysis of aqueous solutions
- Process in which an aqueous solution is electrolysed using inert electrodes, with the ions discharged depending on the relative reactivity of the elements involved.
- Reactivity series and selective discharge
- Relative reactivity of the elements involved decides which ions are discharged, since at the cathode the more easily reduced positive ion is discharged.
- Identification of electrode products
- The products can be identified by testing gases, for example a lighted splint for hydrogen and damp litmus for chlorine.
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