An application question asks you to explain why something works, using ions, electrodes and electrons. Describe what happens and why, and leave out instructions for doing it.
This lesson is part of SPM Science electrochemistry. It reuses the electrode rules from interpreting electrolysis observations.
How does electroplating work?
Electroplating is electrolysis used to coat an object with a thin layer of metal. The object becomes the cathode, and the electrolyte contains ions of the coating metal.
Metal ions in the solution move to the object, gain electrons and deposit as metal atoms on its surface.
Worked example: a silver-coated spoon
An invented diagram shows a steel spoon connected to the negative terminal of a battery. A silver bar connects to the positive terminal, and both dip in silver nitrate solution.
Why is the spoon at the cathode? It is connected to the negative terminal, so it attracts positive ions.
What happens at the spoon? Silver ions gain electrons: Ag⁺ + e⁻ → Ag. A layer of silver forms on the spoon.
What happens at the silver bar? Silver atoms lose electrons and dissolve as Ag⁺ ions: Ag → Ag⁺ + e⁻. This replaces the ions used at the cathode, so the concentration of the solution stays nearly constant.
What does a battery do differently?
In a battery, the direction of energy is reversed. A chemical reaction between two different metals and an electrolyte pushes electrons through the circuit and lights a bulb.
The more reactive metal loses electrons at the negative terminal. The electrons travel through the wire and are used at the positive terminal. This builds on the lesson on comparing simple cell designs.
The mistake that costs marks
The slip is to answer “explain how” with a procedure: “Connect the spoon to the battery and put it in the solution.” This describes an action, not the science.
| Answer | Type | Marks earned |
|---|---|---|
| Connect the spoon to the battery and put it in the solution | Procedure | Low |
| The spoon is the cathode, so Ag⁺ ions gain electrons and form a silver layer | Explanation | Full |
The fix is to replace every action verb with a cause. Change “connect” to “the spoon is the cathode because it is joined to the negative terminal”.
Check yourself
A diagram shows a copper-plated key. Explain why the key is connected to the negative terminal, and why copper(II) sulfate is used as the electrolyte.
Answer
The key is connected to the negative terminal so that it becomes the cathode. Positive Cu²⁺ ions are attracted to it and gain electrons to form a copper layer.
Copper(II) sulfate is used because the solution contains Cu²⁺ ions, which are the ions of the metal being plated.
What to study next
Test all four skills on new questions in the electrochemistry practice set. Log each slip in the mistake log and paper-error review to see which lesson to revisit.
If you want a teacher to rework your written explanations, see online one-to-one Science tuition.