GCSE · Chemistry · Edexcel · Spec 1CH0

Electroplating

How do you give a cheap spoon a real silver surface? You don't paint it on — you switch on a current and let the silver build itself up.

Follow the silver: from bar to spoon

Power supplye⁻Solution containing silver ions+Anode (+)Silver bar−Cathode (−)Spoon

Electrolyte: Solution containing silver ions (aqueous). Cathode (−), made of Spoon, half-equation silver ion + electrons → silver atom, product a thin layer of silver on the spoon, observation the silver coat on the spoon slowly builds up. Anode (+), made of Silver bar, half-equation silver atom → silver ion + electrons, product silver ions, which go into the solution, observation the silver bar slowly gets smaller. + (positive) migrates to the cathode. e⁻ in the external circuit: anode to cathode. static

silver moves bar → spoon

Cathode (−)

silver ion + electrons → silver atom

Product: a thin layer of silver on the spoon

Observation: the silver coat on the spoon slowly builds up

Anode (+)

silver atom → silver ion + electrons

Product: silver ions, which go into the solution

Observation: the silver bar slowly gets smaller

Each amber + bubble is a positive silver ion, and e⁻ marks the electrons in the wire. Watch where the ions start and where they end up: they leave the silver bar (the anode), drift through the solution and arrive at the spoon (the cathode). The two cards show what each electrode does with electrons.

Watch out: The solution isn't simply being used up. Silver ions leave it at the spoon, but the bar keeps putting new ones in — so the concentration stays about the same while the bar gets smaller.

Predict, then check

Think about the charge on the metal ions before you choose.

You want to coat a cheap metal ring with gold. How should you set up the cell?

Chemistry · Why we plate

Looks, or protection?

What is the thin metal layer mainly for?

Still to sort

Mainly for appearance (0)

A thin layer of an attractive metal makes a cheaper object look good.

Where the line is: Ask what the layer is doing. If it is there so a cheaper object looks attractive, it is appearance. Some platings do both jobs — sort by the main one.

Mainly for resistance to corrosion (0)

The layer is a barrier between the steel and the air and water around it.

Where the line is: If the layer is there to keep air and water off the iron in steel, it is corrosion resistance — even if it happens to look nice too.

4 of 4 still to sort.

Sort each plated object by the main job its thin metal layer is doing.

Chemistry · Put it in writing

Explain it in your own words

A steel sheet is to be coated with tin by electroplating. Describe how the cell is set up, and explain what happens at each electrode — including why the electrolyte does not run out of tin ions. [5 marks]

0 words · your answer stays on this page and is not sent anywhere.

Exam line: Set-up first — object, anode, electrolyte — then one sentence for each electrode: ions GAIN electrons at the cathode, atoms LOSE electrons at the anode.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How a current moves metal from a bar onto an object — and why we bother

What you need to know

  • Electroplating uses electrolysis to coat the surface of a metal object with a thin layer of another metal.
  • The set-up: the object to be plated is the cathode (negative electrode); the anode (positive electrode) is made of the plating metal; the electrolyte is a solution containing ions of the plating metal.
  • At the cathode, positive ions of the plating metal gain electrons and are deposited as a layer of metal on the object.
  • At the anode, atoms of the plating metal lose electrons and go into solution as ions, replacing the ions used up. So the concentration of the electrolyte stays about the same while the anode gets smaller.
  • Electroplating can improve an object's appearance (for example silver or gold on a cheaper metal) and/or its resistance to corrosion (for example chromium, nickel or tin on steel, forming a barrier that keeps air and water away from the iron underneath).

The big picture

Electroplating uses electrolysis to coat a metal object with a thin layer of another metal. The object is the cathode, the anode is made of the plating metal, and the electrolyte contains ions of that metal. Positive plating-metal ions move to the object, gain electrons and build up as a metal coat. Meanwhile, anode atoms lose electrons and go into solution as ions, replacing those used up — so the electrolyte stays about the same while the anode shrinks. The metal is moved, not made. We plate things to improve their appearance, their resistance to corrosion, or both.

Key points

1Object = cathode (−). Anode (+) = the plating metal. Electrolyte = a solution of the plating metal's ions.
2Cathode: plating-metal ions gain electrons and become a layer of metal on the object.
3Anode: plating-metal atoms lose electrons and go into solution as ions.
4The anode shrinks, the coat grows, and the electrolyte stays about the same — the metal is moved from anode to object.
5Why plate? To look better, to resist corrosion, or both.

Worked example

Problem

A bike maker wants to nickel-plate a steel bracket so that it resists corrosion. Decide how to set up the cell, and explain how the nickel layer protects the steel.

⚠ Watch out

Connecting the object to the positive terminal. The plating-metal ions are positive, so they are pulled to the negative electrode — the object must be the cathode. The positive electrode is where metal leaves, not where the coat builds up.

🧠

Memory hook

The CAThode gets the Coat. The Anode gives its metal Away. Positive metal ions head for the negative object, and the anode quietly shrinks to keep them coming.

✓

Check yourself

A plating tank has run all afternoon. What has happened to the object, the anode and the electrolyte's concentration? Explain each one using 'gain' or 'lose' electrons.

Flashcards

(10)
What is electroplating?
Using electrolysis to coat the surface of a metal object with a thin layer of another metal.
In electroplating, which electrode is the object?
The cathode — the negative electrode.
What is the anode made of in electroplating?
The plating metal (the metal you want on the surface of the object).
What must the electrolyte contain?
Ions of the plating metal, in solution.
What happens to plating-metal ions at the cathode?
They gain electrons and are deposited as a layer of metal on the object.
What happens to plating-metal atoms at the anode?
They lose electrons and go into the solution as ions.
Why doesn't the electrolyte run out of plating-metal ions?
The anode's atoms become ions that replace the ions used up, so the concentration stays about the same — while the anode gets smaller.
Give the two reasons for electroplating an object.
To improve its appearance, and/or to improve its resistance to corrosion.
How does a coat of chromium, nickel or tin protect steel?
It forms a barrier that keeps air and water away from the iron underneath.
Give an example of plating for appearance.
A thin layer of silver or gold on a cheaper metal, so it looks attractive.

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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