IB Chemistry HLTopic 6 — Electron TransferPaper 1 & 2Practical skill~8 min read
Electroplating
Electroplating is electrolysis used on purpose to coat something. A cheap metal object goes in one end, a thin skin of silver, chromium or gold goes on it, and it comes out looking — and behaving — like the expensive stuff. The chemistry is the same as copper purification, just aimed at a different goal.
📚 What you need to know
Electroplating uses active electrodes and a solution containing ions of the plating metal.
The object being plated is the cathode (negative), because that is where reduction deposits metal.
The anode is made of the plating metal (positive), and it slowly dissolves.
Because the anode replaces what the cathode takes, the ion concentration stays constant.
Thickness depends on current and time; evenness depends on a clean surface, a low steady current and good stirring.
Two reasons to plate: decoration and corrosion protection.
Non-conductors such as plastics can be plated once the surface has been etched and made conducting.
Setting the cell up
There are only three decisions, and they all follow from one idea: metal ions must be able to arrive at the object and pick up electrons there.
🧩 Building any plating cell
Cathode = the object. Connect it to the negative terminal. Reduction happens here, so metal builds up on it.
Anode = the plating metal. Connect it to the positive terminal. It is oxidised and dissolves into the solution.
Electrolyte = a solution of the plating metal’s ions. Silver plating needs silver ions in solution, nickel plating needs nickel ions.
Clean the object first. Grease or oxide stops the coating sticking, and you get a patchy finish.
Use a low current for longer. Rushing it gives a rough, powdery layer that flakes off.
Silver plating usually uses a cyanide-based electrolyte rather than silver nitrate, because it releases Ag+ slowly and gives a smoother, better-stuck coating.
Why the anode has to be the right metal
You could plate with an inert graphite anode — for a while. Here is what goes wrong:
Silver ions leave the solution at the cathode and nothing puts them back.
The concentration falls, so the plating gets slower and patchier.
With no metal to oxidise, the anode starts oxidising water instead, so you get oxygen bubbles and the solution turns acidic.
The neat bit: the anode reaction is the exact reverse of the cathode reaction. Ag → Ag+ + e− at one end, Ag+ + e− → Ag at the other. Silver is simply being transported from one electrode to the other through the solution.
Controlling the coating
What you change
Effect on the coating
Practical reason
Increase the current
Metal deposits faster
More electrons per second means more ions reduced per second
Increase the time
Thicker coating
Total metal deposited depends on total charge passed
Use too high a current
Rough, powdery, flaky layer
Ions cannot reach the surface fast enough to pack neatly
Stir the solution
More even coating
Keeps the ion concentration the same all over the object
Clean and degrease first
Coating actually sticks
Metal cannot bond to a layer of grease or oxide
Think of it like spray-painting. A slow, even pass gives a smooth finish; blasting the paint on gives runs and a rough surface. Same object, same paint, different result — the rate is what matters.
Plating a plastic
Plastics do not conduct, so they cannot be a cathode as they are. The trick is to make the surface conducting first:
The surface is etched, usually with acid, to roughen it so the coating has something to grip.
A very thin layer of metal particles is deposited chemically, making the surface conduct.
The object is now electroplated normally, as if it were metal.
This is how plastic car badges, tap handles and bathroom fittings get their chrome look at a fraction of the weight and cost.
Worked examples
WORKED EXAMPLE
Describe how you would nickel-plate a steel key. Give the electrode reactions.
Step 1: Decide the electrodesThe key is the object, so it is the cathode, connected to the negative terminal. The anode is a piece of nickel.Step 2: Choose the electrolyteA solution containing Ni2+, such as nickel(II) sulfate.Step 3: Write the two reactionsAnode: Ni(s) → Ni2+(aq) + 2e−Cathode: Ni2+(aq) + 2e− → Ni(s)Nickel dissolves from the anode and coats the key at the cathodeclean the key first, and keep the current low for an even layer
WORKED EXAMPLE
A student silver-plates a spoon but uses a graphite anode instead of silver. Predict two differences they would observe.
Step 1: Ask what the anode can oxidiseGraphite is inert, so water is oxidised instead of the metal.2H2O(l) → O2(g) + 4H+(aq) + 4e−Step 2: Follow the consequencesNothing replaces the Ag+ being used up, so its concentration falls.Gas bubbles at the anode, and plating slows and thins as Ag+ runs outthe solution also becomes acidic, so pH falls
WORKED EXAMPLE
A steel bolt is galvanised by coating it with zinc. Explain why this protects the steel even if the coating gets scratched.
Step 1: Compare the two Eθ valuesZn2+/Zn = −0.76 V and Fe2+/Fe = −0.45 VStep 2: More negative means easier to oxidiseZinc is more negative, so zinc is oxidised in preference to iron.Step 3: Say what that means for the scratchThe zinc corrodes instead of the iron — sacrificial protectiona scratched tin coating would be worse, because tin is less reactive than iron
💡 Exam tip
State the electrodes as “object at the cathode, plating metal at the anode” — that phrasing earns the mark cleanly.
Say the electrolyte must contain ions of the plating metal, and name a specific salt if you can.
Give the anode and cathode half-equations, and notice that they are the reverse of each other.
Explain the constant concentration: the anode replaces what the cathode removes.
For “how do you get an even coating”, mention a low current, a clean surface and stirring.
Link plating to corrosion protection when the question mentions rusting or galvanising.
⚠ Common mix-up
Putting the object at the anode. Then it would dissolve, not get coated. The object is always the cathode.
Forgetting the electrolyte must match. You cannot silver-plate from copper sulfate solution.
Saying the anode is negative. In electrolysis the anode is positive. Oxidation still happens there.
Claiming the solution gets more concentrated. With a matching active anode, it stays roughly the same.
Thinking a thicker coat needs a bigger current. Time is the safer variable; too much current ruins the finish.
Assuming any coating protects a scratched surface. Only a more reactive metal gives sacrificial protection.
Up next: Free Radicals — you have spent this whole topic moving electrons in pairs from one species to another. Next, they start moving one at a time, and the chemistry changes completely.
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