Electroplating

Electroplating questions are a gift: three labelling marks and one or two reason marks, with a set-up that never changes from one exam series to the next. The marks still get dropped, though, and almost always the same way: the object and the plating metal swapped between electrodes. Fix that one connection and this subtopic is banked.

The set-up: three fixed choices (Core)

Electroplating coats an object with a thin layer of metal using electrolysis. Three components, three rules:

Component	Choice	Why
Cathode (−)	The object to be plated	Positive metal ions are attracted to it and deposited
Anode (+)	The plating metal itself	It dissolves to replace the ions used up
Electrolyte	Aqueous salt of the plating metal	Supplies the metal ions that coat the object

Worked example for silver-plating a spoon: the spoon is the cathode, a bar of pure silver is the anode, and the electrolyte is silver nitrate solution. For chromium-plating a tap: tap as cathode, chromium anode, solution of a chromium salt. Swap the metal and the pattern holds. That consistency is why a labelled sketch earns marks in under a minute.

What happens at each electrode

At the cathode, metal ions gain electrons and deposit as atoms on the object’s surface. At the anode, the plating metal loses electrons and dissolves into the solution, topping up the ions. The concentration of metal ions in the electrolyte therefore stays constant, a detail that appears as a 1-mark “explain why the colour of the copper(II) sulfate solution does not change” question.

(S) Extended candidates support this with half-equations. For copper-plating:

• Cathode: Cu2+ + 2e− → Cu
• Anode: Cu → Cu2+ + 2e−

Note the symmetry: the anode equation is the cathode equation reversed. Electrons sit on the left where the ion gains them (reduction at the cathode) and on the right where the atom loses them (oxidation at the anode). The same electron bookkeeping runs through all of Redox and Oxidation Numbers.

The mass transferred also balances: the mass lost by the anode equals the mass gained by the object. Questions quoting before-and-after electrode masses are testing whether you can say so.

Why plate anything? (Core)

Two reasons score in 0620:

1. Appearance: a thin layer of silver or chromium looks better than the cheap metal underneath.
2. Corrosion resistance: the coating keeps air and water away from the metal below.

A cost point sometimes supports these: plating gives the look and protection of an expensive metal while using only a thin layer of it. Keep the answer concrete: “to make it look shiny and stop it corroding” maps onto both marking points; “to make it better” maps onto neither.

How this connects to the rest of the topic

Electroplating is ordinary aqueous electrolysis with one twist: the anode is active, not inert. Compare it with copper(II) sulfate between carbon electrodes in Electrolysis of Aqueous Solutions: same electrolyte, different anode, completely different anode product. Exam questions exploit exactly that contrast, and the wider topic context lives on the Electrochemistry pillar.

Worked exam question

A steel fork is to be electroplated with silver. (a) Draw and label a diagram of the apparatus. Your labels should identify the cathode, the anode and a suitable electrolyte. [3] (b) Give one reason why the fork is electroplated. [1] (c) (Extended) Write the half-equation for the reaction at the cathode. [1]

Model answer: (a) Fork connected to the negative terminal, labelled cathode (1); pure silver bar connected to the positive terminal, labelled anode (1); electrolyte labelled silver nitrate solution, or any named soluble silver salt (1). (b) To improve its appearance, or to resist corrosion (1). (c) Ag+ + e− → Ag (1).

Mark-by-mark: each label in (a) is an independent mark, so a rough sketch with three correct labels beats a beautiful drawing with one. The electrolyte mark requires a silver compound in solution: “salt water” or “dilute sulfuric acid” scores zero because neither contains silver ions. In (c), silver is 1+, so one electron only; copying the 2e− from copper costs the mark.

The mistakes that cost marks

1. Making the object the anode. The object must be the cathode: positive metal ions deposit on the negative electrode. PANIC: Positive Anode, Negative Is Cathode.
2. Choosing an electrolyte without the plating metal’s ions. Silver-plating needs a silver salt in solution; sodium chloride solution cannot deposit silver.
3. Using an inert anode and not noticing the question expects the plating metal. With a carbon anode the ion concentration falls and plating soon stops; the metal anode keeps it constant.
4. Vague reasons. “Looks nicer” and “stops corrosion” score; “makes it stronger” does not (a thin metal layer adds no strength).

How examiners want it phrased

Student wording	Mark-scheme wording
”The fork goes on the minus end"	"The fork is the cathode; Ag+ ions are attracted to it and deposited"
"Use silver liquid"	"The electrolyte is aqueous silver nitrate, which contains Ag+ ions"
"The silver bar shrinks"	"The silver anode dissolves: Ag → Ag+ + e−, replacing the ions discharged"
"It keeps the solution the same"	"The concentration of Ag+ ions remains constant”

The full-mark formula is the same one that runs through every electrolysis answer: name the ion, name the electrode, state electron gain or loss. If you can recite the set-up but freeze when a question reverses it (“explain why the anode loses mass”), that is an exam-technique gap, not a knowledge gap, and it is exactly what a free 1-hour trial lesson with a Chemistry specialist is built to find and fix.

Test yourself

Sketch the cell in your head before each answer. The answers stay hidden until you click.

Q1 (3 marks). A steel tap is to be electroplated with nickel. State what should be used as the cathode, the anode and the electrolyte.

Show answer

• Cathode: the steel tap (the object to be plated) [1] • Anode: a bar of (pure) nickel [1] • Electrolyte: an aqueous solution of a nickel(II) salt, e.g. nickel(II) sulfate solution [1]

Q2 (2 marks). During the plating, the anode loses mass but the concentration of metal ions in the electrolyte stays constant. Explain these observations.

Show answer

• The anode dissolves: metal atoms lose electrons and enter the solution as ions [1] • Ions enter the solution at the same rate as they are deposited on the cathode, so the concentration is unchanged [1]

Q3 (2 marks, Supplement). Write the half-equation for the reaction at the cathode and at the anode during nickel-plating.

Show answer

• Cathode: Ni2+ + 2e− → Ni [1] • Anode: Ni → Ni2+ + 2e− [1]