Describe how metals are electroplated

Electrochemistry – Electrolysis & Electro‑plating

Learning Objectives

• Define electrolysis and explain how metals are electro‑plated.
• Identify the anode, cathode and electrolyte in an electrolytic cell and describe the flow of charge.
• Predict the products obtained when the three prescribed electrolytes are electrolysed (molten PbBr₂, concentrated NaCl, dilute H₂SO₄).
• State the practical purposes of electro‑plating (appearance, corrosion resistance, conductivity).
• Explain the factors that affect the quality of an electro‑plated deposit.
• Recall the key safety precautions for laboratory electrolysis and electro‑plating.

What is Electrolysis?

Electrolysis is the decomposition of an ionic compound (either molten or in aqueous solution) by passing an electric current through it. In an electrolytic cell the external DC power source forces a non‑spontaneous redox reaction to occur.

Charge Transfer in an Electrolytic Cell

• Electrons travel through the external circuit from the positive terminal (anode) to the negative terminal (cathode).
• Within the electrolyte, cations migrate toward the cathode and anions migrate toward the anode, completing the circuit.

Polarity in an Electrolytic Cell

Electrode	Charge	Process occurring
Anode	Positive	Oxidation (loss of electrons)
Cathode	Negative	Reduction (gain of electrons)

Typical Products at the Electrodes

• Cathode: a metal is deposited, or hydrogen gas is evolved when H⁺ is reduced.
• Anode: a non‑metal (e.g., Cl₂, Br₂, O₂) is liberated, or the anode metal dissolves to give metal ions.

Prescribed Electrolytes – Half‑reactions and Overall Products

Electrolyte	Half‑reactions (e⁻ shown)	Overall Products
Molten PbBr₂	Anode (oxidation): Br⁻ → ½ Br₂ + e⁻ Cathode (reduction): Pb²⁺ + 2 e⁻ → Pb(s)	Pb(s) on cathode; Br₂(g) at anode
Concentrated NaCl (aq)	Anode (oxidation): 2 Cl⁻ → Cl₂(g) + 2 e⁻ Cathode (reduction): 2 H₂O + 2 e⁻ → H₂(g) + 2 OH⁻	Cl₂(g) at anode; H₂(g) at cathode (solution becomes alkaline)
Dilute H₂SO₄ (aq)	Anode (oxidation): 2 H₂O → O₂(g) + 4 H⁺ + 4 e⁻ Cathode (reduction): 2 H⁺ + 2 e⁻ → H₂(g)	O₂(g) at anode; H₂(g) at cathode (solution remains acidic)

Practical tip: To confirm the gases evolved, collect the cathodic gas over water (hydrogen burns with a ‘pop’), test bromine with starch solution (blue‑black colour), and test chlorine with moist potassium iodide (pale yellow iodine vapour).

General Rule for Molten Binary Salts

For any molten binary salt MX, the metal M is deposited at the cathode and the non‑metal X is liberated at the anode:

\[ \text{Cathode: } \; \text{M}^{n+} + n\text{e}^- \rightarrow \text{M(s)} \qquad \text{Anode: } \; \text{X}^- \rightarrow \tfrac{1}{2}\text{X}_2 + \text{e}^- \]

What is Electro‑plating?

Electro‑plating is the intentional use of electrolysis to coat a conductive object (the cathode) with a thin, uniform layer of a metal. The main purposes are:

• Improving appearance (bright, decorative finish).
• Increasing corrosion resistance.
• Enhancing electrical conductivity or wear resistance.

Key Components of an Electro‑plating Cell

• Anode: usually a bar or plate of the metal to be plated; it dissolves to supply metal ions.
• Cathode: the work‑piece to be plated (e.g., a steel nail, jewellery).
• Electrolyte: aqueous solution of a soluble metal salt, often containing an acid and/or additives that improve conductivity and deposit quality.
• Power source: a DC supply; anode to the positive terminal, cathode to the negative terminal.

Overall Reaction (generic metal Mⁿ⁺)

\[ \begin{aligned} \text{Anode (oxidation)} &: \; \text{M(s)} \rightarrow \text{M}^{n+} + n\text{e}^- \\ \text{Cathode (reduction)} &: \; \text{M}^{n+} + n\text{e}^- \rightarrow \text{M(s)} \\ \text{Net effect} &: \; \text{Transfer of metal from anode to cathode} \end{aligned} \]

Step‑by‑Step Example: Copper Plating

1. Prepare the work‑piece: clean, degrease and rinse thoroughly to remove oils, oxides or dust.
2. Set up the cell:
   • Place a copper anode in a copper‑sulphate bath (CuSO₄·5H₂O) with a small amount of H₂SO₄ to increase conductivity.
   • Connect the object to the negative terminal (cathode) and the copper anode to the positive terminal.
3. Apply current: typical current density for copper is 2–5 A dm⁻². Adjust according to the surface area of the object.
4. Control the time: the thickness of the deposit can be estimated with Faraday’s law \[ t = \frac{M I t_{\text{exp}}}{n F A \rho} \] where
   • \(M\) = molar mass of Cu (g mol⁻¹)
   • \(I\) = current (A)
   • \(t_{\text{exp}}\) = electrolysis time (s)
   • \(n = 2\) for Cu²⁺
   • \(F = 96\,485\) C mol⁻¹
   • \(A\) = cathode surface area (cm²)
   • \(\rho = 8.96\) g cm⁻³ (density of Cu)
5. Finish: switch off the supply, remove the plated object, rinse with distilled water and dry.

Factors Influencing the Quality of the Deposit

• Current density: too high → rough, burnt or porous coating; too low → very slow deposition and poor adhesion.
• Temperature of the electrolyte: higher temperature increases ion mobility, usually giving a smoother coat, but may accelerate anode corrosion.
• Concentration of metal ions: must remain sufficient; low concentration causes uneven thickness.
• pH of the solution: extreme pH can precipitate metal hydroxides or dissolve the anode.
• Additives (brighteners, levelers): organic molecules that modify crystal growth, producing a bright, uniform finish.
• Cathode cleaning: any oil, oxide or dust prevents adhesion and leads to flaking.

Common Metals Used for Electro‑plating

Metal	Typical Anode	Electrolyte (common salt)	Typical Applications
Copper	Copper rod	CuSO₄·5H₂O + H₂SO₄	Printed‑circuit boards, decorative plating
Nickel	Nickel plate	NiCl₂·6H₂O + H₃BO₃	Corrosion‑resistant coating, battery electrodes
Chrome	Chromium metal	CrO₃ + H₂SO₄	Automotive trim, hard wear‑resistant surfaces
Silver	Silver bar	AgNO₃ + NH₄NO₃	Jewellery, electrical contacts
Gold	Gold foil	HAuCl₄ + HCl	High‑value jewellery, aerospace connectors

Safety Considerations

• Wear appropriate PPE: lab coat, safety goggles and chemical‑resistant gloves.
• Work in a well‑ventilated area or fume hood – many baths emit hazardous gases.
• Specific hazards:
  • Molten PbBr₂: bromine vapour (toxic, irritating); use a bromine‑scrubbing system and avoid inhalation.
  • Concentrated NaCl solution: chlorine gas at the anode (corrosive, respiratory irritant); ensure adequate ventilation and use a chlorine‑absorbing trap.
  • Copper plating baths: may generate hydrogen sulfide (H₂S) if sulphide impurities are present; detect with lead acetate paper and keep away from sources of ignition.
• Handle acids and metal salts with care – they can cause burns and skin irritation.
• Neutralise any spills (e.g., with sodium bicarbonate for acids) before disposal and follow local hazardous‑waste regulations.

Practical Skills Checklist (for the lab)

• Set up an electrolytic cell with clearly labelled anode and cathode.
• Measure the volume of gas evolved at each electrode (e.g., over‑water collection, gas syringe).
• Observe colour changes in the electrolyte (e.g., blue‑black starch‑bromine test, yellow chlorine‑iodide test).
• Record the current, time and temperature to allow calculation of deposited mass using Faraday’s law.
• After electro‑plating, test the adhesion of the deposit by gentle tapping or a “tape test”.

Summary

Electrolysis decomposes ionic compounds by forcing a redox reaction with a DC current. In an electrolytic cell electrons flow externally from the anode to the cathode, while ions migrate within the electrolyte. The three prescribed electrolytes give characteristic products that can be predicted with half‑equations. Electro‑plating exploits these principles: a sacrificial metal anode dissolves to supply ions, and the work‑piece cathode receives a thin, uniform metal coat. The appearance, corrosion resistance and conductivity of the object depend on controlling current density, temperature, ion concentration, pH and additives. Understanding the safety hazards—especially the gases produced by each electrolyte—ensures that students can carry out the experiments safely and interpret their results confidently.