L4 - Electroplating

objectives

Understandings

• Electroplating involves the electrolytic coating of an object with a metallic thin layer.

Applications and Skills

• Explanation of the process of electroplating.

Why Electroplate?

• An important use for electrolytic cells is in electroplating.
• Electroplating results in a thin coating of one metal on top of a conducting surface.
• Reasons for electroplating include making the object more corrosion resistant, strengthening the surface, producing a more attractive finish, or for purifying metal.
• The metals commonly used in electroplating include cadmium, chromium, copper, gold, nickel, silver, and tin.
• Common consumer products include silver-plated or gold-plated tableware, chrome-plated automobile parts, and jewelry.
• We can get an idea of how this works by investigating how silver-plated tableware is produced (Figure 3).

• In the figure, the anode consists of a silver electrode, shown on the left.
• The cathode is located on the right and is the spoon, which is made from inexpensive metal.
• Both electrodes are immersed in a solution of silver nitrate.
• As the potential is increased, current flows. Silver metal is lost at the anode as it goes into solution.

• The mass of the cathode increases as silver ions from the solution are deposited onto the spoon

• The net result is the transfer of silver metal from the anode to the cathode.
• The quality of the object is usually determined by the thickness of the deposited silver and the rate of deposition.

Participating Electrodes

When non-inert electrodes are used in electrolysis they can interact with the solutions being electrolysed.

This specifically is important when electrolysis of ions of relatively unreactive metals are electrolysed using the same metal as an electrode. In this case the anode can participate in the reaction by losing atoms in the form of ions to the solution, leaving behind their own electrons.

This is probably best understood by looking at an example. In the electrolysis of copper(II) sulfate solution using copper electrodes. The reaction that would proceed at the anode with inert electrodes would be:

4OH-(aq) - 4e 2H2O(l) + O2(g)

This requires migration of hydroxide ions, which are at a fairly low concentration, to the anode for liberation. The alternative possibility in the case of a copper anode, is for the following reaction to take place:

Cu(s) - 2e Cu2+(aq)

This second reaction has the advantage of not requiring any migration (the copper atoms are already there, they make up the electrode), and the reaction itself is energetically more advantageous. For both of these reaons, the second reaction takes preference.

Consequently, the following reactions occur:

At the anode (positive electrode): Cu(s) - 2e Cu2+(aq)

At the cathode: (negative electrode) Cu2+(aq) + 2e Cu(s)

The overall effect is for copper ions to be liberated as copper at one electrode at the same time as they are being produced by the other electrode. The concentration of copper(II) in solution does not change. However, the mass of copper at the anode decreases and the mass of copper at the cathode increases.

This is summarised in the following table:

Which metals can be Electroplated?

Any metal lower than hydrogen in the electrochemical series, i.e. with a positive electrode potential, can be deposited at the cathode during electrolysis. This means that these metals can be used to electroplate other substances. The requirements for successful electroplating are quite stringent in that the deposit of metal often fails to adhere successfully to the material of the cathode, unless the conditions are optimised.

Suitable electroplating metals include:

• Copper
• Chromium
• Nickel
• Silver