D42.2 Electroplating

An important application for electrolytic cells is electroplating, which forms a thin coating of metal on top of a conducting surface. Metals typically used in electroplating include cadmium, chromium, copper, gold, nickel, silver, and tin. As an example of electroplating, consider how silver-plated tableware is produced in the setup shown below.

This figure contains a diagram of an electrochemical cell. One beakers is shown that is just over half full. The beaker contains a clear, colorless solution that is labeled “A g N O subscript 3 ( a q ).” A silver strip is mostly submerged in the liquid on the left. This strip is labeled “Silver (anode).” The top of the strip is labeled with a red plus symbol. An arrow points right from the surface of the metal strip into the solution to the label “A g superscript plus” to the right. A spoon is similarly suspended in the solution and is labeled “Spoon (cathode).” It is labeled with a black negative sign on the tip of the spoon’s handle above the surface of the liquid. An arrow extends from the label “A g superscript plus” to the spoon on the right. A wire extends from the top of the spoon and the strip to a rectangle labeled “Voltage source.” An arrow points upward from silver strip which is labeled “e superscript negative.” Similarly, an arrow points down at the right to the surface of the spoon which is also labeled “e superscript negative.” A plus sign is shown just outside the voltage source to the left and a negative is shown to its right.
Figure: Silver-plating. The metal spoon is connected to the negative terminal of the voltage source and acts as the cathode. The anode is a silver electrode. Both electrodes are immersed in a silver nitrate solution. When electric current is passed through the solution, the net result is that silver metal is removed from the anode and deposited on the spoon.

The anode consists of a silver electrode. The cathode is a spoon made from a less expensive metal. Both electrodes are immersed in a solution of silver nitrate. As the potential from the voltage source is increased, current starts to flow.

Silver metal is lost at the anode as Ag+ goes into solution:

anode:          Ag(s) ⟶ Ag+(aq) + e–

The mass of the cathode increases as Ag+ ions from the solution are deposited onto the spoon:

cathode:          Ag+(aq) + e– ⟶ Ag(s)

The net result is the transfer of silver metal from the anode to the cathode. The quality of the electroplated object depends on the thickness of the deposited silver and the rate of deposition.

Quantitative Aspects of Electrolysis

The quantity of current that flows in an electrolytic cell is dictated by the amount (moles) of electrons transferred in a redox reaction, which is in turn related to quantities of reactants and products via reaction stoichiometry. Recall that current, I, is related to the total charge, Q, by:

I = \dfrac{Q}{t} \;\;\;\;\; \left(\text{SI units:}\;\ A = \dfrac{C}{s}\right)

Hence:

Q = (amount of e– transferred) × F = I × t

where F is the Faraday constant.

Exercise: Electroplating

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