Electrolysis of Copper(II) Sulfate, and Applications of Electrolysis

1Same solution, different electrodes

Electrolyse blue copper(II) sulfate solution with graphite electrodes and the colour slowly fades as pink copper builds up on the cathode and bubbles rise from the anode. Repeat with copper electrodes and the colour stays exactly the same, no gas appears, and the anode gets thinner while the cathode gets thicker. The second arrangement is how almost all the copper in electrical wiring is purified, and the same principle coats steel with chromium, zinc or tin.

2Copper(II) sulfate with inert electrodes

$$\text{cathode: }\mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{u}(\mathrm{s})\text{anode: }2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to {\mathrm{O}}_2(\mathrm{g})+4{\mathrm{H}}^{+}(\mathrm{a}\mathrm{q})+4{\mathrm{e}}^{-}\backslash text\{cathode:\; \}\; \backslash mathrm\{Cu^\{2+\}(aq)\; +\; 2e^\{-\}\; \backslash rightarrow\; Cu(s)\}\; \backslash qquad\; \backslash text\{anode:\; \}\; \backslash mathrm\{2H\_2O(l)\; \backslash rightarrow\; O\_2(g)\; +\; 4H^\{+\}(aq)\; +\; 4e^\{-\}\}$$

$$\text{overall: }2\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})+2{\mathrm{H}}_2\mathrm{O}(\mathrm{l})\to 2\mathrm{C}\mathrm{u}(\mathrm{s})+{\mathrm{O}}_2(\mathrm{g})+2{\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{a}\mathrm{q})\backslash text\{overall:\; \}\; \backslash mathrm\{2CuSO\_4(aq)\; +\; 2H\_2O(l)\; \backslash rightarrow\; 2Cu(s)\; +\; O\_2(g)\; +\; 2H\_2SO\_4(aq)\}$$

$\mathrm{C}{\mathrm{u}}^{2+}\backslash mathrm\{Cu^\{2+\}\}$ is reduced in preference to water ($E^{\circ }=+0.34E^\{\backslash circ\}\; =\; +0.34$ V), and sulfate is not oxidised, so water is. As $\mathrm{C}{\mathrm{u}}^{2+}\backslash mathrm\{Cu^\{2+\}\}$ is removed, the blue colour fades, and the solution becomes sulfuric acid.

3Copper(II) sulfate with copper electrodes

The copper anode is oxidised more easily than water, so the anode itself dissolves:

$$\text{anode: }\mathrm{C}\mathrm{u}(\mathrm{s})\to \mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\text{cathode: }\mathrm{C}{\mathrm{u}}^{2+}(\mathrm{a}\mathrm{q})+2{\mathrm{e}}^{-}\to \mathrm{C}\mathrm{u}(\mathrm{s})\backslash text\{anode:\; \}\; \backslash mathrm\{Cu(s)\; \backslash rightarrow\; Cu^\{2+\}(aq)\; +\; 2e^\{-\}\}\; \backslash qquad\; \backslash text\{cathode:\; \}\; \backslash mathrm\{Cu^\{2+\}(aq)\; +\; 2e^\{-\}\; \backslash rightarrow\; Cu(s)\}$$

Copper is simply transferred from anode to cathode. For every $\mathrm{C}{\mathrm{u}}^{2+}\backslash mathrm\{Cu^\{2+\}\}$ ion leaving the solution at the cathode, one enters at the anode, so the concentration and colour do not change, and with pure electrodes the anode loses exactly the mass the cathode gains.

4Applications

1. Refining copper. Copper from smelting (about 99 % pure) conducts electricity poorly. It is cast into anodes; thin sheets of pure copper (or stainless steel) are the cathodes; the electrolyte is copper(II) sulfate in sulfuric acid.

Figure 13.3: Electrolytic refining of copper.

More reactive impurities (zinc, iron, nickel) are oxidised and go into solution, but they are not deposited, because $\mathrm{C}{\mathrm{u}}^{2+}\backslash mathrm\{Cu^\{2+\}\}$ is reduced more easily. Less reactive metals (silver, gold, platinum) are not oxidised; they fall as anode sludge, which is collected and refined, often paying for much of the process. The cathode copper is about 99.99 % pure.

2. Electroplating. The object to be plated is the cathode; the anode is usually the plating metal; the electrolyte contains its ions. Chromium on taps and bicycle parts resists corrosion and looks bright; tin on steel food cans is non-toxic; silver and gold on cutlery and jewellery look attractive; zinc on steel protects it from rust. The object must be clean and grease-free, and a small current gives an even, adherent coat.

3. Extraction of reactive metals. Metals above zinc in the reactivity series cannot be obtained by reducing their oxides with carbon economically, so they are made by electrolysis of molten compounds: sodium from molten NaCl, magnesium from molten $\mathrm{M}\mathrm{g}\mathrm{C}{\mathrm{l}}_2\backslash mathrm\{MgCl\_2\}$, and aluminium from aluminium oxide dissolved in molten cryolite at about 950 °C (S6 Unit 2).

4. The chlor-alkali industry (Lesson 2) and hydrogen from water.

5. Anodising. Aluminium made the anode in dilute sulfuric acid grows a thick oxide layer, which resists corrosion and can be dyed, as on window frames and cooking pots.