Copper displacement deposition

In your previous chemistry course, you compared the reactivities of metals. You may recall that, when a piece of zinc is placed in an aqueous solution of copper(II) sulfate, the zinc displaces the copper in a single displacement reaction. This reaction is shown in Figure 10.1. As the zinc dissolves, the zinc strip gets smaller. A dark red-brown layer of solid copper forms on the zinc strip, and some copper is deposited on the bottom of the beaker. The blue colour of the solution fades, as blue copper(ll) ions are replaced by colourless zinc ions. 

This reaction is called displacement deposition, because the nickel ions in solution simply displace the silicon at the surface. The substrate. Si, acts here as a reducing agent, as discussed in Chapter 9. Copper may be deposited on Si from HF acid solutions (69). In the presence of HF, Si is oxidized into [SiF5] . 

The kinetics and mechanisms of the displacement deposition of Cu on a Zn substrate in alkaline media was studied by Massee and Piron (5). They determined that at the beginning of the deposition process, the rate is controlled by activation. The activation control mechanism changes to diffusion control when the copper covers enough of the Zn surface to facilitate further deposition of copper. This double mechanism can explain the kinetic behavior of the deposition process. 

Reaction (9.14) is a very important displacement deposition reaction in the printed circuit industry. It is used to help the soldering capability of copper. 

However, aluminium which is more electropositive than copper does not displace copper from copper salt solutions, due to the presence of a very stable film of aluminium oxide on the surface of the aluminium which has a lower electrode potential. This oxide film can be destroyed by the presence of added chlorine ions. Thus, in the presence of added chloride ions in the solution, copper is deposited and the aluminium dissolves. 

When metallic zinc is introduced into a solution of copper dichloride, the green colour of the solution slowly fades, and a red layer of copper is deposited on the zinc. The zinc has evidently displaced the copper from the solution of its chloride ... 

Even not recognized as such, the galvanic displacement deposition of noble metals such as Au or Ag onto Fe, Zn, Cu, or similar substrates is known since the times of early Mediterranean cultures and, possibly, before. In the sixteenth century, the recovery of copper from copper mine waters by contacting dilute process streams with iron scrap was successfully achieved [2]. Since that time, many different galvanic displacement deposition processes have been developed. Examples used on industrial scale include application of aluminum, iron, or zinc powders for the removal of copper, silver, gold, or other noble metals from waste solutions. Similar approaches are used for the solution purification in hydrometallurgical plants, electronics, electrochemical experiments, etc. 

Considering that during the galvanic displacement deposition of copper the aluminum substrate is dissolved, alternatively, the rate of this process can be studied in the following way. Due to dissolution of aluminum, the concentration of Al(III) ions in the solution should increase with time. Eventually, complete aluminum substrate could be dissolved under proper conditions, during the deposition of copper. By applying Faraday s law to Eq. (9.8), the concentration of Al(III)... 

Let s recall that with ammonia, Cu + gives the deep blue cation complex hexam-mine copper(II) (cupritetrammine). In analytical toxicology, Cu° is searched for in the liquid resulting from the sulfonitric mineralization and is identified in it with ammonia. Because of its weak concentration in the mineralization liquid, Cu " " is first displaced by metallic iron, on which it deposits in the form of a red coating of metallic copper. The deposit is purely and simply the result of the following redox reaction ... 

In the electrochemical series of elements, copper is near the noble end and will not normally displace hydrogen, even from acid solutions. Indeed, if hydrogen is bubbled through a solution of copper salts, copper is slowly deposited (more rapidly if the process is carried out under pressure). (See Section 1.2 for thermodynamic considerations.)... 

These have been developed for special uses. For example, since petroleum-based materials harm natural rubber, a grease based on castor oil and lead stearate is available for use on the steel parts of rubber bushes, engine mountings, hydraulic equipment components, etc. (but not on copper or cadmium alloys). Some soft-film solvent-deposited materials have water-displacing properties and are designed for use on surfaces which cannot be dried properly, e.g. water-spaces of internal combustion engines and the cylinders or valve chests of steam engines. 

Zinc metal reacts spontaneously with an aqueous solution of copper sulfate when they re placed in direct contact. Zinc, being a more reactive metal than copper (it s higher on the activity series of metals presented in Chapter 8), displaces the copper ions in solution. The displaced copper deposits itself as pure copper metal on the surface of the dissolving zinc strip. At first, the reaction may appecir to be a simple single replacement reaction, but it s also a redox reaction. 

---