Metals displacement from solutions

ACTIVITY SERIES- Also referred to as the electromotive series or the displacement series, this is an arrangement of the metals (other elements can be included) in the order of their tendency to react with water and acids, so that each metal displaces from solution those below itiu the series and is displaced by those above it. See Table 1. Since the electrode potential of a metal in equilibrium with a solution of its ions cannot be measured directly, the values in the activity series are, in each case, the difference between the electrode potential of the given metal tor element) in equilibrium with a solution of its ions, and that of hydrogen in equilibrium with a solution of its ions. Thus in the table, it will be noted that hydrogen lias a value of 0.000. In experimental procedure, the hydrogen electrode is used as the standard with which the electrode potentials of other substances are compared. The theory of displacement plays a major role in electrochemistry and corrosion engineering. See also Corrosion and Electrochemistry. 

Tetrammino-platinous Hydroxide, [Pt(NH3)4](OH)2, the base of the series, is isolated by treating a solution of tlie sulphate with baryta. The liquid is filtered and evaporated in vacuo, when the hydroxide separates in white deliquescent needles. It is a strong base, absorbs carbon dioxide from the air, displaces ammonia from its salts, and precipitates metallic hydroxides from solution of their salts. It decomposes at 110° C., and ammonia is not liberated from it by potassium hydroxide. 

Electromotive Series of the Metals, (a) Place a few pieces of zinc in 5 cc. of 0.2 A solution of copper sulphate, shake the mixture frequently, and after about 15 minutes withdraw 1 cc. of the solution and test it for copper and for zinc ions, applying the information obtained from the preceding experiments. If any copper ions are still present let the remainder of the mixture stand for 15 minutes more with frequent shaking, and repeat the test. Continue until you have reached a conclusion as to whether copper ions can be completely displaced from solution by zinc. 

Finely divided copper has been used as a reducing agent for Fe(III). Activated copper,prepared by reducing copper(II) oxide with hydrogen, has been used to displace cadmium but not zinc from a cyanide solution. In cyanide solution, copper is a strong reducing agent (.F° = —1.09 V). Its formal potential hes between those of zinc (—1.26 V) and cadmium (—0.90 V) and several heavy metals (Pb, Bi, Sn, Ag, Hg) are displaced from solution. 

A strip of zinc metal was placed in a blue solution of copper(II) sulfate, CUSO4. The copper has been displaced from solution and has fallen to the bottom of the beaker. The resulting zinc sullate, ZnSO, solution is colorless. 

A metal displaces another metal ion from solution. Direct comparisons of metal reactivity are clearest in these reactions. For example, zinc metal displaces copper(II) ion from (actually reduces Cu " in) copper(II) sulfate solution, as the total ionic equation shows ... 

Can metals be displaced from solutions of their salts. ... 

It is sometimes possible to remove a metal such as copper from an inorganic solution by stirring in the presence of a more reactive metal such as zinc. The copper displaced from solution plates out on the surface of the zinc. An equivalent amount of zinc passes into solution, of course. 

Figure 4.15 A more reactive metal (Cu) displacing the ion of a less reactive metal (Ag )from solution.

The removal of a metal M from solution by chemical displacement it, cementation of a metal onto a more base metal M substrate) is widely used in hydrometallurgy ... 

If a cell is constructed with two metals dipping into solutions of their own ions, the metal which appears lower in the table is the positive pole and current flows inside the cell from the metal higher in the table to that lower. Thus metals with more negative potentials tend to displace from solution metals of more positive potential, e.g. in the Daniell cell (q.v.)... 

Immersion or displacement plating is the deposition of a metallic coating (M) on a metallic substrate (S) from a solution that contains the metal being plated, and the metal on the substrate is displaced by a metal ion from solution that has a lower standard electrode potential than the displaced metal ion. The displaced substrate material enters the solution in ionic form (S" ) and the metal ion (M" ) deposits onto the substrate in its place. 

PRACTICE EXAMPLE B Draw a voltaic cell in which silver ion is displaced from solution by aluminum metal. Label the cathode, the anode, and other features of the cell. Show the direction of flow of electrons. Also, indicate the direction of flow of cations and anions from a KN03(aq) salt bridge. Write an equation for the halfreaction occurring at each electrode, write a balanced equation for the overall cell reaction, and write a cell diagram. 

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Cementation. A metal can be removed from solution by displacing it with a mote active metal. This simple, inexpensive method has been commonly used to recover copper from dilute (1—3 kg/m ) solution using shredded iron and de-tinned iron cans as reducing agent. 

The capability of zinc to reduce the ions of many metals to theh metallic state is the basis of important appHcations. However, metals are removed from zinc solutions by displacement with finely divided zinc before winning by electrolysis. Gold and silver are displaced from cyanide leach solutions with zinc and the following metals are similarly recovered from various solutions platinum group, cadmium, indium, thallium, and sometimes copper. 

Immersion plating Displacement of metal ions in solution by metal to be coated + M2 - M 1 + yMf. where xn =ym M, Cu from aqueous CuSO M2 Fe... 

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

Zinc metal displaces copper ions from aqueous solution. The blue color signals the presence of copper ions. The color fades and copper metal appears as the reaction proceeds. 

Many other metal displacement reactions can be visualized, but not all of them occur. Some metals are oxidized readily, but others are highly resistant to oxidation. Likewise, some metal cations are highly susceptible to reduction, but others resist reduction. Zinc displaces copper ions from aqueous solutions, but copper will not replace zinc ions, because Cu is easier to reduce than Zn . Zinc will not displace ions, because... 

When a metal is immersed into the solution of salt of another metal farther to the right in the electromotive series, the first metal dissolves (is oxidized) while the second metal is deposited (its ions are reduced). Thus, the first metal displaces the second from its solution. 

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