Oxidation of Metals by Acids and Salts

The reaction between a metal and either an acid or a metal salt conforms to the general pattern 

These reactions are called displacement reactions because the ion in solution is displaced (replaced) through oxidation of an element. 

Many metals undergo displacement reactions with acids, producing salts and hydrogen gas. For example, magnesium metal reacts with hydrochloric add to form magnesium chloride and hydrogen gas (A FIGURE 4.13)  

The oxidation number of Mg changes from 0 to +2, an increase that indicates the atom has lost electrons and has therefore been oxidized. The oxidation number of in the acid decreases from +1 to 0, indicating that this ion has gained electrons and has therefore been reduced. Chlorine has an oxidation number of — 1 both before and after the reaction, indicating that it is neither oxidized nor reduced. In fact the CC ions are spectator ions, dropping out of the net ionic equation  

Metals can also be oxidized by aqueous solutions of various salts. Iron metal, for example, is oxidized to Fe by aqueous solutions of NP such as Ni(N03)2(flq)  

The oxidation of Fe to Fe in this reaction is accompanied by the reduction of Ni to Ni. Remember Whenever one substance is oxidized, another substance must be reduced. 

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Although catalytic hydration of ethylene oxide to maximize ethylene glycol production has been studied by a number of companies with numerous materials patented as catalysts, there has been no reported industrial manufacture of ethylene glycol via catalytic ethylene oxide hydrolysis. Studied catalysts include sulfonic acids, carboxyUc acids and salts, cation-exchange resins, acidic zeoHtes, haUdes, anion-exchange resins, metals, metal oxides, and metal salts (21—26). Carbon dioxide as a cocatalyst with many of the same materials has also received extensive study. 

The first product of the oxidation of alcohol is acetaldehyde and an important end-product is fulminic add, which latter can, however, only be isolated if silver or mercury ions are present. With these ions it forms salts—fulminates—which are stable towards nitric add in them, it must be presumed, the linkage with the metal is homopolar and non-ionogenic, as in mercuric cyanide. The formation of fulminic acid takes place because the carbonyl group of the aldehyde confers reactivity on the adjacent methyl group which then forms a point of attack for the nitrous acid. The various stages in the process are indicated by the following formulae ... 

Acidity and basicity are paired concepts that are very often invoked to explain the catalytic properties of divided metal oxides and zeolites. The concept of acids and bases has been important since ancient times. It has been used to correlate large amounts of data and to predict trends. During the early development of acid-base theory, experimental observations included the sour taste of acids and the bitter taste of bases, color changes in indicators caused by acids and bases, and the reaction of acids with bases to form salts. 

Metavanadates of the alkalis are white or colourless, and give colourless aqueous solutions which rapidly become yellow, and, on addition of acids, red or orange. These coloured solutions contain polyvanadates, the formation of which is comparable to that of the polychromates and other salts formed by condensation of weakly acid oxides of metals, e.g. molybdates and borates. Thus, under definite conditions of temperature and concentration, potassium metavanadate is converted into the acid salt 2K20.3Va05, in accordance with the equation ... 

Heteropoly catalysts have significant activities for the oxidation of isobutane into methacrolein and methacrylic acid. The yield increased up to 6% by vanadium substitution or salt formation, as follows. With Cs2.5Ni0.08H0.34+JrPV,Mo12 - O40, the highest conversion and selectivity were observed at x 1 (355). Increases in the reaction temperature to 613 K led to increased yields, up to 9.0%. A similar increase in the yield resulted from the substitution of As for P as a heteroatom or from the addition of various transition metals (106, 356). 

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