Sulfuric acid reacts differently with metals depending on factors such as metal type, acid concentration, and temperature. Dilute acid reacts with metals above hydrogen in reactivity series, while some metals form a protective oxide layer. Solubility of the metal sulfate formed and water content also affect the reaction. Metals below hydrogen do not react with dilute acid, but concentrated acid can act as an oxidizing agent when heated. Gold and platinum do not react with sulfuric acid.
The effects of sulfuric acid on the metal depend on a number of factors including the type of metal, the concentration of the acid and the temperature. Dilute sulfuric acid will, in theory, react with any metal above hydrogen in the series of reactivity to displace the hydrogen from the acid, releasing it as a gas, and forming the metal’s sulfate salt. Metals that fall into this category include alkali metals, such as sodium and potassium, and alkaline earth metals, such as magnesium and calcium, as well as many other common metals, such as iron, nickel, and zinc. Because hydrogen has very low solubility in water and acids, it will produce bubbles; the resulting effervescence is greater with the more reactive metals. For example, dilute sulfuric acid (H2SO4) and magnesium react vigorously: Mg + H2SO4 -> MgSO4 + H2.
In practice, not all of these metals will react with sulfuric acid under normal circumstances. Although pure metals react, some elements quickly acquire an oxide layer when exposed to air. The effects of this acid on metal oxides vary, but in some cases the oxide layer is chemically very inert and prevents any reaction from taking place. For example, although titanium ranks above hydrogen in the series of reactivity, it normally has a thin coating of titanium dioxide making it unreactive towards sulfuric acids and most others. Aluminum also forms a protective oxide layer; however, the sulfuric acid and aluminum will react after some time to produce hydrogen gas and aluminum sulfate.
Another factor that can affect the combination is the solubility of the salt, or metal sulfate, formed by the reaction. Some metal sulphates, such as those of iron, zinc and aluminum, are very soluble in water or acids while others, such as calcium and barium sulphates, are not. When the sulfate has low solubility, the reaction will slow down or stop rapidly as a protective layer of sulfate builds up around the metal.
Pure sulfuric acid does not react with metals to produce hydrogen, as the presence of water is required to enable this reaction. The concentrated sulfuric acid used in laboratories is normally 98% acid and 2% water; the small amount of water present allows these reactions to proceed in some cases, albeit slowly. If a more diluted solution is used, the reaction is much faster. Stainless steel, at low temperatures, is not significantly corroded by acid at concentrations above approximately 98%. In industrial plants it is sometimes stored in steel tanks; however, corrosion is rapid if the water content is higher.
The effect of sulfuric acid on metallic elements that are below hydrogen in the series of reactivity is different, as they cannot displace hydrogen from the acid. These metals include copper, mercury, silver, gold and platinum. They will not react with dilute sulfuric acid or concentrated acid at room temperature.
Concentrated sulfuric acid, on the other hand, acts as an oxidizing agent when heated and this causes it to react with copper, mercury and silver. In the case of copper, for example, the following reaction takes place: Cu + 2H2SO4 -> CuSO4 + SO2 + 2H2O. Gold and platinum do not react with sulfuric acid at all.
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