The chemical reaction of an acid with a metal is specific for these classes of compounds. In its course, the hydrogen proton is reduced and, in conjunction with the acid anion, is replaced by a metal cation. This is an example of a salt-forming reaction, although there are several types of interactions that do not obey this principle. They proceed as redox and are not accompanied by the release of hydrogen.
Principles of reactions of acids with metals
All reactions of an inorganic acid with a metal lead to the formation of salts. The exception is, perhaps, only the reaction of a noble metal with aqua regia, a mixture of hydrochloric and nitric acid. Any other interaction of acids with metals leads to the formation of salt. If the acid is neither concentrated sulfuric nor nitric, then molecular hydrogen is cleaved as a product.
But when concentrated sulfuric acid enters the reaction, the interaction with metals proceeds according to the principle of the redox process. Therefore, two types of interactions of typical metals and strong inorganic acids were experimentally identified:
- the interaction of metals with dilute acids;
- interaction with concentrated acid.
The reactions of the first type proceed with any acid. The exception is only concentrated sulfuric acid and nitric acid of any concentration. They react according to the second type and lead to the formation of salts and reduction products of sulfur and nitrogen.
Typical interaction of acids with metals
Metals to the left of hydrogen in the standard electrochemical series react with dilute sulfuric acid and other acids of various concentrations except nitric acid to form a salt and release molecular hydrogen. Metals located to the right of hydrogen in the series of electronegativity cannot react with the above acids and interact only with nitric acid, regardless of its concentration, with concentrated sulfuric acid and with aqua regia. This is a typical interaction of acids with metals.
Reactions of metals with concentrated sulfuric acid
With a sulfuric acid content in the solution of more than 68%, it is considered concentrated and interacts with the metals to the left and right of hydrogen. The principle of the reaction with metals of various activity is shown in the photo below. Here, the oxidizing agent is the sulfur atom in the sulfate anion. It is reduced to hydrogen sulfide, a 4-valent oxide, or to molecular sulfur.
Dilute nitric acid reactions
Diluted nitric acid reacts with metals to the left and right of hydrogen. During the reaction with active metals, ammonia is formed, which immediately dissolves and interacts with the nitrate anion, forming another salt. Acid reacts with metals of medium activity with the release of molecular nitrogen. With low activity, the reaction proceeds with the release of 2-valent nitric oxide. Most often, several sulfur reduction products are formed in a single reaction. Examples of reactions are proposed in the graphic application below.
Concentrated nitric acid reactions
In this case, nitrogen also acts as an oxidizing agent. All reactions end with salt formation and nitric oxide evolution . Schemes for the occurrence of redox reactions are proposed on a graphical application. At the same time, the reaction of aqua regia with inactive elements deserves special attention. This interaction of acids with metals is not specific.
Metal reactivity
Metals react with acids willingly, although there are several inert substances. These are noble metals and elements having a high standard electrochemical potential. There are a number of metals, which is built on the basis of this indicator. It is called a series of electronegativity. If the metal is in it to the left of hydrogen, then it is able to react with dilute acid.
There is only one exception: iron and aluminum, due to the formation of trivalent oxides on their surface, cannot react with acid without heating. If the mixture is heated, then initially the oxide film of the metal enters the reaction, and then it dissolves in the acid itself. Metals located to the right of hydrogen in the electrochemical series of activity cannot react with inorganic acid, including dilute sulfuric acid. There are two exceptions to the rules: these metals dissolve in concentrated and diluted nitric acid and aqua regia. Only rhodium, ruthenium, iridium, and osmium cannot be dissolved in the latter.