Iron, a common metal with the chemical symbol Fe, and nitric acid, a highly corrosive mineral acid with the formula \(\text{HNO}_3\), react in a manner that is highly dependent on the conditions of their interaction. The outcome of mixing iron and nitric acid is not a single, predictable result but a complex chemical process that can be either vigorous or completely suppressed. The reaction is a classic example of a redox (reduction-oxidation) process. Understanding the specific concentration of the acid and the resulting surface chemistry on the iron is necessary to determine if the reaction will proceed energetically or halt immediately.
Reaction with Dilute Nitric Acid
When iron is exposed to dilute nitric acid, a swift and energetic chemical reaction occurs, where the iron readily dissolves. This interaction is a strong oxidation-reduction process, with the iron metal being oxidized and the nitrogen in the nitric acid being reduced. The iron typically loses electrons to form iron(III) nitrate, a soluble salt in the solution, though under certain conditions, iron(II) nitrate can also form.
This redox reaction generates various gaseous byproducts, such as nitric oxide (\(\text{NO}\)) or nitrous oxide (\(\text{N}_2\text{O}\)), depending on the acid’s exact dilution and temperature. The production of nitric oxide gas is often observed as colorless bubbles rising from the reaction mixture. This colorless gas reacts instantly with oxygen in the air upon release, forming brown nitrogen dioxide (\(\text{NO}_2\)) fumes.
The reaction is exothermic, meaning it releases heat energy, which can accelerate the reaction rate. The overall process is characterized by the consumption of the iron metal as it is converted into a dissolved salt. The vigorous dissolution continues until one of the reactants is completely consumed.
The Phenomenon of Passivation
A dramatically different result occurs when iron is introduced to concentrated nitric acid, a phenomenon known as chemical passivation. In this scenario, the iron metal becomes chemically inert, effectively stopping any further reaction with the acid. This passivity is caused by the formation of an extremely thin, dense, and non-porous layer of metal oxide on the iron’s surface.
The concentrated nitric acid acts as a powerful oxidizing agent, rapidly converting the surface layer of the iron into a protective film, primarily consisting of iron(III) oxide (\(\text{Fe}_2\text{O}_3\)). This oxide layer is insoluble in the concentrated acid and adheres tightly to the underlying metal.
This protective oxide film acts as an impenetrable barrier, physically separating the bulk iron metal from the highly corrosive acid. Since the acid molecules can no longer make direct contact with the underlying iron atoms, the oxidation process is instantly halted. The iron is rendered “passive” and will not dissolve.
The iron remains in this passive state even if the concentrated acid is subsequently diluted. This induced passivity is a property shared with other metals like chromium and aluminum when exposed to concentrated nitric acid.
How Acid Concentration Determines the Outcome
The concentration of the nitric acid is the primary factor that dictates the reaction pathway between the acid and iron. The outcome is a delicate balance between the rate of iron dissolution and the rate of formation of the protective oxide layer. Nitric acid has a modest oxidizing strength when dilute but becomes a potent oxidizing agent at high concentrations.
Concentrated Acid
Highly concentrated acid promotes the rapid formation of the dense, protective iron oxide film on the metal surface. The strong oxidizing power of concentrated \(\text{HNO}_3\) quickly converts the surface iron atoms into the stable oxide before the acid can dissolve the metal. This quick sealing of the surface leads to immediate passivation, halting the reaction.
Dilute Acid
Conversely, dilute nitric acid has a weaker oxidizing capability, allowing the acid to attack and dissolve the iron more quickly than a stable protective layer can form. In this case, the acid’s ability to dissolve the metal dominates over its ability to passivate it. This results in the vigorous, exothermic dissolution of the iron and the release of nitrogen oxide fumes.
The threshold between these two outcomes typically falls around a medium concentration, where the reaction can sometimes start vigorously before the iron passivates, or it can proceed as a vigorous reaction depending on factors like temperature and the presence of trace impurities. The vigorous reaction with dilute acid is hazardous due to the heat generated and the release of toxic nitrogen oxide gases, highlighting the importance of concentration control.