Copper forms nitrogen oxide with dilute nitric acid but with concentrated nitric acid it gives: (a) Nitrogen dioxide (b) Nitrogen oxide (c) Nitrogen peroxide (d) Nitrogen trioxide

Understanding the formation of nitrogen dioxide (NO₂) during the reaction of copper with nitric acid is crucial for students studying chemistry. When copper is introduced to nitric acid, a vigorous reaction occurs. With dilute nitric acid, copper predominantly forms nitrogen monoxide (NO), but with concentrated nitric acid, the reaction is more intense, leading to the production of nitrogen dioxide.

Nitrogen dioxide is a red-brown gas known for its characteristic sharp, biting odor and is a significant air pollutant that can be hazardous to health. The formation of nitrogen dioxide during the reaction can be identified by the visible brown fumes that are released. The chemical equation representing this reaction is:





Cu(s) + 4HNO₃(conc.) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)
Using this equation, students can see how copper (Cu), when combined with concentrated nitric acid (HNO₃), transforms to produce copper nitrate (Cu(NO₃)₂), nitrogen dioxide (NO₂), and water (H₂O).

Copper is a versatile metal that undergoes various types of chemical reactions. When reacting with nitric acid, copper demonstrates different behaviors depending upon the concentration of the acid. With dilute acid, as mentioned earlier, it forms nitrogen monoxide. However, with concentrated nitric acid, it forms nitrogen dioxide.

This difference is not just in the nitrogen oxides produced but also in the substances' oxidation states during the reaction. Nitric acid acts as an oxidizing agent, and the copper's oxidation state changes from 0 in the metal form to +2 in copper nitrate. The more concentrated the nitric acid is, the more pronounced the oxidizing power, leading to more complex nitrogen oxides like NO₂. This behavior highlights copper's ability to take part in redox reactions, which is essential for students to understand as it helps explain many biological and geological processes where copper plays a role.

Moreover, the reaction demonstrates the law of conservation of mass, as there is a balance between the reactants' mass and the total mass of products formed. This is a pivotal concept in chemical reactions, and keeping track of each element's presence in the reactants and products is a key skill in chemistry.

The concentration of nitric acid is a determining factor in the outcome of its reaction with copper. This is because nitric acid can behave differently when in a concentrated form compared to a dilute form. Concentrated nitric acid has a higher purity and less water, making it a stronger oxidizing agent capable of producing more nitrogen dioxide, as opposed to the less aggressive nitrogen monoxide that forms with dilute acid.

Students should recognize that 'dilute' generally implies the acid is mixed with more water, reducing its reactivity and concentration of hydrogen ions (H⁺). This diminished concentration leads to a slower reaction rate and alters the nature of the products. The copper-catalyzed decomposition of nitric acid also varies in temperature between the reactions with dilute and concentrated forms. This is an example of how reaction conditions can dramatically change chemical behavior, an essential principle in the study of chemistry.

Understanding this distinction is vital in laboratory work and industrial processes, where the wrong concentration of an acid can lead to unexpected results or even dangerous incidents. Therefore, grasping the effects of concentration on chemical reactions is an indispensable part of a chemist's education.