Question

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

Short answer

(a) nitrogen dioxide

Step-by-step solution

Understanding the Reaction with Dilute Nitric Acid

To approach this problem, we need to understand how copper reacts with nitric acid. When copper reacts with dilute nitric acid, it forms copper nitrate, water, and nitrogen oxide (NO). The balanced chemical equation is: \[ \text{3 Cu} + \text{8 HNO}_3 \rightarrow \text{3 Cu(NO}_3\text{)}_2 + \text{2 NO} + \text{4 H}_2\text{O} \]

Analyzing Reaction with Concentrated Nitric Acid

Now, let's consider the reaction of copper with concentrated nitric acid instead of dilute acid. When concentrated nitric acid is used, nitrogen dioxide (NO₂) is produced instead of nitrogen oxide. The balanced chemical reaction is: \[ \text{Cu} + \text{4 HNO}_3 \rightarrow \text{Cu(NO}_3\text{)}_2 + \text{2 NO}_2 + \text{2 H}_2\text{O} \]

Identifying the Correct Option

Given the reaction of copper with concentrated nitric acid, we see that the main gaseous product is nitrogen dioxide (NO₂). Based on the options provided in the exercise, option (a) nitrogen dioxide is the correct answer.

Key concepts

Copper Reactions

Copper is a relatively unreactive metal, meaning it usually does not react with plain water or oxygen unless other factors are involved. However, when it comes into contact with acids, it behaves quite differently. With nitric acid, whether dilute or concentrated, copper undergoes a chemical transformation resulting in the formation of copper nitrate, water, and a gaseous nitrogen compound.

When reacting with acids, the transformation is not just a simple displacement reaction. Instead, a redox reaction occurs where oxidation and reduction happen simultaneously. Copper gets oxidized from its metallic state (Cu) to a copper ion \( ext{(Cu}^{2+}\)) forming copper nitrate, while nitrogen in the nitric acid gets reduced to form various nitrogen oxides, depending on the concentration of the acid involved.

• In dilute nitric acid, nitrogen oxide or NO is formed.
• In concentrated nitric acid, nitrogen dioxide or NO₂ is produced.

Dilute Nitric Acid

Dilute nitric acid, as the name suggests, contains a lower concentration of nitric acid in water. When copper is exposed to dilute nitric acid, a visible reaction takes place marked by effervescence due to the release of a gas.

The chemical reaction between copper and dilute nitric acid is an example of a redox reaction. In this process, copper donates electrons and reacts with nitric acid producing copper nitrate and colorless nitrogen monoxide (NO), also called nitric oxide.

The balanced chemical process is:
\[ 3 ext{Cu} + 8 ext{HNO}_3 ightarrow 3 ext{Cu(NO}_3)_2 + 2 ext{NO} + 4 ext{H}_2 ext{O} \]
This reaction is not only a great demonstration of copper's reactive capabilities with acids but also serves as a textbook example for students learning about redox processes in their chemistry studies.

Concentrated Nitric Acid

Concentrated nitric acid is a potent oxidizing agent, more concentrated than the dilute form, capable of influencing the course of copper's reaction. In the presence of this highly potent acid, copper undergoes an intense reaction that not only leads to the formation of copper nitrate but also produces a vivid brown gas, nitrogen dioxide NO₂.

The distinctive feature of concentrated nitric acid is its ability to oxidize copper more vigorously, resulting in nitrogen dioxide as a byproduct. This gas is responsible for the characteristic brown fumes observed during the reaction.

The specific balanced reaction for concentrated nitric acid with copper is:
\[ ext{Cu} + 4 ext{HNO}_3 ightarrow ext{Cu(NO}_3)_2 + 2 ext{NO}_2 + 2 ext{H}_2 ext{O} \]
The nitrogen dioxide gas produced reflects the highly oxidative environment created by concentrated nitric acid prompting quick evolution of the gas.

Nitrogen Dioxide Formation

The formation of nitrogen dioxide NO₂ in reactions involving concentrated nitric acid is an important educational model to illustrate gas evolution reactions in chemistry. When the concentrated acid reacts with copper, the resultant nitrogen dioxide is observed as brown gas.

The significance of the formation of NO₂ doesn't just stop at its visual presentation. It’s a proof of the oxidizing nature of concentrated nitric acid. This gas serves as an oxidizing agent with various industrial applications, although in this reaction its formation mostly serves as an identification marker for the reactive conditions present.

• The brown gas signifies the strong oxidizing environment.
• Nitrogen dioxide’s production confirms concentrated nitric acid reactivity.

Nitric Acid Reactions

Nitric acid, known chemically as \( ext{HNO}_3 \), is a powerful acid commonly used in laboratories for conducting redox reactions. Excitingly reactive, it forms various nitrogen oxides when it reacts with metals like copper.

The type of nitrogen oxide formed in the reaction relies heavily on the concentration of the acid used. While dilute nitric acid results in nitrogen oxide (NO), concentrated nitric acid produces nitrogen dioxide (NO₂).

• These reactions are vital in demonstrating how concentration affects chemical reactions.
• Understanding these reactions helps in comprehending the properties of reactive environments in chemistry.

Moreover, the study of nitric acid reactions broadens understanding of acid-base reactions and redox chemistry, fundamental principles in chemistry education. These reactions serve as a practical application of theoretical principles taught in schools, helping students connect theory with real-world chemical behavior.