Question

Explain why, under normal conditions, the reaction of zinc with nitric acid does not produce hydrogen.

Short answer

Nitric acid's oxidizing nature prevents hydrogen gas production, forming nitrogen oxides instead.

Step-by-step solution

Identify reactants and products

In the reaction between zinc (Zn) and nitric acid (HNO₃), the reactants are zinc and nitric acid. Typically, when metals react with acids, hydrogen gas (H₂) is a potential product. However, we need to consider the products specific to this reaction.

Consider the concentration of nitric acid

Nitric acid is a strong oxidizing agent, especially in concentrated form. Unlike other acids, it can lead to more complex reactions than just forming hydrogen gas. The concentration of HNO₃ is crucial in determining the products of this reaction.

Identify the oxidation-reduction nature

Zinc is a reducing agent, and nitric acid acts as an oxidizing agent. When zinc reacts with nitric acid, the nitric acid is reduced instead of simply donating hydrogen ions. As a result, instead of producing hydrogen gas, other nitrogen oxides, such as NO or N₂O, are formed.

Balance the reaction equation

In a typical reaction of zinc with nitric acid, the balanced equation is Zn + 4HNO₃ → Zn(NO₃)₂ + 2H₂O + 2NO₂. As shown, nitrate ions are reduced to NO₂ rather than producing H₂ gas.

Conclusion

The reaction of zinc with nitric acid does not produce hydrogen gas because the strong oxidizing properties of nitric acid lead to the reduction of nitrate ions instead, resulting in nitrogen oxides as products.

Key concepts

Zinc and Nitric Acid Reaction

In typical metal-acid reactions, metals like zinc displace hydrogen ions to form hydrogen gas. However, when zinc reacts with nitric acid, the situation is different. Nitric acid is not just a regular acid; it is a powerful oxidizing agent. This means it can cause oxidation-reduction reactions, changing the standard outcome. During the reaction between zinc and nitric acid, zinc acts as a reducing agent. It donates electrons, which in turn, reduces the nitric acid. The key distinction here is the ability of nitric acid to accept electrons, unlike other acids that primarily release hydrogen ions. When zinc gives away its electrons, these electrons do not lead to the formation of hydrogen gas. Instead, they participate in the reduction of nitrate ions present in the nitric acid. This process leads to the creation of nitrogen oxides instead of hydrogen gas. This peculiar behavior of nitric acid distinguishes it from other acids when reacting with metals like zinc.

Concentration of Nitric Acid

The concentration of nitric acid plays a crucial role in determining the path of the chemical reaction with zinc. Nitric acid can exist in various concentrations, ranging from dilute to highly concentrated forms. Each concentration level affects the way it interacts with zinc.

• In concentrated form, nitric acid acts more aggressively as an oxidizing agent, which enhances its ability to attract electrons from zinc.
• With increased concentration, the likelihood of forming complex nitrogen oxides rather than simple hydrogen gas increases.
• Dilute nitric acid can still oxidize zinc but may lead to different nitrogen-based products.

Concentrated nitric acid is key to the formation of nitrogen oxides over hydrogen gas. By accepting electrons, concentrated nitric acid mainly undergoes reduction, preventing hydrogen ion release as free hydrogen gas.

Nitrogen Oxides Formation

When zinc reacts with nitric acid, the primary products formed are nitrogen oxides. These compounds are a result of the reduction of nitrate ions by the electrons provided by zinc. Let's look into how this transformation occurs. Nitric acid contains nitrate ( NO₃⁻ ) ions, which are reduced during the reaction. Instead of releasing hydrogen gas, these ions form nitrogen oxides, such as nitrogen dioxide ( NO₂ ) and sometimes other species like nitrous oxide ( N₂O ) or nitric oxide ( NO ).

• **Nitrogen Dioxide ( NO₂ )**: Often appears in the reaction as a brown gas. It's the most common oxide formed.
• **Nitric Oxide ( NO )**: Produced under certain conditions, useful in industrial processes.
• **Nitrous Oxide ( N₂O )**: Occasionally generated, known as "laughing gas."

Each oxide has distinct properties and forms based on reaction conditions and nitric acid concentration. This highlights the complexity of the oxidation-reduction reaction between zinc and nitric acid.