Problem 15 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following combination will produce \(\mathrm{H}_{2}\) gas? (a) Fe metal and conc. \(\mathrm{HNO}_{3}\) (b) Cu metal and conc. \(\mathrm{HNO}_{3}\) (c) \(\mathrm{Zn}\) metal and \(\mathrm{NaOH}(\mathrm{aq})\) (d) Au metal and \(\mathrm{NaCN}(\mathrm{aq})\) in the presence of air

Step-by-Step Solution

Answer

(c) \( \mathrm{Zn} \) metal and \( \mathrm{NaOH} \) produce \( \mathrm{H}_{2} \) gas.

Step 1: Understand the Reaction Involving Fe and HNO3

Fe (iron) reacting with concentrated \( \mathrm{HNO}_{3} \) (nitric acid) typically does not produce \( \mathrm{H}_{2} \) gas. Instead, nitric acid acts as an oxidizing agent and oxidizes iron to \( \mathrm{Fe}^{3+} \), releasing nitrogen oxides (such as \( \mathrm{NO}_{2} \)). Thus, no hydrogen gas is formed in this reaction.

Step 2: Examine Cu and HNO3 Reaction

Copper (Cu) does not react with concentrated \( \mathrm{HNO}_{3} \) to produce \( \mathrm{H}_{2} \) gas. Instead, copper is oxidized by nitric acid, releasing nitrogen oxides like \( \mathrm{NO} \) and \( \mathrm{NO}_{2} \), and forming \( \mathrm{Cu(NO}_{3})_{2} \). Therefore, no hydrogen gas is produced in this case as well.

Step 3: Check Zn Reaction with NaOH

Zinc (\( \mathrm{Zn} \)) reacts with aqueous sodium hydroxide (\( \mathrm{NaOH} \)) to produce hydrogen gas (\( \mathrm{H}_{2} \)). The reaction is as follows: \[ \mathrm{Zn} + 2\mathrm{NaOH} + 2\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Na}_{2}\mathrm{[Zn(OH)}_{4}\mathrm{]} + \mathrm{H}_{2} \] This reaction shows that hydrogen gas is indeed formed.

Step 4: Analyze Au and NaCN Reaction in Air

Gold (Au) does not react with \( \mathrm{NaCN} \) in the presence of air to produce \( \mathrm{H}_{2} \) gas. In fact, such conditions are more likely associated with the cyanide leaching process for extracting gold, where cyanide and oxygen dissolve gold to form a gold-cyanide complex, but hydrogen gas is not a product.

Key Concepts

Redox Reactions

Redox reactions, or oxidation-reduction reactions, involve the transfer of electrons between chemical species. Generally, one compound is oxidized, losing electrons, while another is reduced, gaining electrons. Understanding this electron transfer is crucial as it drives many chemical processes. For example:

• In the reaction between iron (Fe) and concentrated nitric acid (\( \mathrm{HNO}_{3} \)), iron is oxidized by losing electrons and becomes \( \mathrm{Fe}^{3+} \) ions.
• Nitric acid, on the other hand, is reduced, but instead of producing hydrogen gas, it produces nitrogen oxides due to the complex nature of nitric acid as an oxidizing agent.

This type of redox reaction is common but doesn't result in the formation of hydrogen gas. Instead, the release of nitrogen oxides occurs, showcasing how variations in redox reactions can yield different products.

Metal-Nonmetal Reactions

Metal-nonmetal reactions typically involve the transfer of electrons from the metal to the nonmetal, resulting in ionic bonds. These reactions often lead to the formation of new compounds. For example:

• In a typical reaction like that of zinc (\( \mathrm{Zn} \)) with sodium hydroxide (\( \mathrm{NaOH} \)), the metal donates electrons resulting in ionic compounds.
• When zinc reacts with sodium hydroxide in water, a fascinating reaction occurs, producing zincate ions and releasing hydrogen gas (\( \mathrm{H}_{2} \)).

Such reactions are crucial in understanding how metals behave with bases and the conditions under which hydrogen gas can be evolved. This particular reaction harnesses the reactive nature of zinc in an alkaline environment to produce a gas evolution, showing not all metal reactions are straightforward or involve only acids.

Gas Evolution Reactions

Gas evolution reactions are chemical reactions where a gas is released as a product. This type of reaction is very common in the laboratory and plays significant roles in various industrial processes.

• The typical hallmark of these reactions is the visible effervescence or bubbling as gas escapes from the solution.
• For instance, during the reaction of zinc with sodium hydroxide as mentioned earlier, the release of hydrogen gas (\( \mathrm{H}_{2} \)) is a classic example of a gas evolution reaction.
• Understanding the conditions that encourage gas production, such as reactant concentrations and specific metal behavior, is essential in leveraging these reactions effectively.

Gas evolution reactions not only demonstrate fundamental chemical principles, such as how gases form and escape but also provide practical applications like hydrogen gas production or detecting reactant completion in synthesis processes.