Problem 6 - Chemistry in 30 days

Question

Hydrogen is evolved by the action of cold dil. \(\mathrm{HNO}_{3}\) on (a) \(\mathrm{F}\) (b) \(\mathrm{Mg}\) (c) \(\mathrm{Cu}\) (d) \(\overrightarrow{\mathbf{A l}}\)

Step-by-Step Solution

Answer

Hydrogen is evolved when Mg reacts with cold dil. \( \text{HNO}_3 \).

Step 1: Understanding the Nature of Dilute HNO3

Dilute nitric acid is a weak oxidizing agent. Typically, it reacts with metals to produce various products, often involving the reduction of nitrate ions. Examining which metals can evolve hydrogen when reacting with dil. HNO3 involves understanding their activity in the reactivity series.

Step 2: Analyzing Reaction with Non-metals

Element \( \text{F} \) is a non-metal, specifically fluorine. Non-metals generally do not react with acids to release hydrogen gas. Thus, fluorine will not evolve hydrogen when reacted with dilute \( \text{HNO}_3 \).

Step 3: Examining the Reactivity of Mg with Dilute HNO3

Magnesium is a highly reactive metal that lies above hydrogen in the reactivity series. It can react with dilute \( \text{HNO}_3 \) to displace hydrogen. Therefore, hydrogen gas is evolved when magnesium is reacted with cold, dilute \( \text{HNO}_3 \).

Step 4: Assessing Cu Reactivity with HNO3

Copper is less reactive than hydrogen and does not react with dilute \( \text{HNO}_3 \) to produce hydrogen gas. Instead, copper reacts with concentrated nitric acid to produce nitrogen dioxide or other nitrogen oxides, but not hydrogen.

Step 5: Evaluating Aluminum's Reaction with Dilute HNO3

Aluminum is a metal that lies above hydrogen in the reactivity series. In the presence of a conductive environment and sufficient reaction time, aluminum may theoretically produce hydrogen. However, practical observations show that it typically forms complex oxides with nitric acid, hence no hydrogen gas is evolved.

Key Concepts

nitric acid reactions

Nitric acid (\( \text{HNO}_3 \)\u00a0) is a strong acid known for being an oxidizing agent. This means it not only donates protons (like most acids) but also accepts electrons from substances it comes into contact with. A product of this electron acceptance is usually in the form of a gas, typically involving nitrogen oxides like NO or NO\(_2\).
To help understand how nitric acid reacts, it's important to note that with metals, nitric acid usually does not release hydrogen gas. Instead, it tends to engage in redox reactions where other products are formed.

• For example, iron (Fe) may react with concentrated nitric acid to produce ferric nitrate, while copper (Cu) mostly forms nitrogen oxides.
• In specific cases, such as with magnesium (Mg), you might see hydrogen gas due to the initial reduction of nitrate followed by its reaction in varying conditions.

metal reactivity series

The metal reactivity series is a list of metals arranged in order of decreasing reactivity towards chemical reactions like oxidation and displacement. This series plays a crucial role in predicting and understanding how different metals will react with acids, such as nitric acid.
The basic idea is:

• Metals that are higher in the series react more vigorously with acids like hydrogen chloride (HCl), often evolving hydrogen gas.
• Metals below hydrogen in the series generally do not release hydrogen gas when reacting with dilute acids.

In the context of nitric acid:

• Magnesium (Mg), higher in the series, can react with dil. \( \text{HNO}_3 \) to produce hydrogen gas in certain cold conditions.
• Aluminum (Al), although above hydrogen, tends to form a protective oxide layer rapidly, reducing its reactivity in such situations.
• Copper (Cu) sits below hydrogen in the series and ordinarily won't release hydrogen gas when reacting with nitric acid.

evolution of hydrogen gas

The evolution of hydrogen gas is a fascinating process in reactions involving acids and metals. When a metal reacts with an acid, it can displace hydrogen ions in the acid, as the metal tends to give up electrons to these hydrogen ions, forming hydrogen gas (\( \text{H}_2 \)\u00a0).
Typically, only those metals capable of losing electrons more readily than hydrogen can engage in this kind of reaction.
Let's break this down:

• The reactivity of a metal is determined by its position in the metal reactivity series. Metals like magnesium (\( \text{Mg} \)\u00a0), which is very reactive, readily undergo this process.
• When \( \text{Mg} \)\u00a0 interacts with cold, dilute nitric acid, it can lead to hydrogen gas production, given the product depends on conditions like concentration and temperature.

In conclusion, the evolution of hydrogen gas is specific to certain metals and underlines the importance of the metal's efficiency in undergoing oxidation, as well as the nature of the surrounding conditions (like the specific type or concentration of acid involved).